Genetics Chapter 3

8/13/2019 Genetics Chapter 3

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Mitosis and Meiosis

Chapter 3



Prokaryotes, Archaea, and

Eukaryotes

The major difference between prokaryotes and

eukaryotes is the presence of a true nucleus in

eukaryotes



Eukaryotic Cell Organelles

GolgiNuclear

envelope Chromosomal

DNA Nucleus Nucleolus

Polyribosomes Ribosome Rough endoplasmicreticulum Cytoplasm Membrane protein

Plasma membrane Smooth endoplasmicreticulum

Mitochondrion MitochondrialDNA Centriole Microtubule

Lysosome

•Nucleus – contains the genome

- linear, double stranded DNA

•Golgi apparatus

- protein modification

•Mitochondria

- energy

production (ATP)



Eukaryotic Cell Organelles

•Endoplasmic reticulum

– lipid production and some protein translation

•Chloroplast (plant, algae, and some protozoans)

– photosynthesis



The Centrosome

• Found in most animals and lower plant cells

• Not membrane bound and is found in the cytosol

• Composed of cylindrical structures - centrioles



The centrosome functions to

organize the microtubules

•Microtubules – dynamic

protein polymers

•Composed of subunits:

• α tubulin

• β tubulin



The centrosome functions to

organize the microtubules

• Polarity

(-) or fixed end associated with the centrosome

(+) end extends toward cell periphery

• Spindle fibers that attach to chromosomes

during the early stages of mitosis and meiosis

are composed of microtubules



Chromosome – “Colored Body”

• Chromosomes are composed of double stranded DNA

and protein called chromatin• Euchromatin – loosely packed

• Heterochromatin – condensed and readily visible

• At interphase (period between nuclear division),chromosomes are diffuse and not readily visible

• Classified by location of spindle fiber attachment siteat the centromere

• On the surface of the centromere is a proteinaceous

substance where the microtubules attach - kinetochore



Giemsa-Stained Chromosomes

Telocentric

h l ifi d b d



Chromosomes are classified based on

centromere placement

For any particular chromosome the placement of thecentromere is fixed

- divides chromosome into 2 parts (p : short arm , q : long arm)



Chromosome Complement: Diploid

• Diploid – contain two sets of chromosomes

– Homologous chromosomes (homologs) – One chromosome comes from each parent

• humans have 23 homologous chromosome pairs

2n

= 46• dogs : 2n = 78

• fruit fly : 2n = 8

• garden pea : 2n = 14

• house mouse : 2n = 40

• lily : 2n = 24

• Indian fern : 2n = 1260 !

2n indicates cells are diploid



The physical location of a gene on a

chromosome is called its locus.

Homologouspair of

chromosomes

Gene loci (location)

A b c

A B c

Homologous Chromosomes



• Haploid cells – some eukaryotic cells and

human gametes (sex cells)

- only 1 set of chromosomes

- meiosis products: egg and sperm

- contain one member of each

homologous pair: n = 23

Chromosome Complement : Haploid



Karyotype Analysis

• Detect gross chromosomal abnormalities• Determine sex of the individual

• Homogametic sex (♀) – all gametes contain Xchromosomes

• Heterogametic sex (♂) – gametes contain an X or a Ychromosome



An Example of a Karyotype

A normal - Homogametic



The Cell Cycle

4 principle stages:

•Gap 1 (G1)

•DNA replication (S)•Gap 2 (G2)

•Mitosis (M)

G1, G2, and S = interphase

Go = leaves cell cycle, remains

viable



Mitosis: Cell Division

• Mitosis-Promoting Factor (MPF): initiates the

mitotic phase of the cell cycle

• Composed of 2 proteins

– Cyclin B: oscillates in quantity

– Constant gene product: CDC2

• kinase : enzyme that performs a phosphorylation

reaction

• CDC2 only functional when combined with

cyclin cyclin dependent kinase (CDK)

• Degradation triggered by anaphase promoting

complex (APC)



Mitosis: Cell Division



Cell Cycle Checkpoints•G1 /S

•Checks cell size•Examines DNA for damage

•S-phase promoting factor (SPF)

•G2 /M•Determines if DNA replication is

completed

•Examines for damaged DNA

•M

•Spindle fibers properly assembled

and attached to kinetochore



Preparing for Mitosis : S phase

DNA replication

A pair of sister chromatids

Kinetochore

(proteins attachedto the centromere)

Centromere

(DNA that ishidden beneaththe kinetochore

proteins)

Onechromatid

Onechromatid



Mitosis Begins - Prophase

•Shortening and thickening of the chromosomes

•Sister chromatids become visible

•Nuclear envelope and nucleolus disintegrates

D i h th t di id



During prophase, the centrosome divides

and moves to opposite poles of the cell

•Each newly divided centrosome radiates out microtubulesforming spindle fibers and asters.

•Microtubules elongate, shrink at their “+” end and attach to the

kinetochore

Mi t b l f diff t l tt h



Microtubules from different poles attach

to sister kinetochores

•Purpose – ensure

sister chromatidsmove to opposite

poles during

anaphase



During metaphase, the chromosomes

move to the equator of the cell

•Chromosomes

align themselves at

the metaphase

plate marking the

end of metaphase

(help to push

poles apart)



During anaphase, sister chromatids

move to opposite poles

• Physical separation of the chromatids

– check point in the cell cycle

• Migration to opposite poles of the cell

– destruction of cohesins

R l ti f i t h tid



Regulation of sister chromatid

separation

S i dl fib



Spindle fibers

separate the

daughterchromosomes

•The kinetochore

microtubules contract,

dragging the

chromosomes towardopposite poles

In anaphase the chromosomes are pulled



In anaphase, the chromosomes are pulled

through the cytosol

Two mechanisms:

• Proteins within the kinetochore act as a microtubule motor –

hydrolyzes ATP and moves toward the (-) end of the centrosome

• The chromosome is pulled toward the centrosome as the (+) end

of the centrosome disintegrates

In telophase the 2 daughter cells



In telophase, the 2 daughter cells

begin to return to the interphase state

- chromosomes uncoil direct protein synthesis

- nuclear envelope and nucleolus reforms

- cytokinesis : 2 daughter cells form



Mitosis in Onion Root Cells



Mitosis in an Animal Cell

Mitosis in a Plant Cell

Meiosis: generation of

http://localhost/var/www/apps/conversion/tmp/scratch_5/18.4-animal_cell_division.wmv


http://localhost/var/www/apps/conversion/tmp/scratch_5/18.3-plant_cell_division.wmv








Meiosis: generation of

haploid cells

• Haploid animal gametes and spores are formed

from diploid cells

• As a result of halving the diploid number there is

one copy of every chromosome present in the

haploid cell

• DNA replication occurs before meiosis as well as

before mitosis

Two Nuclear Divisions Are Necessary



Two Nuclear Divisions Are Necessary

without Further Replication of DNA

•Meiosis is a two-division process that produces 4

haploid cells

•Meiosis I

•Meiosis II

•Meiosis I: Stages include –

•Prophase I: Leptomena, Zygonema, Pachynema,

Diplonema, Diakinesis

•Metaphase I and Anaphase I

•Telophase I

•Meiosis II: Stages same as mitosis

M i i P h I



Meiosis : Prophase I

• chromosomes condense and tips are attached to the

nuclear membrane

• pairing of homologous chromosomes – bivalents

Recombination: Exchange of DNA between



Recombination: Exchange of DNA between

Nonsister Chromatids

•Crossing over in the tetrad

•Note the exchange of DNA

•Recombination nodules –

Enzymatic machinery that

correlates with recombination

Prophase I in Meiosis



Meiosis : Prophase I

• Nucleolus vanishes

• Nuclear membrane breaks down

• Spindle fibers attach to one kinetochorein each tetrad



Anaphase I of Meiosis



Anaphase I of Meiosis

Figure 3.25

• Meiosis I : Reductional Division

• Reduces the number of chromosomes from 2n to 1n

• Two sister chromatids represent one chromosome

• Dyad or monovalent present at pole of the cell

Dyad

or Monovalent

(half of tetrad)

M i i II A E ti l Di i i



Meiosis II: An Equational Division

• A mitotic division with the chromatids pulled to opposite poles

•For each cell entering meiosis, 4 haploid cells are produced



Meiosis: A Summary

• Reduces the diploid number of chromosomes by half- separation of alleles ; Mendel’s Law of Segregation

• Randomness of how the tetrads align on the metaphaseplate and separate during meiosis I

- Mendel’s Law of Independent Assortment

• Produces genetic diversity

- Recombination during Prophase I

Meiosis: Segregation



Homologous

chromosomes

segregate from each

other.

This leads to the

random segregation

of the alleles into

separate gametes.

Metaphase

Anaphase

Telophase

Meiosis II

Meiosis I

Prophase

Haploid cells

Heterozygous (Aa) cell

a

a

a

a a

a

a

A A

A

A

A A

A

Meiosis: Segregation

half the gametes receive one

homolog, the other half receive

the other homolog

Meiosis: Independent Assortment



Meiosis: Independent Assortment

Ry

Heterozygous diploidcell (YyRr ) toundergo meiosis

y

y

y

y y

y

y y

y

Y

Y Y

Y

Y Y

Y

Y Y

R

R

R r

r

y

r

r R

R

Y

R

R R

R

y

R

r

r r

r

Y

r

Y y

R r

R

r r

rY ry RY

Meiosis I

(two possiblearrangements

in metaphase)

Meiosis II

Ry rY ry RY

each gamete has an equal probability of receiving a particular combination of alleles

Independent Assortment and



Independent Assortment and

Recombination Lead to Genetic Diversity

•Recombination and

independent

assortment occur

simultaneously

** both can produce allele

combinations not present in

either parent

In a 2n cell with 2 sets of homologous chromosomes :

a single recombination event in 1 homologous pair 8 different

gametes possible

2 ABCD2 abcd

2 ABcd

2 abCD

1 ABCD

1 abcd

1 AbCD

1 aBcd

1 ABcd

1 abCD

1 Abcd1 aBCD



2 ABCD

2 abcd

2 ABcd

2 abCD

1 ABCD

1 abcd

1 AbCD

1 aBcd

1 ABcd

1 abCD

1 Abcd

1 aBCD

* Similarities : Mitosis & Meiosis



Similarities : Mitosis & Meiosis

• before both Mitosis &

Meiosis I :DNA replication occurs

producing sister

chromatids from each

chromosome

• Meiosis II &

Mitosis :

Spindle fibers separate

sister chromatids

* Diff Mi i & M i i



* Differences :Mitosis & Meiosis

•Haploid and diploid cells enter mitosis, onlydiploid cells complete meiosis

•Homologous chromosomes pair in Prophase

I of meiosis with no pairing in mitosis

•Kinetochore attaches to spindle fibers on

•sister chromatids going to opposite poles(mitosis)

•homologous chromosomes (meiosis)

* Diff Mi i & M i i



• Centromere

– mitosis – splits, sister chromatids go to

opposite poles

– meiosis I – does not split ; sister chromatids

go to the same pole

• Mitosis – a conservative process

• Meiosis – generates genetic diversity

* Differences : Mitosis & Meiosis

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio17.swf::Comparison%20of%20Meiosis%20and%20Mitosis



Comparison of Mitosis and Meiosis

Spermatogenesis Takes Place in the Testes





Spermatogenesis Takes Place in the Testes

The male produces 4 functional sperm for each

spermatogonium that enters meiosis

Oogenesis Takes Place in the Ovary



Oogenesis Takes Place in the Ovary

The female produces only one functional egg for each primary

oocyte that completes meiosis

Lif C l i th C Pl t



Life Cycle in the Corn Plant

•Diploid sporophyte – meiosis gives

rise to spores

•Spores germinate into alternate

generation•Haploid gametophyte which

produces gametes by mitosis

•In corn – mature corn plant is the

sporophyte

Generation of a corn plant requires a double fertilization event.

Some organisms exist as haploids in their



Some organisms exist as haploids in their

mature form (many fungi, protists)

• Use mitosis to produce morehaploid cells

• Some haploid cells have the

ability to fuse with other cells

• therefore, in these organisms,

gametes are produced by

mitosis

• Fusion generates a diploid cellthat can go through meiosis

•Purpose?

Increase in genetic diversity



Mitosis in fertilized frog cell

http://localhost/var/www/apps/conversion/tmp/Cell%20and%20Molecular%20Biology/Movies/01.1-developing_egg_cells.wmv

http://localhost/var/www/apps/conversion/tmp/Cell%20and%20Molecular%20Biology/Movies/01.1-developing_egg_cells.wmv

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Genetics Chapter 3

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