meiosis and genetic diversity

Question 1

what is mitosis

Answer 1

two successive divisions to form 4 daughter cells

number of chromosomes is halves

one complete set of chromosomes contains information to code for all parts of an organism - some organisms have only this - haploid organisms - haploid number = N

most organisms are diploid - two sets of chromosomes - 2N

diploid organisms inherit one complete set of maternal chromosomes from their mother and one complete set of chromosomes from their father

Question 2

stages of meiosis

Answer 2

interphase

meiosis 1 - PMATC
meiosis 2 - PMATC

Question 3

interphase

Answer 3

Interphase
• Synthesis of proteins, organelles, ATP etc (G1 phase).
• DNA Replication (S-phase).
• DNA is present in the decondensed form, i.e., long & threadlike (chromatin).

Question 4

prophase 1

Answer 4

Prophase I
• Chromatin condenses into chromosomes.
• Homologous Chromosomes will pair up, we call them bivalents.
• The homologous chromosomes exchange DNA by forming chiasma. This is known as crossing over.
• This process mixes the maternal and paternal alleles and creates new combinations.
• Crossing over increases genetic variation.
• Nuclear membrane disintegrates.
• Centrioles move towards opposite poles.

Question 5

metaphase 1

Answer 5

Metaphase I
• Bivalents line up along the equatorial plane of the cell (still in homologous pairs).
• Spindle fibres attach from the centrioles to the centromere of each chromosome.

Question 6

anaphase 1

Answer 6

Anaphase I
• Spindle fibres contract.
• One chromosome from each homologous pair is pulled to opposite poles.
• Independent assortment of chromosomes: cause of genetic variation.
• (Note that the centromere does not split in Anaphase I.)

Question 7

telophase 1

Answer 7

Telophase I
• New nuclear envelopes are formed (phospholipid synthesis).
• Centrioles are replicated.
• Chromosomes start to decondense.

Question 8

cytokinesis 1

Answer 8

Cytokinesis I
• A cleavage furrow is formed.
• Phospholipids are added to make new cell membrane to separate the two cells.
• Chromosomes decondense and become thread-like (chromatin).

Question 9

meiosis 2

Answer 9

At the end of Meiosis I, we have two daughter cells. These cells go into Prophase II without doing an Interphase.
The plane of division in Meiosis II is at right angles to the plane of division in Meiosis I. (Last time, the centrioles moved from the side to top and bottom, therefore the equator was horizontal. This time, the centrioles are at top (or bottom) of the cell, so they will move to the sides, so the equator will be vertical.)

Question 10

prophase 2

Answer 10

Prophase II
• Chromosomes become condensed, i.e., visible into individual chromosomes.
• Each chromosome consists of a centromere with two chromatids attached. But there is only one chromosome from each pair.
• The nuclear envelope starts to disappear.
• Centrioles become prominent and move to opposite poles.

Question 11

metaphase 2

Answer 11

Metaphase II
• Chromosomes line up along the equatorial plane of the cell.
• Spindle fibres attach from the centrioles to the centromere of each chromosome.
• The nuclear envelope is lost completely.

Question 12

anaphase 2

Answer 12

Anaphase II
• The spindle fibres contract.
• The centromeres are split.
• Chromatids from each chromosome are pulled towards opposite poles.
• This is known as independent assortment of chromatids: a cause of genetic variation.

Question 13

telophase 2

Answer 13

Telophase II
• New nuclear envelopes are formed (phospholipid synthesis).
• Centrioles are replicated.
• Chromosomes start to decondense.

Question 14

cytokinesis 2

Answer 14

Cytokinesis II
• A cleavage furrow is formed.
• Phospholipids are added to make new cell membrane to separate the two cells.
• Chromosomes decondense and become thread-like (chromatin).

Question 15

variations in meiosis

Answer 15

prophase 1 - we did crossing over

anaphase 1 - separated the pairs

anaphase 2 - separated the chromatids

Question 16

what should we do if question is about number of chromosomes

Answer 16

use the words DIPLOID and HAPLOID

Question 17

what should we do if question is about mass of DNA

Answer 17

use the words DOUBLE and HALVED in anser

Question 18

stages

Answer 18

G1 (each chromosome has 1 chromatid)

S-phase

G1 (each chromosome has 2 chromatids) 2x2N

meiosis 1 - separate the homologous chromosomes
each chromosome has 2 chromatids

meiosis 2 - separate the chromatids

Question 19

meiosis

Answer 19

1. Meiosis produces (4) cells with half the number of chromosomes as the parent cell.
   a. A DIPLOID CELL (2N) will produce HAPLOID gametes with N chromosomes.
   b. Many crop plants are tetraploid (4N) or hexaploid (6N) or octaploid (8N) or …c. Plants that are triploid (3N) or pentaploid (5N) cannot do meiosis, therefore cannot produce gametes - no sexual reproduction possible. They can only reproduce asexually by mitosis.

Question 20

gametes

Answer 20

GAMETES have half the number of chromosomes. Important because when two gametes fuse together (by fertilisation), the original number of chromosomes is restored in the zygote.

Question 21

genetic variation

Answer 21

2. The 4 gametes are genetically different from the parent cell AND FROM EACH OTHER, ie, Meiosis is a source of genetic variation. This occurs in three stages:
   a. Prophase I: crossing-over.
   b. Anaphase I: independent assortment of chromosomes.
   c. Anaphase II: independent assortment of chromatids.

Question 22

homologous chromosomes

Answer 22

a pair of chromosomes that contain the same genes but not necessarily the same alleles

Question 23

genes

Answer 23

a section of DNA at a particular locus coding for a polypeptide or for a functional RNA eg gene for hair texture, gene for insulin protein

Question 24

allele

Answer 24

an alternative form of a gene eg the gene for hair texture could have two alleles, one allele that codes for straight hair and another allele that codes for curly hair

Question 25

genome

Answer 25

all the DNA (includes genes and non-coding DNA) in an individual

The genome is the same in all cells (because cells divide by mitosis, therefore have the same DNA), but the proteome is different in different cells at different times (because different genes can be switched on / off for cells to specialise).

Question 26

proteome

Answer 26

all the proteins that can be coded by a cell

Question 27

non-disjunction during meiosis

Answer 27

Either Anaphase I or Anaphase II can go wrong, leading to one cell with too many chromosomes and another with too few. = Known as non-disjunction.
(This can also happen in Anaphase of Mitosis - such cells are removed by the immune system.)

nondisjunction at ana 1 - two gametes have 1 extra and 2 gametes have 1 less chromosome than normal

nondisjunction at ana 2 - 2 gametes have usual number, one gamete has 1 extra, one gamete has 1 less chromosome than normal

Question 28

determining is foetus has chromosomal abnormality

Answer 28

To determine if a foetus has a chromosomal abnormality, a procedure called amniocentesis is done, in which cells shed by the growing foetus into the amniotic fluid are removed. (Look up amniocentesis to see how they detect Down Syndrome foetuses.) The chromosomes in these cells are stained using a dye and then photographed and arranged to produce a KARYOTYPE.
Karyotypes are a pictorial representation of all chromosomes in a cell - tells us about the number of chromosomes, the length (additions and deletions), the banding pattern, ie, the appearance of chromosomes (to detect mutations that may not affect overall length).

Question 29

variation

Answer 29

differences between the individuals of the same species

Question 30

genetic variation

Answer 30

all the different alleles in a species (gene pool)

Question 31

species

Answer 31

a group of individuals that can interbreed to produce fertile offspring

Question 32

causes of variation

Answer 32

Causes of Variation

1. Mutation
2. Crossing-over in Prophase I of Meiosis
3. Independent Assortment of Chromosomes in Anaphase I of Meiosis
4. Independent Assortment of Chromatids in Anaphase II of Meiosis
5. Random fertilisation of gametes
6. Environment

Question 33

mutation

Answer 33

Mutation
Change in DNA base sequence.
Only source of new alleles.
Caused naturally or artificially.

errors in DNA replication and DNA recombination

due to mutagens eg chemicals or radiation