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Cell Division
Sexual Reproduction = egg & sperm
OR
Asexual Reproduction = single parent, no egg/sperm
Cell Division: reproduction of cells; “cells come from cells”
* Basis of all life2 Main Roles:
1) development of fertilized egg
2) continuation of life (growth, repair)
Prokaryotes = binary fission (split in half)
OR
Eukaryotes = more complex; more genetic material
chromosome: structure which contains DNA (deoxyribonucleic acid)
chromatin: long, thin fibers of DNA & protein clumping together to form chromosomes
gene: specific region of DNA on chromosomes
somatic cell: all body cells except egg & sperm; contain chromosomes
(humans= 46)
Human egg & sperm (gametes) have 23 chromosomes
Prior to Cell Division…
* All chromosomes duplicate…result in 2 identical parts = sister
chromatids (X-shaped)
* joined at centromere
When Cells Divide
* sister chromatids separate..each goes to separate cell (daughter
cell)
* each daughter cell has complete set of chromosomes
Overview of Cell Division
* eukaryotic cells divide according to cell cycle
cell cycle: sequence of events including time a cell divides until its daughter cell divide
Phases in the Cell Cycle
1) Interphase: most of cycle here
- chromosomes duplicate
- cell grows
2) Mitotic Phase: cell division phase
Includes Mitosis & Cytokinesis
* Mitosis unique to eukaryotes* Mitosis = continuous process but separated into defined stages
Stages of Mitosis
1) Prophase
- chromatin fibers coil to form discrete chromosomes
- sister chromatids
- nuclear membrane breaks near end2) Metaphase
- sister chromatids line up along center of cell
Stages of Mitosis
3) Anaphase
- sister chromatids separate & migrate to opposite ends of cell4) Telophase
- nuclear membrane reforms around chromosomes
Cytokinesis: division of cytoplasm
- usually occurs along with telophase
- daughter cells separate
- Certain drugs can stop cell cycle by preventing DNA synthesis, or inhibiting synthesis of necessary proteins
Ex: cancer drugs target rapidly dividing cells – including hair follicles and digestive tracts
homologous chromosome: matched pair of chromosomes; same length, genes for same traits at same loci
e.g., each chromosome has gene for hair color at same loci, but the gene may be for any color of hair … impt pt = gene results in some color of hair
locus (loci = plural): specific location of a gene on a chromosome
• homologous chromosomes have matching loci &
• One chromosome of each pair inherited from mother & father
Human Example
Somatic cells = 46 chromosomes
23 pairs of homologous chromosomes
22 pairs = autosome chromosomes (F & M)
1 pair = sex chromosomes; specific to the sex (M or F)
Sex Chromosomes
Human females 1 pair (2 XX)
Human male 1 pair (1X, 1Y)
• Are human male sex chromosomes homologous?
diploid cells: cells with 2 homologous sets of chromosomes in nucleus
total # chromosomes = diploid # = 2n
human diploid # = 46 (2x23=46)
• Humans = diploid animals because most of our cells = diploid (e.g., somatic cell)
• But, eggs & sperm are not diploid
gametes: egg & sperm cells (sexual reproduction only)
haploid cells: cells with 1 homologous set of chromosomes
haploid # = n
human haploid # = 23
• Human gametes are haploid
• Fertilized egg = zygote = ????
Why is there so much variety among species? (e.g., diversity in humans)
1) Independent orientation of chromosomes
- in Metaphase I --- way that tetrads line up is due to chance (random)
- Results in different possible combinations of chromosomes in gametes
- For humans = 8 million possible combos.!
2) Random fertilization (1 egg & 1 sperm)
What is probability that 1 of 8 million possible sperm fertilizes 1 of 8 million possible eggs????
Humans = (8 M) * (8 M) = 64 trillion possible combinations of chromosomes due to random fertilization!
3) Crossing Over
- can result in genetic recombination
genetic recombination: producing gene combinations different from those carried by original chromosomes
* During synapsis, tetrad formed – crossing over possible
1) homologous chromatids break at similar locations & chromatids join
2) h. chrom. separate at Anaphase I – crossing over
3) Meiosis II, sister chromatids separate
Mendelian Genetics
genetics = science of heredity
gene: specific region of genetic material (DNA) that provides provides the cell with a “map”
Goal: determine patterns of inheritance
Mendelian Genetics
Gregor Mendel – 1860’s monk
significant findings = offspring obtain discrete heritable factors (genes) from their parents
Mendelian Genetics
Gregor Mendel – 1860’s monk
-carefully chose organisms to study (garden pea), controlled pollinations, chose traits that were easy to observe, used statistical methods to analyze data
-significant findings = offspring obtain discrete heritable factors (genes) from their parents
Terms
self-fertilization: plant’s egg fertilized by it’s own pollen
cross-fertilization: plant’s egg fertilized by another plant’s pollen (hybridization)
P generation: parental generation
F1 generation: filial generation; hybrid offspring of the P generation
F2 generation: offspring produced by F1 generation via self-fertilization
Mendel’s Principles
1) Principle of Segregation – pairs of genes segregate during gamete formation; fertilization pairs genes again
monohybrid cross: cross of 2 individuals that differ in 1 trait
allele: alternate form of a gene found at same loci of homologous chromosomes
1) Principle of Segregation
Ex: Flower color (P = purple, p = white)
P = 1 Purple (PP) & 1 white (pp)
F1 = all Purple (Pp)
F2 = ¾ Purple (PP & Pp) ¼ white (pp)
homozygous: identical pair of alleles
heterozygous: 2 different alleles for a trait
phenotype: physical trait; appearance of organism; expressed as phenotypic ratio
genotype: genetic makeup of organism; expressed as genotypic ratio
• In the flower color example…..
What is the phenotypic ratio?
What is the genotypic ratio?
** For monohybrid cross… phenotypic ratio is always 3:1 & genotypic ratio is always 1:2:1
2) Principle of Independent Assortment
• each pair of alleles segregates independently during gamete formation
dihybrid cross: cross of 2 individuals that differ in 2 traits
2) Principle of Independent Assortment
ExampleP generation: Round (RR) & Yellow (YY) seeds = RRYY
Wrinkled (rr) & Green (yy) seeds = rryy
Gametes = RY and ry
F1 gen: All RrYy (Round & Yellow seeds)
Gametes = RY, Ry, rY, ryFemale
Male
RY
ry RrYy
2) Principle of Independent Assortment
Example (continued)F2 gen: (Do Punnett Square
Female
Male
RY ryRy rY
RY
Ry
rY
ry
2) Principle of Independent Assortment
Example (continued)F2 gen: (Do Punnett Square
Female
Male
RY ryRy rY
RY
Ry
rY
ry
RRYY
2) Principle of Independent Assortment
Example (continued)F2 gen: (Do Punnett Square
Female
Male
RY ry
RRYy
Ry rY
RY
Ry
rY
ry
RRYY RrYY RrYy
2) Principle of Independent Assortment
Example (continued)F2 gen: (Do Punnett Square
Female
Male
RY ry
RRYy
Ry rY
RY
Ry
rY
ry
RRYY
RRYy
RrYY RrYy
RRyy RryyRrYy
RrYY
rryyrrYyRryyRrYy
rrYyrrYYRrYy
Probabilities• Probability (chance) of an event occurring ranges
from 0 to 1
Probability = 0 = event will not occur
Probability = 1 = event will occur always
Tossing a Coin
What is the probability of getting a “tails”?
= 0.5 (1/2)
What is the probability of getting a “heads”?
= 0.5 (1/2)What is the probability of getting a “heads” or a “tails”?
= P(heads) + P(tails) = 0.5 + 0.5 = 1.0
Tossing 2 Coins
What is the probability of getting a “heads” on both coins?
= P(heads) x P (heads) = (0.5)*(0.5) = 0.25
Flower Color Example
F1 = Pp = 0.5 P & 0.5 p gametes
F2 = Pp x Pp
1 P (female) x 1 P (male) = 0.5 * 0.5 = 0.25 PP
1 P (female) x 1 p (male) = 0.5 * 0.5 = 0.25 Pp
1 p (female x 1 P (male) = 0.5 * 0.5 = 0.25 Pp
1 p (female) x 1 p (male) = 0.5 * 0.5 = 0.25 pp
• What is the probability of getting a heterozygote?• What is the probability of getting a homozygote?
Why are some flowers pink?
• Complete dominance = dominant & recessive alleles
• Incomplete dominance = F1 offspring have phenotype somewhere between that of the 2 parents = both alleles expressed
Ex: Flower color (R = red, r = white)
P = 1 Red (RR) & 1 white (rr)
F1 = all Reddish-White = Pink (Rr)
F2 = ¼ Red (RR), ¼ white (rr), ½ pink (Rr)
Incomplete Dominance
Pleiotropy vs. Polygenic Inheritance
• pleiotropy = 1 gene influence many traits
e.g., sickle-cell anemia = homozygous recessive disease
sickle-cell gene influences:
- shape of RBC’s
- health of heart, brain, spleen, kidneys• polygenic inheritance = many genes
influence 1 trait, e.g., skin color
- many genes interact to give diverse skin color ranging very dark to very light
Chromosomal Basis
Review Problem / “Test-Like Question”
Cross the following (assume complete dominance)
AABbCC X AabbCc
Where: A = no anemia a = anemia
B = brown eyes b = blue eyes
C = climb trees c = don’t climb trees
1) Find genotypes & phenotypes of F1 offspring & express in ratio format
2) Do a F1 X F1 cross using the AABbCC (one of the F1 offspring) & find the genotypes and phenotypes of the F2 offspring; express in ratio format
3) What is the probability of a F1 offspring having anemia, brown eyes, and tree-climbing abilities?