Meiosis

Key Terms

- Meiosis- Gametes- Recombination / Crossing Over- Chiasma- Independent Assortment

Chromosomes contain genetic information

Human somatic cells have 46 chromosomes

Divided into 23 pairs of homologous chromosomes, or homologs; one set of 23 from mother, one set of

23 from father.Specific traits are coded for in genes

Example: eye colour, hair colour, etc

Learning the Basics…

Each of your parents contribute to genes towards a specific trait, but they are different forms of the same gene, called alleles. Mom’s gene codes for blue eyes Dad’s gene codes for green eyes Both are alleles for the “eye colour

gene”

Recall:Mitosis involves cell division that

ensures both daughter cells receive a full set of chromosomes (diploid cells)

Sexual reproduction involves the “merging” of two gametes (sex cells) together (eg. sperm and egg)

Twice the chromosomes = New species?!?

Meiosis:

Division process that prevents this “doubling” of genetic material from occurring.

Creates gametes with half the number of chromosomes (haploid cells)

Fertilization is the fusion of 2 gametes (male + female) to get back to the diploid number

Stages of Meiosis

Prior to the start of meiosis, cell undergoes S Phase (chromosome replication)

Meiosis has two rounds of cell division:Meiosis IMeiosis II

Each round is divided into 4 sub phases:Prophase, Metaphase, Anaphase,

Telophase

Meiosis I

Interphase (S-phase)

Prophase I Chromosomes condense

& shorten, visible

Spindle fibres form

Centrioles moving towards poles of the cell

Homologous chromosomes form a tetrad made of 4 chromatids Process of forming a

tetrad is called synapsis

Tetrad of a homologous pair

Prophase I

In the tetrad, chromatids often break at the end and swap places with the sister chromatid

This process is called recombination or crossing over and allows for genetic variation

The location of recombination is called the chiasma

Recombination/Crossing Over:

Homologous pair of chromosomes(tetrad)

Metaphase I

Tetrads line up along equator randomly

= Independent Assortment

Spindle fibres attach to the pair of sister chromatids

Anaphase IThe sister chromatids do NOT

separate here (different than mitosis)Instead, the pairs of chromosomes

move apart to opposite poles

Telophase IChromosomes

condense slightly, nuclear membrane may form

Cytokinesis occurs forming two genetically different daughter cells

Each daughter cell has half the number of chromosomes as the parent cell (haploid - n)

Prophase I(early)

(diploid)

Prophase I(late)

(diploid)

Metaphase I(diploid)

Anaphase I(diploid)

Telophase I(diploid)

NucleusSpindlefibres

Nuclearenvelope

Meiosis I

Meiosis II

Meiosis II

Similar to mitosis, but no duplication of chromosomes during a very short interphase (no G1 or S phase)

Each chromosome (made of two "mixed" chromatids) lines up at equator, centromeres split, and each chromosome is pulled to opposite poles

End result – four haploid cells

Prophase II

Nuclear envelope begins to break down

Spindle fibres begin to form

Centrioles begin to move to poles

Metaphase II

Chromosomes align along equator of cell.

Spindle fibres attach to centromeres of sister chromatids

Anaphase II

Spindle fibres contract and pull sister chromatids apart

Telophase IINuclear

envelope assembles

Chromosomes decondense

Spindle disappears

Cytokinesis divides each cell into two

Results of Meiosis:

4 haploid gamete cells

1 copy of each chromosome

1 allele of each gene

Different combinations of alleles for different genes along the chromosome

Prophase II(haploid)

Metaphase II(haploid)

Anaphase II(haploid)

Telophase II(haploid)

Four Non-

identical haploid daughter

cells

Meiosis II

Sexual reproduction

Growth and repair

Role

At sexual maturityThroughout lifeWhen

Germline cellsSomatic cellsWhere

Half of parentSame as parentChromosome #

NoYesGenetically identical?

42Number of

daughter cells

21Number of

divisions

MeiosisMitosisVIDEO http://www.youtube.com/watch?v=kVMb4Js99tA

Chromosome Accounting

INSTRUCTIONS:o Complete the table below to show the number of chromosomes present

in different stages of cell division in a variety of species.

# of Chromosomes Present in Meiosisat Beginning of Each Phase

# ofChromosomesin Daughter

Cells ofMitosis

Diploid # Haploid # # of Pairs ofHomologous

Chromosomes Prophase I ProphaseII

TelophaseII

Cabbage18 18 9 9 18 9 9

Trillium5

Blackbear

38

Human23

Fruit Fly8

Peanut40