Topic 10.1 - Meiosis

Question 1

Chromosome replication

Answer 1

Like mitosis, meiosis has an interphase and during S, the chromosomes are replicated.

However, synapsing occurs where homologous chromosomes align beside each other. This is called a tetrad/bivalent as there are four chromatids/two homologous chromosomes.

In most eukaryotic cells, a protein-based structure forms between chromosomes (synaptonemal complex)

Question 2

Exchange of genetic material

Answer 2

Crossing over - occurs between non-sister chromatids of homologous chromosomes. (Sister chromatids are identical - swapping genes would not do anything)

During prophase I, breaks may occur in chromosomes and non-sister chromatids of homologous chromosomes may bind to these breaks.

Connection points of crossing over are called chiasmata.

Crossing over causes genetic variation within a species.

Question 3

Chiasmata

Answer 3

Chiasmata formation causes increased bivalent support

Crossing over causes swapping of paternal and maternal genes and can cause an unlinkage of linked genes and can cause an independent assortment of alleles.

Chromatids that have been crossed over are called recombinant chromosomes.

Question 4

Meiosis

Answer 4

Reduction division of a diploid nucleus to form haploid nuclei.

Has two stags: Meiosis I and meiosis II

Question 5

Prophase I

Answer 5

Chromosomes condense and thicken and are visible.

Homologous chromosomes (chromosomes with similar size with the same type of genes in the same place) line up together with a synaptonemal complex between them.

Crossing over occurs here, creating recombinant chromosomes.

Question 6

Metaphase I

Answer 6

Chromosomes lined up in pairs in the middle of the cell

Random orientation of bivalents occurs (causes genetic variation)

Question 7

Anaphase I

Answer 7

Chromosomes pulled away by spindle fibres

Question 8

Telophase I

Answer 8

Two new nuclei formed,

Cytokinesis follows, two new cells formed.

Question 9

Prophase II

Answer 9

(crossing over doesn’t occur again - there are not non-sister chromatids to have parts to cross over)

Chromosomes are condensed, spindles form.

Question 10

Metaphase II

Answer 10

Chromosomes lined up in the middle in a single line.

Question 11

Anaphase II

Answer 11

Chromatids pulled away by spindle fibres.

Question 12

Telophase II

Answer 12

Four nuclei are formed,

Cytokinesis follows, four new cells formed.

Question 13

Chromosome numbers

Chromatid numbers

Answer 13

Before I - after I - after M 1 - after M 2

46 - 46 - 23 - 23
46 - 92 - 46 - 23

(I = interphase, M = meiosis)

Question 14

Non-disjunction

Answer 14

Chromosomes do not correctly separate during anaphase (can be I or II)

Often leads to a syndrome (trisomy 21 - down’s)