meiosis and mitosis difference

7/31/2019 Meiosis and Mitosis Difference

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Meiosis A Source of Distinction

Why do you share some but not all characters of each parent?

What are the rules of this sharing game?

At one level, the

answers lie inmeiosis.


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Meiosis does two things -

1) Meiosis takes a cell with two copies of everychromosome (diploid) and makes cells with a

single copy of every chromosome (haploid).

This is a good idea if youre going to combine

two cells to make a new organism. This trick

is accomplished by halving chromosome

number.

In meiosis, one diploid cells produces four

haploid cells.


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Why do we need meiosis?

Meiosis is necessary to halve the number

of chromosomes going into the sex cells

Why halve the chromosomes in gametes?

At fertilization the male and female sex

cells will provide of the chromosomeseach so the offspring has genes from

both parents


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2) Meiosis scrambles the specific forms of

each gene that each sex cell (egg or sperm)receives.

This makes for a lot ofgenetic diversity. Thistrick is accomplished through independent

assortment and crossing-over.

Genetic diversity is important for the evolution

of populations and species.


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Meiosis

Parent cell

chromosome pair

Chromosomescopied

1st division - pairs split

2nd division produces

4 gamete cells with

the original no. of

chromosomes


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Meiosis mouse testes

Parent cell

4 gametes

1st division

2nd division


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The Stages of Meiosis:

aka: Reduction Division


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Meiosis I : Separates

Homologous Chromosomes

Interphase

Each of the chromosomes replicate

The result is two genetically identicalsister chromatids which remain

attached at their centromeres


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Prophase I

This is a crucial phase for mitosis.

During this phase each pair ofchromatids dont move to the equatoralone, they match up with their

homologous pair and fasten together(synapsis) in a group of four called atetrad.

Extremely IMPORTANT!!! It is during thisphase that crossing over can occur.

Crossing Over is the exchange ofsegments during synapsis.


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Metaphase I

The chromosomes line up at the

equator attached by their

centromeres to spindle fibers fromcentrioles.

Still in homologous pairs


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Anaphase I

The spindle guides the movement of the

chromosomes toward the poles

Sister chromatids remain attached

Move as a unit towards the same pole

The homologous chromosome moves

toward the opposite pole

Contrasts mitosis chromosomes appear as

individuals instead of pairs (meiosis)


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Telophase I

This is the end of the first meiotic cell

division.

The cytoplasm divides, forming two

new daughter cells.

Each of the newly formed cells has

half the number of the parent cells

chromosomes, but each chromosomeis already replicated ready for the

second meiotic cell division


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Cytokinesis

Occurs simultaneously withtelophase I Forms 2 daughter cells

Plant cells cell plate Animal cells cleavage furrows

NO FURTHER REPLICATION OF

GENETIC MATERIAL PRIOR TOTHE SECOND DIVISION OFMEIOSIS

Fi 13 Th f i i ll di i i M i i I


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Figure 13.7 The stages of meiotic cell division: Meiosis I


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Meiosis II :Separates sister chromatids

Proceeds similar to mitosis

THERE IS NO INTERPHASE II !


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Prophase II

Each of the daughter cells forms a

spindle, and the double stranded

chromosomes move toward theequator


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Metaphase II

The chromosomes are positioned

on the metaphase plate in a mitosis-

like fashion


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Anaphase II

The centromeres of sister

chromatids finally separate

The sister chromatids of each pairmove toward opposite poles

Now individual chromosomes


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Telophase II and Cytokinesis

Nuclei form at opposite poles of the

cell and cytokinesis occurs

After completion of cytokinesis thereare four daughter cells

All are haploid (n)

Figure 13 7 The stages of meiotic cell division: Meiosis II


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Figure 13.7 The stages of meiotic cell division: Meiosis II


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One Way Meiosis Makes Lots of

Different Sex Cells (Gametes)

Independent Assortment

Independent assortment produces 2n

distinct gametes, where n = the number

of unique chromosomes.

Thats a lot of diversity by this

mechanism alone.

In humans, n = 23 and 223 = 6,000,0000.


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Another Way Meiosis Makes Lots of Different

Sex Cells Crossing-Over

Crossing-over multiplies the already huge number of different gamete

types produced by independent assortment.


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The Key Difference Between Mitosis and Meiosis is

the Way Chromosomes Uniquely Pair and Align in

Meiosis

Mitosis The first (and

distinguishing)

division of meiosis


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meiosis and mitosis difference

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