2.6.5 The Stages Of Meiosis

Question 1

Stages of Meiosis

Answer 1

• meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells
• it produces gametes in plant and animals that are used in sexual reproduction
• it had many similarities to mitosis however it has 2 divisions: meiosis I and meiosis II
• within each division there are the following stages: prophase, metaphase, anaphase and telophase

Question 2

Prophase 1

Answer 2

DNA condenses and becomes visible as chromosomes
DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere
The chromosomes are arranged side by side in homologous pairs
- a pair of homologous chromosomes is called a bivalent
As the homologous chromosomes are very close together the crossing over of non-sites chromatids may occur
- the point at which crossing over occurs is called the chiasma (chiasmata for plural)
In this stage centrioles migrate to opposite poles and the spindle is formed
The nuclear envelope breaks down and the nucleolus disintegrates

Question 3

Metaphase 1

Answer 3

The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
The maternal and paternal chromosomes in each pair position themselves independently of the others; this is independent assortment
- this means that the proportion of paternal or maternal chromosomes that end up on each side of the equator is due to chance

Question 4

Anaphase 1

Answer 4

The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
The centromeres do not divide

Question 5

Telophase 1

Answer 5

The chromosomes arrive at opposite poles
Spindle fibres start to break down
Nuclear envelopes form around the two groups of chromosomes and the nucleoli reform
Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

Question 6

Cytokinesis

Answer 6

This is when the division of the cytoplasm occurs
Cell organelles also get distributed between the two developing cells
In animal cells, the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, diving the cytoplasm in half
In plant cells, vesicles from the Golgi Apparatus gather along the equator of the spindle (the cell plate)
The vesicles merge with each other to from the new cell surface membrane and also secrete a layer of calcium Pentateuch which becomes the middle lamella
Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cells
The end product of cytokinesis in meiosis 1 is two haploid cells

Question 7

Second Division of Meiosis: Meiosis II

Answer 7

There is no interphase between meiosis I and meiosis II so the DNA is not replicated
The second division of meiosis is almost identical to the stages of mitosis

Question 8

Prophase II

Answer 8

The nuclear envelope breaks down and chromosomes condense
A spindle form at a right angle to the old one

Question 9

Metaphase II

Answer 9

Chromosomes line up in a single file along the equator of the spindle

Question 10

Anaphase II

Answer 10

Centromeres divide and divide al chromatids are pulled to opposite poles
This creates 4 groups of chromosomes that have half the number of chromosomes compared to the original parent cell

Question 11

Telophase II

Answer 11

Nuclear membranes form around each group of chromosomes

Question 12

Cytokinesis (in Meiosis II)

Answer 12

Cytoplasm divides as new cell surface membranes are formed increasing four haploid cells