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1. Cell Division
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* Introduction* The need for new cells* The Cell Cycle
* Types of Cell Division1. Mitosis
2. Meiosis*Summary
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Introduction
1. Definition of a Cell:2.Growth and Development:3. Organization:
4. Function of a cell:
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The Need for New Cells
1).Growth
2).Replacement3).Repair
4).Reproduction
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The CELL CYCLE
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The Cell Cycle
To divide, each new cell has to undergo aphase of GROWTH and DEVELOPMENT.
It is after this phase that the cell attainsenough maturity and can complete allmetabolic processes that are necessarybefore entering the phase of cell division.
This preparatory phase of cells is termed asInter-phasefollowed by cell divisionMITOSIS
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INTERPHASE
It is the long phase in a body / somaticcell.
Before dividing, each cell has to attain thesame size as its mother cell.
It has 3 sub-stages:
a). G-1 Phaseb). S Phase (Synthesis Phase)
c). G-2 Phase
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G-1 PHASE
Its an initial growth phase or first growthphase of the cell.
The amount of cytoplasm increases and cyto-
plasmic organelles like mitochondria etc.replicates.
S-PHASE (Synthesis PHASE)-Chromosomes are duplicated by the synthesis
of more DNA
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G-2 PHASE This growth phase is shorter. Proteinrequired for cell division are synthesized.
Cell increases in size. Cells prepares itself to enter the next phase of
division Mitosis The nucleus of cell enlarges and a definite number
of fine, coiled thread like structures calledchromosomes become visible.
In fact at the beginning of mitosis the nuclearmaterial has already doubled. This stage is calledINTERPHASE, also called the Resting Stage when infact the cell is synthesizing the chromatinsubstance.
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CELL CYCLE
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What happens in INTERPHASE?
Cellsizeincreases.
Nucleusof dividing cellenlargesto its maximumsize.
Chromosomesreplicatei.e. a copy of eachchromosome is created.
Nucleolusisvery clear.
Theenergyfor cell division is stored as proteins.
Certainsubstancesnecessary forinitiationofcell division are formed.
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Cell Division When the cell has reached certain level of growth
it enters the division phase Where through a sequence of events the mature
parent cell divides into two daughter cells.
These phases are of various durations in differentcells, but the basic outline of sequences is thesame.
Cell division consists of two major stages:
1). Karyo-kinesis: Nuclear division where nucleusdivides into two.
2). Cyto-kinesis: Entire cells splits separating eachnuclei.
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Types of Cell Division
Cells in animals and plants divide in oneof the two ways described below:
1). Mitosis: that takes place in body cellsresulting in growth and development.
2). Meiosis: that takes place in gamete
producing cells resulting in gametes
that fuse forming zygote.
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MITOSIS Mitos = Thread and Osis = State Cell division process was first described by
SCHLEIDEN. This process takes place in Body or Somatic
Cells of organisms. The basic principle is The Chromosomenumber of cells is not altered (notchanged) in this process. Its justduplication.
Its a complex process where mother / parentsomatic cell divides to form two identicaldaughter cells.
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Phases of Mitosis
1). Prophase2). Metaphase
3). Anaphase
4). Telophase
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1). Prophase- Pro means First
- Nucleus is visible withlarge, shortened much likespring & thickenedChromosomes.- Nuclear Membrane &Nucleolusdisappear.- Centrioles two minutebodies outside the nucleusseparates & forms spindlefibres or astral rays &attaches to centromere of
chromosomes.- Most plant cells do nothave centrioles but stillhave a spindle network
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AsPROPHASEendsfollowingfeaturesare noticed
Chromosomes: thick, clearly visible, eachchromosomes split into 2 chromatids & remainin contact throughout at the centromere.
Nucleolus & Nuclear Membrane startsdisappearing.
Continuous spindle fibresjoin both centrioles.
Chromosomesattach to spindle bycentromeres and start moving towardsequatorial position.
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KINETOCHORE is the site
of attachment of spindlefibres.
CENTROMERE is the
center of attachment ofthe chromatids
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2). Metaphase
- Metameans After
- Short phase- Chromosomesbecomes most distinct.
- Duplication of
Chromosomes occursbefore prophase, butbecomes more evidentduring metaphase.
- Chromosomesarrange themselves atthe equatorial plane.
A B k
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3). Anaphase- Anameans Back
- Very active phase ofMitosis. Main features are
a). Homologous daughterchromosomes (2chromatids) of metaphasenow separate and begin tomove in the opposite
direction towards the polewith the contraction ofspindle fibres.
b). Chromosomes bend
around the centromereshowing as V or J or Lshaped structure based onthe length of its arms.
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4). Telophase
- Telo means End
- As soon as thechromatids cometogether at the endsof the spindle
network, thetelophase begins.
The various changes observed during
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The various changes observed during
TELOPHASE are:1). Chromosomes reach opposite poles. Spindle
network disintegrates but centrioles remain.2). Chromosomes uncoil, lengthen & thus becomes
thinner turning into a network of less distinctchromatin threads.
3). Nuclear membrane & Nucleolus startsforming around 2 sets of chromosomes inboth the newly formed daughter cells.
4). With the formation of nuclear membrane, 2 nucleiare formed & the process of nuclear division orKARYO-KINESIS is over. This is followed byCYTO-KINESIS.
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Cyto-Kinesis
- At this point the cytoplasm
between the two daughternucleiconstricts i.e. afurrow starts forming in theplasma membrane at themiddle.
- This deepens towards theinterior of the celluntil thecytoplasm is split into 2similar daughter cells.
- In plant cells constriction isnot observed duringcytokinesis.
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Difference in Plant and AnimalMitosis
PlantMitosis
1. Centrioles are absent inPlant Mitosis.
2. No contractile ring is found.
3. No cell constriction isformed.
4. In Telophase there is aformation of Cell Plate for
the process of cytokinesis.5. Nuclear and Cell division
are found in special regionscalled Meristems.
AnimalMitosis
1. Centrioles are present inAnimal Mitosis.
2. Formation of contractile ring
between 2 nuclei of thedividing cell
3. Cell constriction is formed inthe late Telophase for the
process of Cytokinesis.4. No cell plate is formed.
5. Cells divide everywhere, allthe time
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Importance of Mitosis
- Helpful in growth of tissues, organsand thus organism.
- Used for repair of damaged tissues andorgans.
- It is the method by which lower animalsreproduce.
- Hereditary characters are maintainedby the replication of chromosomes.
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Characteristics of Mitosis
- It takes place in Somatic / Body cells.
- It helps in tissue growth.
- 2 equal similar daughter cells areformed.
- Chromosome number is preserved.
- Daughter cells are identical to parentcell.
M i i
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Meiosis- Meiosis means To reduce.
- It is a specialkind of cell division that takesplace in reproductive / germ cellsonly.
- Cells in the reproductive tissuesmultiply innumber by mitotic divisions.
- The final division that producesmaturegametes however, are not mitotic.
- In fact these division are of the reducing
type.- The number of chromosomes in the cell is
halved in gametes.
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- When these haploid gametes fuse to forma zygote the diploid chromosome numberof organism is restored.
- In man, meiosisoccurs in testes producing
sperm and in female it occurs in ovariesproducing ova.
- In plant kingdommeiosis occurs in anthers
and ovaries of angiosperms (floweringplants) producing pollen grains and ovulesrespectively.
In Meiosis the cell divides in two
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In Meiosis the cell divides in two
sequences as below1). Heterotypicor
1st Meiotic Division- Here the diploid
chromosome number ofparent cell is reduced to
half (haploid) in daughtercells.
2). Homotypicor2nd Meiotic Division
- This is normal mitoticdivision with no change inthe number ofchromosome.
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1).Heterotypic Division (Meiosis-I)
It is also known as First Meiotic Division
Here the chromosome number is halved.
It consists of following phases1. Prophase I
2. Metaphase I
3. Anaphase I4. Telophase I
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All these phases be summed up as under:-
a). Pairing of Chromosomes:Homologous chromosomespairs seem to attract each other. They are similar
chromosomes-one each from either parent. Each pair ofhomologous chromosomes is also known as tetrad becauseit has four chromatids.
b). Crossing Over:While the homologous chromosomes are inthe tetrad formation the chromatids seem to cross each other
at one or at more points. The chromatids actually break andrejoin at these points of intersection known as Chiasmata.Alongs with parts of chromatids exchanged, genetic materialgets recombined into new combinations of genes.
c).Nuclear envelope, nucleolietc disappearand the
chromosomes move apart to the opposite poles and 2 newcells are formed. Thus at the end of first meiotic division, 2haploid daughter cells are form from a diploid parent cell.
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Significance of Crossing Over
- During Prophase-Icrossing over takes place.- The maternal and paternalhomologouschromosomes of an organism pair up andexchange certain portions of their chromatids.
- This crossing over is the basis of geneticvariations in organisms.
- This is why all offspring of 2organisms are notalike.
- Crossing over makes the genetic informationin each haploid cell unique.
- Each chromosome pair can crossover at many pointsmore than once.
2) H t i Di i i (M i i II)
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2). Homotypic Division (Meiosis-II)- This division is identical to Mitosis.- Here the chromosome number (haploid) is maintained
and at the end, four haploid daughter cells are produced.- The phases of second meiotic division are divisible into
Prophase-II, Metaphase-II, Anaphase-II and Telophase-II.
- Cytokinesis in meiosis varies greatly.- Sometimes two daughter cells produced at end ofMeiosis-I undergo only Karyo-kinesis and go throughMeiosis-II before dividing into four haploid daughtercells.
- At other times Cytokinesis occur at end of Meiosis-I andthus two haploid daughter cellsproduced under goMeiosis-II.
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Characteristics of Meiosis
Occur in germ cells only. Results in formation of gametes.
Results in formation of four haploid
daughter cells.
Chromosome number of parent cell is
halved. Daughter cellsdiffer from parent cell.
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Significance of Meiosis
Due to Meiosis, chromosome number isreduced to half in gametes andchromosome number of species ismaintained.
Due to crossing over in Prophase-I, geneticvariations occur.
Due to Meiosis, the zygote on fertilization
has one maternal and one paternalchromosome in each homologous pair.
Summary
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Summary Cell division is of2 types in higher plants and animals
1). Mitotic 2). Meiotic
Amitosis is the simplest cell division found in groupProtista. E.g. Amoeba, Paramoecium etc..
first: nucleus divides (karyo-kinesis)
second: cytoplasm divides (cyto-kinesis) Mitosis
- it is helpful in growth and repair oftissues
- it results in 2 daughter cells identical to parent cells in
all aspects- it takes place in somatic / body cells
- it takes place in one phase.
S
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Summary Meiosis
- it takes place in 2 phases.- it occurs in germ or reproductive cells.- it results in the formation of 4 daughtercells different from parent cells.
- it results in formation of gametes forreproduction.- it is called reduction division becausechromosome number of parent cell is
halved in daughter cells.- Crossing over in Meiosis is responsible forgenetic variations in individuals.
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CHAP. 2.Structure of Chromosomes
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1. Introduction
2. Discovery of Chromosomes3. Basic structure of
Chromosomes4. Significance of Chromosomes5. The DNA Strand
6. Importance of DNA7. Summary
Introduction
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Introduction Chromosomes are the chromatin
material inside the nucleus. Chromosomes are so called becausethey take up certain basic dye stains
very readily. Chromos = colour and soma =body
Chromosomes consist of Histoneproteins and DNA (deoxy-ribonucleicacid).
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DNA carry information for
synthesis of various proteinsrequired for physiological
functions. These stretches of DNA are
referred to as GENES
Chromosomes are best observedat Metaphase stage.
l h
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Colchicine
(0.3 ml of 1 % Colchicine)
It is used to stop cell cycle during Metaphase.
It is used
To check defects in lengths of arms
To ensure attachments of spindle fibres for
correct separation.
Colchicine is used in chromosome
Karyotype analysis.
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Definition of Chromosome
Chromosomes are chromatin materialthread like structure present inside the
nucleus made of DNA and Histone
proteins and are important in transfer ofgenetic material (genes) from one
generation to the future generation.
Also termed as Hereditary material.
Discovery of Chromosomes
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Discovery of Chromosomes E.Strasburger: Observed thread like structures
during cell division. Balbiani: Described rod like structures in nucleusbefore cell division.
W. Fleming: Described splitting of chromosomecalled stained material Chromatin
Waldeyer: Coined the term Chromosomes
Sutton & Boveri: Said chromosomes are physicalstructures and transmitters ofhereditary traits
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TELOMERE: A TELOMERE is a region of
repetitive DNAat the end of a chromosome,
which protects the end of the chromosomefrom deterioration. Its name is derived from
the Greek words telos "end" and mers
"part". The telomere regions deter the
degradation of genes near the ends of
chromosomes by allowing for the shortening
of chromosome ends, which necessarily
occurs during chromosome replication.
CENTROMERE A C t i
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CENTROMERE: A Centromere is a
region ofDNA typically found near the
middle of a chromosome where twoidentical sister chromatids come
closest in contact. It is involved in celldivision as the point of mitotic spindle
attachment. The sister chromatids are
attached all along their length, but
they are closest at the centromere.
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HISTONES: Histones are stronglyalkaline proteins found in eukaryoticcell nuclei, which package and orderthe DNA into structural units called
nucleosomes. They are the chiefprotein components of chromatin,act as spools around which DNA
winds, and play a role in generegulation.
Basic Structure of Chromosomes
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Basic Structure of Chromosomes1. Chromosomes are thread like bodies
presentin nuclei of animals and plants.2. They are covered with a SHEATH made of
proteins.
3. Inside this sheath is present a granularmatter referred as matrix.
4. Inside the matrix there are two threads
called Chromonemata which are thesubunits of chromatids & are presentduring Prophase.
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5. At Metaphase, the chromosome consists
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p ,of two symmetrical strands calledChromatids.
6. Each chromosome possess a distinctconstriction called Centromere (Primaryconstriction), divides into 2 parts and it
gets attached to the spindle network.7. The ends of chromosome are termed as
Telomeres and it protects from
deterioration orfrom being destroyed.8. Some chromosomes have anotherconstriction called Secondaryconstriction
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Chemical composition of chromosome
Substance AmountDNA 40 % approx
Histone proteins 50 %
Non-Histone proteins 8.5 %
Metallic ions like Mg++,Ca++ etc.
In traces
DNA packaging in Chromosome
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DNA packaging in Chromosome
Ch h k i d ili
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Chromosome: The packaging and coiling
The chromatin fibre is actually a very long DNA
strand coiled again and again.
The coiling is strictly according to the plan.
The DNA strand makes loops around apolymer
of 8 histone protein molecules.
Each such histone & DNA unit is called a
nucleosome
This coiled strand is coiled repeatedly, much like
a telephone cord till it becomes the
chromosome familiar.
Chromosome NumberCommon Name Chromosome No.
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Chromosome NumberCommon Name Chromosome No.
Animals:
1. Round worm 2
2. Mosquito 6
3. Fruit Fly 8
4.Human Beings 46
5.Butter Fly 446
6.Dog 78
7.Monkey 54
8.Mouse 40
Plants:
1. Bread Mould 2
2. Onion 16
3. Wheat 42
4. Sugar Cane 80
5. Grass 265
The number of
chromosomes isconstant for a particular
species.
Each species has its
own chromosome no..
Size of organisms hasno relation with its
chromosome numbere.g. Butterfly has 446
chromosomes than Man
(46)
Chromosome Size
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Chromosome Size
Chromosome size is usually measured at
metaphase when they are clearly visible. Plant generally have larger chromosomes as
compared to animals.
Chromosomes of Monocotyledonous plants arelarger than Dicotyledonous plants.
Chromosomes have a size range of0.20 0.50m
Chromosome vary in size from species to speciesand remain relatively constant for a particularspecies.
Human chromosomes are up to 6m in length.
Chromosome Shape
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Chromosome Shape Usually observed at mitotic
anaphase. The shape ofchromosome may be:
1). Telocentric: centromere at1 end makes chromosomerod shaped.
2). Acrocentric: centromerenear 1 end divides it into
very short and a long arm.3). Submetacentric:
centromere near centregives L or J shapedstructure.
4). Metacentric: centromere atcentre giving V shapedstructure.
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Depending upon the position of
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p g p p
centromere
MONOCENTRIC: If chromosome hasonly1centromerethen it is calledMONOCENTRIC.
DI-CENTRIC: while if there are 2centromeres, it is calledDI-CENTRIC
HOLOCENTRIC: If centromere isdiffused then the chromosome iscalled HOLOCENTRIC.
POLY CENTRIC if th
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POLY-CENTRIC: if there are
more centromeres, thechromosomes is known as
POLY-CENTRIC. ACENTRIC: If centromere is
absent, the chromosome isknown as ACENTRIC
Karyotype: A species is characterized by a set of
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Karyotype: A species is characterized by a set ofchromosomes whose features are constant in
individuals of the same species. This set of
characteristics is called a Karyotype. OR
The appearance of the chromosomal makeup of a
somatic cell in an individual / species (including the
number, appearance, arrangement, size andstructure of the chromosomes)
Idiogram: A Karyotype is depicted by a diagramusually. Here chromosomes of a haploid set arearranged in decreasing order of size. This is called anIdiogram.
Karyotype of a Human:
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Karyotype of a Human:
Chromosomes in Man
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The chromosome number in Man is 46 i.e. 23pairs.
Out of 23 pairs, 22 pairs are common in bothmales and females. These are calledAutosomes.
23rd pair of chromosomes is a small pair and it
determine the sexin the man. In male, 22 pairs are autosomes and 23rd pair
are sex chromosomes that are denoted by X &Y(22pairs of A + XY).
Similarly in females, 22 pairs are autosomesand 23rd pair are sex chromosomes but theyare identical i.e. there are 2 X chromosomes(22 pairs of A + XX).
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- The X and Y chromosomes are
different in shape and size.
- Y chromosome is Jshaped and issmaller than X chromosome. But
they pair up during meiosis.
Significance of Chromosomes
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g1. Chemicals of chromosome remains stable
even though the constituent chemicals of
cytoplasm of a cell are broken down.
2. During Cell division, no other material ofcell is shared as exactly as chromosomes.
3. They are the main source of chemical information that determines that a cellshould become like its parent cell.
4. They give the characteristic features of itsspecies during development.
5. The nuclei of gametes carry a set ofpaternal
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and maternal haploid chromosomes whichunite to form diploid zygote and give rise to a
unique individual.
6. The number of chromosomes is also
important. Sometimes addition or deletionof chromosomes in zygote can cause serious
disorders in the individual (MUTATION).
7. Sex chromosomes are also of greatsignificance as they determine the sex of an
individual.
DNA Strand
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Chromatin material is basically the DNA strand
wrapped around Histonesforming nucleosomes.This coiled strand is further coiled to form
chromatin filament.
DNA itself consists of two filaments or strandshelically coiledwith each other.
It is also called a macromolecule because it is a
very large single molecule.The detailed structure of DNA was worked out by
Watson & Crickin 1953.
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Watson and Crick DNA
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Watson and Crick DNA
model
- Each DNA strand has acomplex chemical structure.
- It is made up small building
units called nucleotides.-Each spiral of DNA has 10
nucleotides.
- Each nucleotide consists of 3
subunits (one molecule each
of pentose sugar, phosphate
and nitrogenous base).
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-The Phosphate and Sugar molecule are
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p garranged one above the other alternately whilethe base is oriented inwards, attached to ribose
sugar.-The base of one strand joins with the base of
the complementary strand with Hydrogen-bond
forming a spiral staircase where the basesforms the steps.-The nitrogenous base join only with its
complementary base thus Adenine (A) joins
with Thymine (T) through double bond &Cytosine (C) joins with Guanine (G) throughtriple bond i.e. A=T and CG
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Main Features of DNA
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There are 2 strands running anti-parallel to eachother & are spirally arranged around an axis so it is
called a double helical arrangement.Each DNA strand is made up of sugar, base &
phosphate.
Sugar are pentose type andbases are of 2 types:
1). Purines:Adenine (A) & Guanine (G).2). Pyrimidines:Cytosine (C) & Thymine (T).
Bases are connected with each other throughHydrogen Bondsi.e.A=T and GC.
Total number ofPurines = Pyrimidines
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i.e. A + G = T + CBase ratio i.e. (A + T) / (G + C) is constant for a given
species. This ratio is called Dissymmetry Ratio.
Bases are inside and sugars are outside and 2 sugars
are connected with Phosphoric Acid.
DNA has a property of Duplication. Each replicates
and from each replicated DNA another new DNA is
formed. This is called Replication of DNA.
Before Replication of DNA, Hydrogen bondsbetween the bases are broken.
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Importance of DNA
Very important because it is Hereditarymaterial.
It acts as director ofProtein Synthesis.
Some DNA of chromosome forms
nucleolus.
DNA in presence of enzymes formmRNA which acts as messenger.
DNA Replication
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DNA Replication
During Cell Cycle chromatin material
duplicated during inter-phase in order toprepare formitosis.
In this process, DNA double helix open up at
one end.When the 2 strands become free at one end,
new nucleotides start forming
complementary pairs and thus new strandsstart forming around each unraveling oldstrand sequentially.
DNA Replication
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DNA Replication
In the replication process the parent DNA molecule
unwinds and unzips. Then each of the old strands serves as the
template for the new strands.
Each daughter DNA molecule receives one parentalstrand and a newly synthesized strand.
This type of DNA replication is commonly called as
semi-conservative replication, because here eachdaughter DNA molecule receives one parental
strand.
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DNA replication requires following three steps:
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Unwinding: - The old strand that makes up the parent DNAmolecule is unwound and unzipped (weak hydrogen bonds
between the paired bases are broken). The hydrogen bondsbetween the molecules are broken with the help of Helicaseenzyme.
Complementary base pairing:- With the help of enzyme DNApolymerase new complementary nucleotides (that arealways present in the nucleus) are positioned adjacent toeach other opposite to the parent DNA template.
Joining:-This step also requires DNA polymerase enzyme for joining the complementary nucleotides. Each daughtermolecule contains an old and a new strand.
Replication of DNA strand has an origin point at which thereplication is initiated. It may also have a terminus point
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p y pwhere the replication of DNA is terminated. A ' Y ' shapedstructure is formed at the point of replication which iscalled as "replication fork ".
Replication of DNA may be unidirectional or bidirectional.During DNA replication, one nucleotide is joined withanother. Each nucleotide already has a phosphate groupat the 5' carbon atom and it is joined to 3' carbon atom of
the sugar molecule. Thus the synthesis of DNA molecule takes place in the
5'->3' direction with the help of DNA polymerase enzyme.But this causes a problem at the replication fork whereonly one of the new strands run in the 5'->3' direction( the template for this strand runs in the 3'->5' direction).This strand is called as LEADING STRAND.
The template for the other strand runs in the 5'->3'direction, but DNA synthesis could only take place in
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, y y p
5'->3' direction. Thus, this poses a problem and due tothis reason synthesis has to begin in the replication
fork.
Replication of the 5'->3' parental strand begins assoon as the DNA molecule unwinds and unzips
replication of this strand is discontinuous. Thereplication of this strand results in segment calledOKAZAKI FRAGMENTS.
Discontinuous replication takes more time thancontinuous replication therefore the new strand inthis case is called the LAGGING STRAND.
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DNA R li ti
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DNA Replication
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Summary Chromosomes are thread like structures in nucleus
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Chromosomes are thread like structures in nucleus.
They are studied generally at Metaphase stage of Mitosis.
Chromosomes are made up ofDNA & proteins. Chromosome number (2n) of species is constant.
Chromosomes may be rod shaped / V / J / L shaped.
DNA is the hereditary material.
It act as director ofProtein synthesis. There are 2 nucleic acids DNA & RNA.
Man has 46chromosomes i.e. 23 pairs.
Chromosomes Common to males and females are referredto as Autosomes (22 pairs in human).
Chromosome determining the sex of an individual are calledsex chromosomes (XY in males & XX in females).
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1). Introduction2) Gregor Johann Mendel
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2). Gregor Johann Mendel
3). Mendels Experiments
4). Terminology5). Monohybrid Cross
6). Di-hybrid Cross
7). Mendels Laws8). Exceptions to Mendels Law
9). Sex Chromosomes in Human
10). Determination of Sex11). Sex Linkage
12). Sex Linked inheritance of diseases
Introduction
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It is commonly observed that children resemble their
parents.
An individual inherits the characteristic features of the
species from its parents.
This resemblance is because the offspring inherits
certain Factors from the parents.
All the characters that offspring can inherit from the
parents are called Hereditary Characters and the
process of their transfer is called Heredity. Thus height, types of leaves, flower color, shape, seed
structure etc..are hereditary characters in plants.
- In the same manner curly hair, eye color, colorblindness etc in man are the hereditary characters
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blindness etc.in man are the hereditary characters.
- These factors are now know as genes that are located
at specific points on chromosomes.- These genes control a particular Character or Trait.
- Heredity: means transmission of genetically based
characteristics from parents to offspring.- Each gamete has a complete haploid set of these
genes.
- Thus, the zygote inherits one half of its geneticmaterial from either parent. This is the one reason
why children resemble both the parents.
Although offspring resemble their parents,they are rarely identical to them and they
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they are rarely identical to them and theyshow some dissimilarities from their parents.
Such dissimilarities in a species are called.
These differences are caused by genetic /
inherited variations and environmental / noninherited differences.
Only inherited variations can be passed to
offspring and have an influence in changingthe genetic constitution of the species.
Gregor Mendel
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Know as Father of GENETICS
Born in 1822 in Czechoslovakia.
He carried out expts on garden pea for 8 yrs.
On the basis of his results, he derived a fewfundamental principles.
He presented his results in a paper Expts in PlantHybridization before Brunn Natural History Society in1865.
Mendel repeated his work with some other plantsalso but failed to repeat his results.
Hugo de Vries, Carl Correns & Erich von Tschermakworking independently rediscovered his work andbrought Mendels expts to the limelight.
Reasons for Mendels Success
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He chose the garden pea for his expts.
Pea plants were easy to cultivate, had a short
lifespan and showed self-pollination as well
as easily distinguishable characters. This
helped him.
His success was also due to his meticulous
planning and laborious observations andrecord that enable him to have enough data
to be analyzed statistically.
Mendels Experiments
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p
& Techniques
Hybridization (crossbreed) involves
crossing of two
individuals with
different desired
characters to produce
an offspring that has
desired characters ofboth parents.
New terms
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New terms
Homozygous: refers to having identicalalleles for a single trait.
Eg. The gene for seed shape in pea plants
exists in two forms, one form or allele forround seed shape (R) and the other for
wrinkled seed shape (r). A homozygous plant
would contain the following alleles for seedshape: (RR) or (rr).
Heterozygous: refers to having two different
alleles for a single trait
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alleles for a single trait.
E.g.The gene for seed shape in pea plants existsin two forms, one form or allele for round seedshape (R) and the other for wrinkled seed shape(r). A heterozygous plant would contain thefollowing alleles for seed shape: (Rr).
Organisms have two alleles for each trait. Whenthe alleles of a pair are heterozygous, one isdominant and the other is recessive. Using the
previous example, round seed shape (R) isdominant and wrinkled seed shape (r) is recessive.Round: (RR) or (Rr), Wrinkled: (rr).
Picture showing Homozygous andHeterozygous
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Heterozygous
Mendels experiment
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He cultivated the pea plants with different
characters and studied till he obtained TRUEBREEDING / HOMOZYGOUS plants (TT). E.g.
Homozygosity for tall plants was tested by
growing seeds of tall plants [ gametes of purebreed tall plants are TT and for dwarf plants
are tt ].
Before experimenting he considered some
characters for his experiments.
Characters Mendel considered are
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1. Stem length: Tall / Dwarf
2. Flower position: Axial /
Terminal
3. Flower color: Red / White
4. Pod color: Green / Yellow
5. Pod shape: Inflated /Constricted
6. Seed shape: Round /
Wrinkled
7. Cotyledon color: Yellow /
White
The next step was HYBRIDIZATION
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He chose 2 parents with alternate forms of a
character. Eg. Red / White flower color, Tall /Dwarf stem length etc..
Then he removed the anthers of the plant, he
designated female, dusted its stigma withpollen of desired male and bagged it.
Such a cross was called
because it involved cross between alternateforms of one character only. The offspring was
F1 generation.
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Further
# Th M d l d i l h
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# Then, Mendel made a reciprocal cross where
the parents in the earlier cross were reserved
i.e. male as female and vice versa.
# In all these, the offspring demonstrated Red
flowers showing dominance of Red color.# The F2 generation was obtained by bagging
complete flowers of F1 generation enabling
them to self pollinate and produce seeds.
TERMINOLOGY
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Term Explanation Example
GENE The basic unit ofinheritance for a given
character
Height of theplant
ALLELE Alternate forms of thesame gene which
determine contrasting
characters
Tall (T) or
Dwarf (t)
Homozygous Diploid condition whereboth the alleles are
identical
TT or tt
Heterozygous Diploid condition whereboth the alleles are
Tt
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both the alleles are
different
Phenotype The physical or external andobservable expression of a
character
Tall,Dwarf
GenotypeThe genetic expression of a
character in terms of alleles
written in symbols
TT, tt, Tt
Dominant An allele which expresses
itself externally whenpresent in homozygous or
heterozygous conditions
TT or Tt
(both representtallness: T is
dominant)
Recessive An allele which expressesitself externally when
t or dwarf
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itself externally when
present in homozygous
condition but remainssuppressed in heterozygous
condition.
Monohybrid When only one pair ofalleles is used during
hybridization.
Tall x Dwarf
(TT) x (tt)
F1
Generation
The generation produced by
crossing two parental stocksis called first filial
generation.
(P) TT x tt
(F1) Tt
F2
G ti
The generation
produced by crossing(P) TT x tt
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Generation produced by crossingtwo individuals of F1
generation is calledsecond filial
generation.
(F1) Tt x Tt
(F2) TT Tt Tt tt
Test Cross A cross between therecessive parent and
an individual of F1
generation.
(P) tt x Tt (F1)
Definition
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Variation: Small differences betweenindividuals due to inheritance
Mutation: Sudden changes in one or more
genes in the progeny, which normally maynot have existed in the parents, grand
parents or even great grand parents.
E.g.Albinism (total loss of skin pigment)
Monohybrid Cross
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Monohybrid cross is one where the parents used
for hybridization differ in only one pair ofcontrasting characters or allele.
Eg: Take Homozygous / pure TALL (TT) & pure
DWARF (tt) plant. The initial plant materials are called parents or P.
These are cross pollinated using emasculation and
bagging. The seeds obtained are sown and plants so
obtained form the F1 or first filial generation.
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All plants of F1 generation are tall, the dwarf characterdoes not appear.
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The character appearing in F1 generation is
DOMINANT and one that is suppressed is RECESSIVE. When plants of F1 generation i.e. Tall hybrids are
allowed to self pollinate the F2 generation is
predominantly tall with some dwarf plants.
Always the ratio of 3 tall to 1 dwarf plant
approximately is observed without any exception.
On allowing F2 plants to self pollinate dwarf plants
produce only dwarfs, 1/3rd of tall plants produce only
tall plants and remaining 2/3rd of hybrid tall plants
again produce tall and dwarfs in 3:1 ratio.
Di-hybrid Cross
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In a dihybrid cross, 2 pairs of alleles or contrasting
characters are considered in parents. In his expt., Mendel used cotyledon color and seed
shape of pea as 2 pairs of allele
He followed the same method as in monohybrid cross
of the two parents one was a true breeding dominant
(yellow colored cotyledons and round seeds, YYRR)
and the other was a true breeding recessive (green
colored cotyledons and wrinkle seeds, yyrr).
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-
1). YYRR: Yellow Cotyledons Round Seeds
2). YYrr: Yellow Cotyledons Wrinkled Seeds
3). yyRR: Green Cotyledons Round Seeds
4). rryy: Green Cotyledons Wrinkled Seeds
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This gave an approximate phenotypic ratio of
9:3:3:1, which is know as di-hybrid ratio.
These results show that though dominant and
recessive alleles were present in same plant in
generation P, in F2 they separated independentlyform new combination of Yyrr and yyRR.
Thus, each pair segregate independent of each
other resulting in new combinations.
MENDELs LAW
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MENDEL s LAW
1). LAW OF DOMINANCE: In a givencross between 2 organisms with
pure contrasting alleles or
characters only one allele isexpressed in F1 generation; that
character that appears is called
dominant and the other recessive.
Law of Dominance
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) h
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2). LAW OF SEGREGATION: When a
pair of contrasting alleles cometogether due to hybridization in a
hybrid individual, the 2 characters
remain together without mixing or
losing their purity. They segregate
(separate) in the gametes of thehybrid.
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3).
When the parentsdiffer in two or more alleles then
the inheritance of one pair of alleles
does not depend on the other pair.
EXCEPTION OF MENDELs LAW
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1).Unlike Mendels F1 generations, in afew cases F1 generation has an
intermediated phenotype betweendominant and recessive alleles e.g. inor
heterozygous F1 Rr flowers are pink as
compared to homozygous P (RR) redand P (rr) white.
Incomplete Dominance
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Incomplete Dominance
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2). Linkage:
G h id
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Genes on same chromosomes are said to
be linked. The closer they are stronger thelinkage. Linked genes are inherited
together.
Mendel was lucky that none of his 7 alleles
showed linkage or his law of independent
assortment would not have been
formulated.
3). Multiple Allelism:
Each character may have more than 2 alleles
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ye.g. rabbits fur color has alleles. This cant
be explained by Mendels laws which statethat each character has 2 alleles only.
4). Gene Interactions: (Epistasis)
Sometimes a character (e.g. complexion inhumans) is controlled by more than 1 geneor factor. This again is an exception to the
Mendelian statement that each character iscontrolled by one gene / allele.
Gene Interaction
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1) D i d i h b
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1). Dominant and recessive characters can be
found.2). A hybrid with desired characters can be
produced easily on the basis of Mendels laws.
3). Crops can be improved.
4). Pure recessive characters can be used where
needed.
5). Genotypes and phenotypes of next generation
can be predicted even before cross is made.
Sex Chromosomes in Humans The chromosome number in Man is 46 i.e. 23
i
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pairs.
Out of 23 pairs, 22 pairs are common in bothmales and females. These are calledAutosomes.
23rd pair of chromosomes is a small pair and it
determine the sexin the man. In male, 22 pairs are Autosomes and 23rd pairare sex chromosomes that are denoted by X &Y (22pairs of A + XY).
Similarly in females, 22 pairs are Autosomesand 23rd pair are sex chromosomes but theyare identical i.e. there are 2 X chromosomes(22 pairs of A + XX).
Sex Chromosomes in Humans
The X and Y chromosomes are different in
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The X and Y chromosomes are different in
shape and size. Y chromosome is J shaped and is smaller
than X chromosome but they pair up during
meiosis. From the given information it is clear that
maleness is due to presence of one X and
one Y chromosome and femaleness ispresence of two X chromosomes.
Determination of Sex
During meiosis the sex chromosomes are
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During meiosis, the sex chromosomes are
also reduced to half like Autosomes forminga haploid gamete.
In female, ova will have one X chromosome +
22 Autosomes making the haploidchromosome number 23.
But in male, sperms half will have one X
chromosome + 22 Autosomes and the otherhalf will have one Y chromosome + 22Autosomes.
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In humans and many other species males produce
two types of gametes i.e. X and Y, hence they are
Heterogametic.
Females produce only one type of gametes i.e. X,hence they are Homogametic.
Sex determination in Human
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Sex Determination in Human
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Sex Linkage
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Sex chromosomes (X & Y) carry sex determining
genes i.e. male & female. But they also carry certain other genes that control
some important traits.
Such characters / traits that are controlled bygenes occurring on sex chromosomes are calledsex-linked characters.
The genes controlling these characters are referred
to as sex-linked genes and the inheritance of thesegenes is called sex-linkage or more commonly sex-linked inheritance.
SEX-Linked inheritance of Diseases
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Certain genes which occur on the X
chromosome are more likely to affect a malethan a female.
This is clearly evident in the case of a certain
form of colour blindness and haemophilia,
two important sex-linked inherited diseases
of human.
l k d h d h d
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It is a sex-linked inherited human disease.
People suffering from this disease are not able todistinguish between the colour red and green.
The gene for red-green colour blindness is carriedon the X-chromosome.
Normal vision is dominant over red-green colourblindness.
the condition was often called daltonism, although
this term is now used for a type of colour blindnesscalled deuteranopia.
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Haemophilia
Haemophilia is an another X linked inherited disease
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Haemophilia is an another X-linked inherited disease.
The people suffering from haemophilia are unable tosynthesize a normal blood protein called Anti-Haemophilic Globulin (AHG) that helps in clotting.
As a result, haemophiliacs can bleed for a long time
even from a very small cut. That is why haemophilia is also called Bleeders
disease.
The gene causing Haemophilia is also a recessive genelocated on the X chromosome similar to that of red-green color blindness.
Haemophilia chart
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Haemophilia chart
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Other X-linked Factors
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It include Brown Enamel on teeth.
Sexual characters like beard and muscularphysique in men, mammary glands, wider pelvis inwomen are not due to sex-linked genes.
Rather they are different expressions of the samegenes.
It is due to the presence of the X or Y chromosome
that creates an environment of maleness orfemaleness causing to the sex chromosomespresent in an individual.
A very few abnormalities are thought to be linkedh h b h
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to the Y chromosome but now even they are open
to more research. It should be remembered that the X-chromosome
of a man always comes from his mother while a
girl inherits one X-chromosome each from hermother and father.
Thus, a man can never transfer a sex-linked gene
directly to his son because the son inherits onlythe Y-chromosome from his father.
Father of Genetics: Gregor Mendel
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at e o Ge et cs: G ego e de
Mendels law: 1). Law of Dominance 2). Law of
segregation 3). Law of independent assortment.
According to Mendel each characters has two alleles:
Dominant and Recessive
Law of Segregation: it states that when a pair of
contrasting alleles come together in an organism they
remain pure and separate again in the gametes.
Law of independent assortment: it states that whenthe parents differ in two or more alleles then
inheritance of a pair of allele is independent of others.
There are exceptions to Mendels law e.g.
Incomplete Dominance, linkage, Multiple
Allelism and Gene Interactions
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Allelism and Gene Interactions.
Mendels law are important because genotypesand phenotypes of next generation can be
predicted even before cross is made.
Chromosomes common to male and femaleindividuals are called Autosomes.
Chromosomes that determine the sex of a
diploid individual are called sex chromosomes.
In humans, there are 46 chromosomes (23 pairs of
chromosomes: 22 pairs are Autosomes and 23rdpair is Sex chromosome, XY in male and XX in
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p ,female.)
Characters controlled by genes on sexchromosomes are called sex-linked characters.
Two important sex-linked inherited human
diseases are haemophilia and colour blindness. They are both due to a recessive gene present on
X-chromosome.
X-chromosome linked recessive diseases are lesscommon in female because of their heterozygosity.