MEIOSISMEIOSIS

MeiosisMeiosis

Division of sex cells, called gametes

MeiosisMeiosis is sexual reproductionsexual reproduction

Humans have 46 chromosomes

Numbers of Chromosomes

Diploid= 2 sets (2n) Haploid= 1 set (n)

Meiosis vs Mitosis

Meiosis is the division of gametes (sex cells)

Mitosis the division of somatic cells

There are two cell divisions during meiosis and only for mitosis!

Gametes

Sperm and egg

Fertilization

When sperm (n) and egg (n) fuse to make a zygote (2n)

A zygote is the product of the fusion of an ovum (egg) and a sperm. It contains two copies of each chromosome, one from each parent. Egg and sperms cells, on the other hand, each contain only one copy of each chromosome. The zygote develops into an embryo.

Sister Chromatids

Exact copies of DNA

Homologous Chromosomes

Pair of chromosomes have 2 chromatids each, with exact same genes on each

Homologous pairs (tetrads) are 2 chromosomes paired together

Homologous Chromosomes Each locus, where alleles or traits

are located, are in the same position on all chromosomes

Homologous Homologous ChromosomesChromosomes

Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are similar in shape and size.

Homologous pairs (tetrads) (tetrads) carry genes controlling the same inherited traits.

Each locuslocus (position of a gene) (position of a gene) is in the same position on homologues.

Humans have 23 pairs of homologous homologous chromosomes.chromosomes.

a. 22 pairs of autosomesautosomesb. 01 pair of sex chromosomessex chromosomes

Phases of Meiosis

Meiosis I (four phases)Meiosis I (four phases)

Cell division Cell division that reduces the chromosomechromosome number by one-half.one-half.

four phasesfour phases:

a.a. prophase Iprophase I

b.b. metaphase Imetaphase I

c.c. anaphase Ianaphase I

d.d. telophase Itelophase I

Interphase Interphase Chromosomes replicate during the

S phase of the cell cycle.

Each duplicated chromosome consists of 2 sister chromatids

atttached at their centromere.

Interphase Interphase

Centrioles duplicate

Chromosomes are visible

Interphase Interphase

NucleusNucleus and nucleolusnucleolus visible.

nuclear membrane

nucleolus

cell membrane

chromatin

Prophase 1Prophase 1

Chromosomes condense.

Synapsis occurs when 2 homologous chromosomes are paired (Which is a tetrad)

Prophase IProphase I - - SynapsisSynapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

Prophase IProphase I

centriolesspindle fiber

asterfibers

Prophase 1 Prophase 1

Crossing Over may occur between chromatids of different chromosomes in a tetrad.

Crossing Over - Crossing Over - variation variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

Metaphase IMetaphase I

TetradsTetrads align in the middle of the cell.

INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Mixes genetic info from maternal and paternal

chromosomes 2. Leads to different combinations of

chromosomes

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Independent Assortment

Anaphase IAnaphase I

Homologous chromosomes Homologous chromosomes separate and move towards the poles.

Sister chromatids Sister chromatids remain attached at their centromerescentromeres.

Anaphase IAnaphase I

Telophase ITelophase I

Each pole now has haploidhaploid set of chromosomeschromosomes.

CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

Telophase ITelophase I

Meiosis IIMeiosis II

No interphase II No interphase II (or very short - no more DNA DNA replicationreplication)

Remember:Remember: Meiosis II Meiosis II is similar to mitosismitosis

Prophase IIProphase II

Metaphase IIMetaphase II

metaphase platemetaphase plate

Anaphase IIAnaphase II

sister chromatids separate, sister chromatids separate, daughter chromosomes move daughter chromosomes move to different ends of the cellto different ends of the cell

Telophase IITelophase II

Nuclei form.

CytokinesisCytokinesis occurs.

Four haploid daughter cells produced.Four haploid daughter cells produced.

gametes = sperm or egggametes = sperm or egg

Telophase IITelophase II

VariationVariation

Important to population as the raw Important to population as the raw material for material for natural selectionnatural selection..

Question:Question:

What are the three sexual What are the three sexual sources ofsources of

genetic variation? genetic variation?

Answer:Answer:

1. crossing over (prophase I)1. crossing over (prophase I)

2. independent assortment 2. independent assortment (metaphase I)(metaphase I)

3. random fertilization3. random fertilization

Remember: variation is good!

Question:Question: A cell containing 20 chromosomes20 chromosomes

(diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

Answer:Answer:

10 chromosomes (haploid)10 chromosomes (haploid)

KaryotypeKaryotype A method of organizing the A method of organizing the chromosomes chromosomes

of a cell in relation to number, size, and of a cell in relation to number, size, and type.type.

Karyotype

Karyotype

Diploid= 2 copies

Triploid= 3 copies

Polyploid= more than 2 copies