5.2.1 - Meiosis

Question 1

DNA

Answer 1

deoxyribonucleic acid

• Strucutred in a double helix (circular in prokaryotes and linear in eukaryotes)
• Chromatin - DNA wrapped around histone proteins. This is how data is contained in the nucleus.

Question 2

Human Karyotype

Answer 2

There are 23 pairs of chromosomes

• Pairs because one is from the mother, one from the father.
  The 23rd chromosome is the sex chromosome (fem: XX, mal: XY)
  The other 22 are called autosomes
• In all body cells of organisms that reproduce sexually, there are is a maternal and paternal copy of each chromosome

Question 3

Homologous pairs

Answer 3

Genetically identical chromosomes:

• Joined by a centromre in the same location
• Length of the chromatids are the same
• Banding are the same
  Genes that are the same, but alelles that are different (from different parents)

Question 4

When to use sister chromatids/chromatids

Answer 4

Chromosome has two sister chromatids:

• Called sister when the chromosomes condense and to describe the chromatids as they are genetically identical

When they separate (centromere breaks), just call them two chromosomes (1 chromatid each)

Question 5

Mitosis and Meiosis similarities

Answer 5

Cell division starting from a diploid cell
Mitosis and meiosis go through the same phases.

• Interphase occurs before nuclear division.
• Chromatin material transforms into chromosomes in the same way during prophase in the first meiotic division.
• Break down of nuclear material and formation of the spindles are the same.
• Cytokinesis in meiosis takes place in the same manner.

Question 6

Meiosis - basic definition

Answer 6

Meiosis is the process that forms haploid gametes.

• 4 haploid daughter gamets are formed from each parent (diploid) cell.
• Genetic variation from crossing over and independent assortment.

Occurs in sex cells to produce gametes - in ovaries or testes

Undergoes two cell divisions
* Meiosis I: reduction division as a diploid cells results in haploid cells.
* Meiosis II: haploid forms two haploid.

Question 7

Interphase in Meiosis

Answer 7

• Before entering meiosis 1, the cell must go through interphase
• DNA replicates, goes through G1, S, G2 just the same.

Question 8

Prophase I

Answer 8

• Chromatin condenses and chromosomes separate into homologous pairs.
• Nuclear membrane breaks down
• Crossing over occurs (father and mother DNA exchanged) - genetic variation introduced
• Centrioles forms and moves to poles

Question 9

Metaphase I

Answer 9

• Homologous chromosomes align in pairs in the middle of the cell
• When organisms produce gametes (sperm or egg cells), the alleles for one gene separate into gametes independently of the alleles for another gene, assuming the genes are located on different chromosomes or are far apart on the same chromosome.
• Law of independent assortment: the inheritance of one trait genetically does not affect the inheritance of another, as alleles of different genes assort independently during metaphase.

Question 10

Anaphase I

Answer 10

• Homologous chromosomes pairs separate and each moves to opposite end of the cell by spindle fibres from the centrioles.
• Law of segregation: each allele in a gene pair separates (segregates) from the other to form gametes to each gamate receives only one allele for each gene.
• Difference with mitosis: the centromere does not split, whole chromosomes go to cell poles.

Question 11

Law of segregation

Answer 11

Each organism has two alleles for each gene (one from each parent).
These alleles segregate during gamete formation so that each gamete receives only one allele for each gene.

Anaphase I: homologus chromsome pairs separate with each chromosome going to a different daghter cell. This sets the foundation to realise the Law of Segregation, as it ensures that in meiosis II individual chromatids, not double stranded chromosomes are being segregated into new gametes.

Anaphase II: sister chromatids separate, ensuring that each gamete has only one allele for each gene

Question 12

Telophase I

Answer 12

• Chromosome number is halved
• Chromosome combinations in cells differ
• Chromosomes begin to diffuse and partly unwind.
• Spindle fibres break
• Nuclear membranes may or may not appear - as it dissolves in prophase II with a significant interphase.
• Contractile ring develops at the equator.

Question 13

Cytokinesis I

Answer 13

Membrane divides by a contractile ring.
Daughter cells are not identical to each other and have half the original number of chromosomes.

Question 14

Prophase II

Answer 14

Spindle fibres start forming again - but at a rotated angle of 90 degrees from original poles.

Question 15

Metaphase II

Answer 15

Chromosomes align at the equator

Question 16

Anaphase II

Answer 16

Daughter chromosomes move to opposite poles by spindle fibres
Centromeres break, and each chromosome has one chromatid.

Question 17

Telophase II

Answer 17

Nuclear membrane forms in daughter cells

Question 18

Cytokinesis II - Tetrad

Answer 18

Four resulting daughter cells are not genetically identical to each other and have half the original chromosome number.

Question 19

Recombinant chromatids

Answer 19

A mixture of paternal and maternal sequences in a chromatid.

Question 20

Non-recombinant

Answer 20

Chromatids that are completely maternal or paternal, and the original combination of alleles remain the same.