10.1 Meiosis10.1 Meiosis

Learning Targets: Learning Targets: Describe Describe chromosomes in the phases chromosomes in the phases

of meiosis. Outline chiasmata of meiosis. Outline chiasmata in crossing over. Explain how in crossing over. Explain how

meiosis results in genetic meiosis results in genetic variety.variety.

What are some advantages and What are some advantages and disadvantages of both asexual and disadvantages of both asexual and sexual reproduction?sexual reproduction?

Asexual – only requires one parent Asexual – only requires one parent (advantage), but offspring are clones (advantage), but offspring are clones (disadvantage for adaptation)(disadvantage for adaptation)

Sexual – requires two parents Sexual – requires two parents (disadvantage), produces unique (disadvantage), produces unique combinations and genetic diversity combinations and genetic diversity (advantage for adaptation)(advantage for adaptation)

Define:Define:Diploid – a cell with two sets of Diploid – a cell with two sets of

chromosomes – symbolized as 2nchromosomes – symbolized as 2n In humans, n=23, so diploid cells In humans, n=23, so diploid cells

have two sets of 23 chromosomes have two sets of 23 chromosomes (2n=46)(2n=46)

Examples: somatic (body) cells, a Examples: somatic (body) cells, a zygote (fertilized egg)zygote (fertilized egg)

Haploid – a cell with a single Haploid – a cell with a single chromosome set – symbolized as nchromosome set – symbolized as n

In humans, haploid cells have one In humans, haploid cells have one set of 23 chromosomesset of 23 chromosomes

Examples: gametes (egg and sperm Examples: gametes (egg and sperm cells formed by meiosis)cells formed by meiosis)

Homologous chromosomes – chromosomes Homologous chromosomes – chromosomes carrying genes for the same traits carrying genes for the same traits

Examples: your diploid cells contain Examples: your diploid cells contain chromosome 1 from your mother, and chromosome 1 from your mother, and chromosome 1 from your fatherchromosome 1 from your father

Gene – a segment of DNA in which the Gene – a segment of DNA in which the sequence carries the code for making an sequence carries the code for making an enzyme or protein (in most cases)enzyme or protein (in most cases)

Each chromosome carries a version of a gene for the same trait (ex. blue vs. brown eyes)

Sister chromatids are identical; Homologous chromosomes are

not

Label:Label:

Label:Label:

Karyotype

Homologous chromosomes

Sister chromatid

Chromosome

Sex chromosomes

Autosomes

1-22

Review: In what phase of the cell cycle Review: In what phase of the cell cycle is DNA synthesized to create is DNA synthesized to create identical sister chromatids? identical sister chromatids?

Interphase (S phase of interphase) Interphase (S phase of interphase) BEFORE mitosis or meiosisBEFORE mitosis or meiosis

Why is it important that meiosis Why is it important that meiosis reduces a cell’s chromosome number reduces a cell’s chromosome number to produce haploid gametes? to produce haploid gametes?

Otherwise a fertilized egg would Otherwise a fertilized egg would have double the normal have double the normal chromosomal material – it would be chromosomal material – it would be 4n, the next generation would be 8n, 4n, the next generation would be 8n, etc.etc.

Meiosis I:Meiosis I: Prophase I: Chromosomes Prophase I: Chromosomes

condense; homologous condense; homologous chromosomes form tetrads; chromosomes form tetrads; crossing over occurs at crossing over occurs at chiasmatachiasmata

Metaphase I: Tetrads line up at Metaphase I: Tetrads line up at midlinemidline

Anaphase I: Homologous pairs Anaphase I: Homologous pairs separateseparate

Telophase I and cytokinesis: 2 Telophase I and cytokinesis: 2 haploid cells form haploid cells form (chromosomes still consist of 2 (chromosomes still consist of 2 non-identical sister non-identical sister chromatids)chromatids)

Meiosis II:Meiosis II: similar to mitosis similar to mitosis Prophase II, Metaphase II: Prophase II, Metaphase II:

chromosomes line up at chromosomes line up at midlinemidline

Anaphase II: sister chromatids Anaphase II: sister chromatids separateseparate

Telophase II and cytokinesis: 4 Telophase II and cytokinesis: 4 haploid cells form haploid cells form

Mitosis All Cell Division Meiosis

One division Forms 2 diploid

daughter cells Daughter cells

are identical to parent cell

Functions in growth, repair

Chromosomes replicate once in interphase before division

Prophase, metaphase, anaphase, telophase

Two divisions Forms 4 haploid

cells Crossing over

occurs in prophase I

Cells are genetically unique

Produces gametes

Sexual reproduction reshuffles genes Sexual reproduction reshuffles genes to increase variation (but mutation is to increase variation (but mutation is the original source of all variation)the original source of all variation)

Sources of genetic variation: Sources of genetic variation:

1. Independent assortment of 1. Independent assortment of homologous chromosomes in Meiosis homologous chromosomes in Meiosis I, and non-identical sister chromatids I, and non-identical sister chromatids in Meiosis IIin Meiosis II 22nn possibilities, so 2 possibilities, so 22323 options for each options for each

human gametehuman gamete

In example below, n=2, so 2In example below, n=2, so 222= 4 different = 4 different possible gametespossible gametes

2. 2. Crossing over – genetic exchange creates – genetic exchange creates recombinant chromosomes with genes recombinant chromosomes with genes from each parentfrom each parent

Tetrad

Recombinant gametes

Chiasmata

3. Random fertilization – creates unique 3. Random fertilization – creates unique combinations of parental genescombinations of parental genes 2223 23 x 2x 223 23 options for each human zygote!options for each human zygote!