MeiosisIB Biology

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Definitions

• Chromosome pairs, one from each parent, that are similar in length, gene position and centromere location.

• A cell that contains one complete set of chromosomes.• A cell that contains two sets of chromosomes. One set of

chromosomes is donated from each parent.• Location of a gene on a chromosome• Different forms of the same gene• Sex cells that unite during sexual reproduction to form a new

cell called a zygote

Homologous pair

HaploidDiploid

Gene locus (loci)

Allele Gamete

Spermatogenesis

2n=46

humansex cell

diploid (2n)

n=23

n=23

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

Interphase I

• Similar to mitosis interphase.

• Chromosomes replicate (S phase).

• Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.

• Centriole pairs also replicate.

Interphase I

• Nucleus and nucleolus visible.

nuclear membrane

nucleolus

cell membrane

chromatin

http://www.lpscience.fatcow.com/jwanamaker/animations/meiosis.html

Meiosis I (four phases)

• Cell division that reduces the chromosome number by one-half.

• four phases:a. prophase Ib. metaphase Ic. anaphase Id. telophase I

Prophase I

• Longest and most complex phase (90%).

• Chromosomes condense.

• Synapsis occurs: homologous chromosomes come together to form a tetrad.

• Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

Prophase I - Synapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

Homologous Chromosomes• Pair of chromosomes (maternal and paternal) that are similar

in shape and size.

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

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

• Humans have 23 pairs of homologous chromosomes.

a. 22 pairs of autosomesb. 01 pair of sex chromosomes

Homologous Chromosomes

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Crossing Over

• Crossing over (variation) may occur between nonsister chromatids at the chiasmata.

• Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.

• Chiasmata (chiasma) are the sites of crossing over.

Crossing Over - variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

Sex Chromosomes

XX chromosome - female XY chromosome - male

Prophase I

centriolesspindle fiber

asterfibers

Metaphase I• Shortest phase

• Tetrads align on the metaphase plate.

• INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n

Example: 2n = 4then n = 2

thus 22 = 4 combinations

Metaphase I

metaphase plate

OR

metaphase plate

Question:

• In terms of Independent Assortment -how many different combinations of sperm could a human male produce?

Answer

• Formula: 2n

• Human chromosomes: 2n = 46 n = 23

• 223 = ~8 million combinations

Anaphase I

• Homologous chromosomes separate and move towards the poles.

• Sister chromatids remain attached at their centromeres.

Anaphase I

Telophase I

• Each pole now has haploid set of chromosomes.

• Cytokinesis occurs and two haploid daughter cells are formed.

Telophase I

Meiosis II

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

• Remember: Meiosis II is similar to mitosis

Prophase II

• same as prophase in mitosis

Metaphase II

• same as metaphase in mitosis

metaphase platemetaphase plate

Anaphase II

• same as anaphase in mitosis• sister chromatids separate

Telophase II

• Same as telophase in mitosis.

• Nuclei form.

• Cytokinesis occurs.

• Remember: four haploid daughter cells produced.

gametes = sperm or egg

Telophase II

Meiosis

2n=4

sex cell

diploid (2n)

n=2

n=2

meiosis I

n=2

n=2

n=2

n=2

sperm

haploid (n)

meiosis II

Variation

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

• Question:What are the three sexual sources ofgenetic variation?

Answer:

1. crossing over (prophase I)

2. independent assortment (metaphase I)

3. random fertilization

Remember: variation is good!

Question:

• A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer:

• 10 chromosomes (haploid)

Fertilization

• The fusion of a sperm and egg to form a zygote.• A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

Question:

• A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer:

• 10 chromosomes

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