Meiosis

Question 1

Define Meiosis

Answer 1

Type of nuclear division that results in the formation of cells containing half the number of chromosomes of the parent cell.

Question 2

How many chromosomes in the human body are there ?

Answer 2

46 chromosomes
23 come from the mother
23 come from the father

Question 3

Main stages of meiosis

Answer 3

During the S phase of interphase, each chromosome was duplicated.

Two divisions and each in each division there are four stages.

Prophase I, metaphase I, anaphase I, telophase I.

The cell then enters a short interphase before starting the second meiotic division.

At the end of the second division, cytokinesis occurs.

Question 4

Meiosis in simple terms

Answer 4

Essentially two rounds of mitosis but with some very important differences.
You start with 1 diploid cell and end up with 4 haploid cells.

Question 5

Interphase

Answer 5

Before meiosis interphase occurs. Consists of two phases:

S phase - DNA replication
Growth phase (G1 + G2) - new organelles and proteins are made.

Question 6

Prophase I

Answer 6

Chromatin condenses and each chromosome supercoils.

Nuclear envelope breaks down, spindle threads of tubulin protein form.

Chromosomes come together in their homologous pair.

Crossing over occurs where non sister chromatids wrap around each other and may swap sections so that alleles are shuffled.

Question 7

Metaphase I

Answer 7

Pair of chromosomes attach along the equator of the spindle (still crossed over)

Each attaches to a spindle thread by its centromere.

Homologous pairs are arranged randomly, pairs facing opposite poles of the cell (independent assortment).

Question 8

Anaphase I

Answer 8

Members of each pair of chromosomes are pulled apart by motor proteins that drag them along the spindle fibres.

Centromeres do not divide.

Crossed over areas separate from each other, resulting in swapped areas of chromosome (allele shuffling)

Question 9

Telophase I

Answer 9

Most animal cells two new nuclear envelopes form around each set of chromosomes and cell divided by cytokinesis.

Then a short interphase when chromosomes uncoil.

Each new nucleus contains half the original number of chromosomes, but each chromosome consists of two chromatids.

In most plant cells, cell goes straight from anaphase I to prophase II.

Question 10

Prophase 2

Answer 10

If nuclear envelopes have reformed, then they break down.

Chromosomes coil and condense

Chromatids of each chromosome are no longer identical, due to crossing over in prophase I.

Spindles form

Question 11

Metaphase 2

Answer 11

Chromosomes attach by their centromere to the equator of the spindle.

Chromatids of each chromosome are randomly arranged.

Way that they are arranged will determine how the chromatids separate during anaphase

Question 12

Anaphase 2

Answer 12

Centromeres divide.

Chromatids of each chromosome are pulled apart by motor proteins that drag them along the spindle, towards opposite poles.

Chromatids therefore randomly segregated.

Question 13

Telophase 2

Answer 13

Nuclear envelopes form around each of the four haploid nuclei.

In animals, the two cells now divide to give four haploid cells.

In plants, a tetrad of four haploid cells is formed .

Question 14

How meiosis produces genetic variation?

Answer 14

Crossing over during prophase I shuffles alleles.

Independent assortment of chromosomes in anaphase I and 2 lead to random distribution of maternal and paternal chromosomes of each pair.

Haploid gametes are produced, which can undergo random fusion with gamete’s from another organism of the same species.