MeiosisSBI 3U

Agenda

1. Lesson Learning Goals2. Meiosis3. Meiosis Bingo

Learning Goals

By the end of today’s lesson, you will be able:

● To use proper vocabulary related to this unit, including meiosis, sexual reproduction, synapsis, etc.

● Understand and describe the specific phases of meiosis● Identify key similarities and differences between meiosis and

mitosis

Meiosis

● The form of cell division by which gametes, with half the regular number of chromosomes are produced

Diploid(2n) → Haploid (n)(complete set of chromosomes) (half the regular # of chromosomes)

● Meiosis is needed for sexual reproduction● Two divisions (meiosis I and meiosis II)

● Reproductive cells divide to produce haploid gametes (sperm or egg) that when combined through fertilization create a diploid zygote.

● Gametes have half the regular # of chromosomes.● This process occurs only in gonads (testes or ovaries).

○ Male: spermatogenesis (23 chromosomes)○ Female: oogenesis (23 chromosomes)

● Meiosis II is similar to mitosis with some chromosomal differences.

MeiosisMeiosis has two key outcomes:

1. Genetic Reduction: Meiosis is a form of cell division that produces daughter cells with half the number of chromosomes of the parent cell (23)

2. Genetic Recombination (Variation): The products of meiosis have different combinations of alleles. Genetic recombination gives rise to offspring that are genetically different from one another and their parents. This greatly increases genetic variation in a population

Sex Chromosomes

Spermatogenesis(Similar toOogenesis)

Interphase I

● Similar to mitotic interphase● Chromosomes replicate (S phase)● Each duplicated chromosome consist of two identical sister

chromatids attached at their centromeres● Centriole pairs also replicate (in animal cells)

Interphase I

● Nucleus and nucleolus visible

Meiosis I (four phases)

● Cell division reduces the chromosome number in daughter cells produced by one-half.

● Four phases:○ Prophase I○ Metaphase I○ Anaphase I○ Telophase I

Prophase I

● Longest and most complex phase (90%)● Chromosomes condense by lining up with their homologous pairs. This is called

synapsis.● Synapsis occurs: Homologous chromosomes come together and form a tetra

○ Homologous chromosomes are a pair of chromosomes (maternal and paternal) that are similar in shape and size, and which carry genes controlling the same inherited traits.

○ Each locus (position of a gene) is in the same position on homologues ■ Eg. chromosomes for moms hair colour and dads hair colour come together.

○ Humans have 23 pair of homologous chromosomes.■ 22 pairs of autosomes■ 1 pair of sex chromosomes

● Tetrad is a pair of homologous chromosomes or four chromatids (sister and non sister chromatids) that align in Prophase I

Homologous Chromosomes

Prophase I-Synapsis

Crossing Over

● Crossing over may occur between non sister chromatids at the chiasmata during Prophase I

● During crossing over segments of non-sister chromatids overlap, break, and reattach to the other.

● Crossing over increases variation in the gametes produced● Chiasmata are the sites of crossing over

Crossing Over - Variation

Meiosis Prophase I Mitosis Prophase

Metaphase I

● Shortest phase● Tetrads align on the metaphase plate. ● Independent assortment occurs:

○ Orientation of homologous pairs to poles is random.

Metaphase I

Mitosis Metaphase

Anaphase I

● Homologous chromosomes separate and move towards the poles● Sister chromatids remains attached at their centromeres

Mitosis Anaphase

Telophase I

● Each pole now has haploid set of chromosomes

● Cytokinesis occurs and two haploid daughter cells are formed

Meiosis II

● There is no interphase II ● There is 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

Anaphase II

● Same as anaphase in mitosis● Sister chromatids separate

Telophase II

● Same as telophase in mitosis● Nuclei form● Cytokinesis occurs● Four haploid daughter cells (gametes) are

produced

Telophase II

Meiosis Overview

MEIOSIS BINGO