Chapter 11: Meiosis and sexual reproduction

What is sexual reproduction?

• The most common method single celled and multicellular organisms use to reproduce

• combines cells (gametes) from 2 parents through fertilization

Compare and Contrast asexual and sexual reproduction

Asexual reproduction:

• all cell division by mitosis (no meiosis),

• offspring genetically identical to parents

Sexual reproduction:

• life cycle includes mitosis and meiosis

• offspring genetically different than parents due to 3 events:

  • crossing over in meiosis

  • independent assortment of homologues in meiosis

  • fertilization brings together genetically different gametes

What are the benefits of sexual reproduction over asexual reproduction?

Asexual reproduction

• faster population growth

• do not need to wait to find a mate

• if parent is well suited to environment, offspring will be similarly well suited

Sexual reproduction

• new genetic combinations

• some combinations likely to be better than parent

• allows fitness to increase over generations

Define meiosis

Reduces the number of chromosomes by ½ to produce haploid (1n) cells

What is the role of meiosis and fertilization?

• Meiosis → Reducing number of chromosomes

• Fertilization → resotoring number of chromosomes

What is fertilization?

Compare and contrast meiosis and mitosis?

Compare: both processes divide the nucleus in eukaryotic cells

Contrast:

• mitosis:

  • single division

  • produces genetically identical clones

• meiosis

  • two divisions

  • Homologous chromosome pair form tetrads

  • Crossing over and random alignment of chromosomes→ creating genetic variation

  • meiosis reduces ploidy (2n→1n)

What are the three major events associated with meiosis?

1. Chromosome replication in interphase prior to start of meiosis

2. Meiosis I

3. Meiosis II

What are the 3 phases in the cell cycle that precede meiosis?

G1 phase, S phase, G2 phase

Explain each stage of meiosis I

Prophase I

• chromosomes condense, nuclear envelope fragments

• homologous chromosomes pair and the synaptonemal complex holds them together in synapsis

• Segments of chromosomes can be exchanged through crossing over

• Visible structures at corss-over points called chiasmata

• the four chromatids held together form a tetrad or bivalent

• spindle fibers emerge from the centrosomes

Prometaphase II

• homologous chromosomes are attached to spindle microtubules at the fused kinetochore shared by sister chromatids

• chromosomes continue to condense, and nuclear envelope completely disappears

Metaphase I

• Homologous chromosomes randomly assemble at the metaphase plate, where they have been maneuvered into place by microtubles

Anaphase I

• spindle microtubles pull the homologus chromosomes apart

• Chiasmata are broken

• sister chromatids still attached at the centromere

Telaphase I & Cytokinesis

• Chromosomes arrive at the poles of the cell and begin to decondense

• nuclear envlelope forms around each nucleus

• cytoplasm divided by a clevage furrow

• result is two haploid cells

• each cell contains one duplicated copy of each homologous chromosome pair

Explain each stage of meiosis II

Prophase II

• chromosomes recondense

• duplicated centrosomes migrate to opposite poles and new spindles form

Prometaphase II

• nuclear envelope completely disappears

• spindle is fully formed

• each sister chromatid forms a kinetochore and attaches to microtubules from opposite poles

Metaphase and Anaphase II

• same as mitosis, sister chromatids separate and move toward opposite poles

Telophase II and Cytokinesis

• chromosomes arrive at opposite poles and decondense

• nuclear envelopes form around chromosomes

• cytokinesis separates the two cells into four unique haploid cells

What is synpasis?

The term for the process of homologues binding together

What is a synaptonenal complex?

Proteins that bind the homologues together during prophase

What is crossing over?

It is the process of exchanging parts of chromosome arms at a point called chiasmata

What is Chiasma?What is its role?

• Point of chromosome where crossing over happens.

• help keep homologous chromosomes attached to each other all the way until they separate

• Exchanges DNA between homologous chromosomes pair, allowing for genetic variation

What are tetrads?

Why is crossing over important?

• Creation of genetic diversity

• To hold homologous chromosomes together during prometaphase and metaphase after the synaptonemal complex has broken down

In prometaphase, why do microtubule kinetochores attach each homolog to only one pole