BIOL 1020 / Topic 7 / Meiosis

Gene

A unit of heredity as a section of DNA on chromosomes.

Protein-coding genes and non-protein coding genes code for different things. What do they individually code for?

• Protein-coding genes code for mRNA.

• Non-protein coding genes code for: tRNA, rRNA, and miRNA, and others.

Locus

A gene’s specific location on a chromosome.

What’s the difference between mitosis and meiosis in terms of energy?

Mitosis doesn’t need as much energy as meiosis because they don’t need to find a mate or court. Therefore, mitosis occurs at a faster rate than meiosis.

Meiosis

The type of eukaryotic cell division in sexually reproducing organisms that reduces the number of chromosomes.

How are chromosomes counted? By the number of what?

Centromeres.

What do we start with in meiosis?

One diploid cell with one nucleus. In that one nucleus, we have a pair of homologous chromosomes formed as a tetrad with an exchange of genetic material.

What do we end with in meioisis?

Four haploid cells, each with one copy of the homologous chromosomes or homologues.

What happens in prophase I?

Chromatin condenses into chromosomes. Each chromosome pairs up forming a tetrad, and crossing over occurs. The nuclear envelope breaks down, and the spindle fibres form and migrate to each end of the cell.

What happens in metaphase I?

Tetrads line up at equator, and the spindle apparatus reaches out to the centromeres of each chromosome, with one fibre attaching per chromosome.

What happens in anaphase I?

Spindles shorten and tetrads separate.

What happens in telophase I?

Homologues are now at opposite poles of the current cell, and they decondense in to chromatin. The nuclear envelop and nucleoli return.

After meiosis I, how many cells do we end up with?

Two haploid cells.

Meiosis II is very similar to meiosis I. The main difference lies in the fact that we didn’t replicate DNA and that we now have haploid cells. What did we separate in meiosis II, and what did we separate in meiosis I?

In meiosis II, we separate sister chromatids. In meiosis I, we separate homologous chromosomes.

These haploid cells… give me some examples of what these haploid cells are! At least three.

• Spores

• Egg

• Sperm

What are three sources of genetic variation?

• Crossing over

• Independent assortment

• Random fertilization

Crossing over

The phenomenon leading to exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids.

Recombinant chromosomes

The chromosomes once they’ve crossed over.

Cohesins

A protein complex that mediates sister chromatid cohesion.

Synaptonemal complex

A zipperlike protein complex aiding in the attachment of each homologue to each other.

Synapsis

The pairing of the homologues.

Chiasma

The location on a chromosome where crossing over occurs.

How does crossing over work?

1. After replication, sister chromatids are bound together via cohesins. However, there are proteins that breaks DNA.

2. The synaptonemal complex forms, attaching the two homologues together as the chromatin condenses into chromosomes.

3. The synaptonemal complex is fully formed, and the two homologues are in synapsis, wherein the DNA breaks are joined together by the corresponding segment of their non-sister chromatid.

4. The synaptonemal complex disassembles but are still kept together due to cohesins.

Independent assortment

The phenomenon wherein different genes are distributed independently during meiosis.

Random fertilization

The phenomenon wherein egg cells and sperm cells fuse by chance during fertilization.