*Meiosis chapter 6

*Mitosis Cell DivisionUsed for growth and repair

Occurs in somatic (body) cells

Produces two new cells identical to the parent cell

*Chromosomes

*ChromosomesChromosomes store genetic information

Human cells have 46 chromosomes: 23 pairs

*Chromosomes

*Chromosomes Duplicated chromosomes are called chromatids

chromatids held together by centromere

Called Sister Chromatids

*Body cells are diploid = 2nGametes arehaploid = n

The pairs are called homologs

*The Problem: How can the cell divide and cut its chromosome number in ?

How can we go from 2n to 1n?

*Meiosis

*Facts About MeiosisTwo divisions- Meiosis I and II

reduction- division

Original cell is diploid (2n)

Four daughter cells are haploid (1n)

*Here is the problem:

How do you cut the chromosome number in ?

How do you divide and get the right chromosomes into each cell?

*A major difference between the divisions of mitosis and meiosis:

The HOMOLOGS pair up before dividinghttp://www.pbs.org/wgbh/nova/baby/divi_flash.html

*Meiosis: Two Cell DivisionsHomologsseparateSister chromatidsseparateDiploidDiploidHaploid

*Prophase I Early prophaseHomologs pair.Crossing over occurs.Late prophaseChromosomes condense.Spindle forms.Nuclear envelope fragments.

*Crossing-OverHomologous chromosomes in a tetrad cross over each otherPieces of chromosomes or genes are exchangedProduces Genetic recombination in the offspring

*Crossing-over multiplies the already huge number of different gamete types produced by independent assortmentCrossing-Over

*Metaphase IHomologous pairs of chromosomes align along the equator of the cell

*Anaphase IHomologs separate and move to opposite poles.

Sister chromatids remain attached at their centromeres.

*Telophase INuclear envelopes reassemble.

Spindle disappears.

Cytokinesis divides cell into two.

*Prophase IINuclear envelope fragments.

Spindle forms.

*Metaphase IIChromosomes align along equator of cell.

*Anaphase IISister chromatids separate and move to opposite poles.EquatorPole

*Telophase IINuclear envelope assembles.

Chromosomes decondense.

Spindle disappears.

Cytokinesis divides cell into two.

*Results of MeiosisGametes (egg & sperm) form

Four haploid cells with one copy of each chromosome

One allele of each gene

Different combinations of alleles for different genes along the chromosome

*Comparing Mitosis and Meiosis

*

MitosisMeiosisWHAT it is used forHow many daughter cellsAre the daughters identical to the parent?If the parent is diploid with 16 chromosomes, what will the daughter be?Does the DNA need to replicate before division?Does crossing over occur?

*Comparison of Divisions

MitosisMeiosisNumber of divisionsNumber of daughter cellsGenetically identical?Chromosome #WhereWhenRole

*Comparison of Divisions

MitosisMeiosisNumber of divisions12

Number of daughter cells24Genetically identical?YesNoChromosome #Same as parentHalf of parentWhereSomatic cellsGerm cellsWhenThroughout lifeAt sexual maturityRoleGrowth and repairSexual reproduction

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