Topic 3: Meiosis

Question 1

The ovum and sperm are two specialised animal cells called _________.

_________ are made by the process of_________.

The sperm and ovum fuse during ____________, to form a ________.

Answer 1

gametes

gametes

meiosis

fertilisation

zygote

Question 2

Most human body cells, including the ones in the ovaries and testes that make gametes, contain __ chromosomes

Half of these are from the ______ and half are from the ______.

Answer 2

46

father

mother

Question 3

A chromosome from the father and a chromosome from the mother, which contain the same _____ at the same ____ ____ (in the same position), with their _________ in the same position form a _____________ _____ ____ __________.

Answer 3

genes

gene loci

centromeres

homologous pair of chromosomes

Question 4

Key definition: define haploid (1 point)

Answer 4

1. HALF the number of chromosomes found in {body/SOMATIC CELLS}.

Question 5

Key definition: define diploid (1 point)

Answer 5

1. The FULL number of chromosomes found in {body/SOMATIC CELLS}.

Question 6

Consider a diploid cell in the ovary with 46 chromosomes, which will produce eggs by _______.

When it divides by meiosis, _ haploid cells, each with __ chromosomes are formed.

__ chromosomes will be needed to make these _ cells.

So the cell needs to carry out ____ ________ BEFORE meiosis.

If a copy of every one of the 46 {chromosomes/DNA molecules} is made, this gives __ {chromatids/DNA molecules}.

Answer 6

meiosis

4

23

92

4

DNA replication

92

Question 7

How does a cell prepare for cell meiosis?

3 steps

Answer 7

(It prepares by doubling the quantity of {chromosomes/DNA})

1. Chromosomes undergo DNA replication – each chromosome makes a copy of itself
2. The two chromosomes are attached to each other at a region called the CENTROMERE.
3. Each chromosome (now an X shape) is now made of 2 SISTER CHROMATIDS WHICH ARE IDENTICAL.

Question 8

What does meiosis form? (1 thing)

Answer 8

1. Gametes e.g. human sperm and egg

Question 9

Where does meiosis take place? (1 point)

Answer 9

1. In the mammalian ovaries and testes

Question 10

Meiosis has one type of ________ division.

Answer 10

nuclear (nucleus)

Question 11

Each diploid gamete producing cell (in ovary/testis) makes __ _______ _______, each with _____ the number of chromosomes as parent cell.

Answer 11

4 haploid gametes

half

Question 12

In meiosis, the gametes produced are NOT ___________ _________.

This leads to _________ _________ in offspring.

Answer 12

genetically identical

genetic variation

Question 13

What is the {role/importance} of {meiosis/making gametes}?

2 points

Answer 13

1. HALVES THE CHROMOSOME NUMBER, PRODUCING HAPLOID {GAMETES/cells} so that the diploid number is restored at fertilisation.
2. PROMOTES GENETIC VARIATION in gametes through INDEPENDENT ASSORTMENT OF CHROMOSOMES and CROSSING OVER OF ALLELES BETWEEN CHROMATIDS.

Question 14

Describe the process of meiosis

4 steps

Answer 14

1. Homologous chromosomes line up in pairs
2. (MEIOSIS I - first division) Homologous chromosomes then separate
3. (MEIOSIS II - second division) Sister chromatids separate
4. 4 haploid gametes are formed

Question 15

In both meiosis I and meiosis II (both divisions), chromosomes attach to _______ _____.

_______ _____ contract and shorten to pull the __________ (meiosis I) or _________(meiosis II) to opposite ends of the cell.

This ensures each gamete only receives one _____ of each _____.

Answer 15

spindle fibres

spindle fibres

chromosomes

chromatids

allele

gene

Question 16

Key Summary: State the two important features of meiosis which lead to genetic variation in the 4 gametes.
(2 points)

Answer 16

1. Crossing over of chromatids
2. Independent assortment OF CHROMOSOMES

NOTE: In exams, if meiosis is not mentioned in the question, for extra mark say: (point 1) + (point 2) + “in meiosis”

Question 17

Independent assortment of chromosomes occurs during ________ _ when the homologous chromosome pairs line up ready to divide

The way that the chromosome pairs line up is _______ and with 23 pairs there are a huge number of different arrangements

The different arrangements result in different combinations of chromosomes which come together in the ________.

Answer 17

meiosis I

random

gametes

Question 18

Key Summary: Explain Independent assortment of chromosomes

3 points

Answer 18

1. RANDOM ALIGNING OF {HOMOLOGOUS/maternal and paternal} CHROMOSOMES on the equator of the cell
2. Creates {new/DIFFERENT} COMBINATIONS OF CHROMOSOMES, SO ALSO NEW COMBINATIONS OF ALLELES, in the gametes
3. OCCURS DURING METAPHASE OF MEIOSIS I when homologous chromosome pairs line up on the equator

Question 19

During ________ __ ________ _, homologous chromosomes come together in pairs and chromatids come into contact.

____-________ __________ may cross over at a point called a _______.

Answer 19

prophase of meiosis I

Non-sister chromatids (chromatids on different chromosomes)

chiasma

Question 20

At each cross over point (chiasma), chromatids _____ and _____, exchanging alleles (+ sections of DNA) between ____-_______ __________.

Several chiasmata may occur along a chromosome.

Crossing over ___________ ________, so produces {_________ _________ / _________ _________} – contain new combinations of alleles from both parents = genetic variation.

Answer 20

break

rejoin

non-sister chromatids

recombines alleles

{recombinant chromatids/recombinant gametes}

Question 21

Key Summary: Explain crossing over

4 steps

Answer 21

1. During MEIOSIS I, when homologous chromosomes pair up, chromatids come into contact and NON-SISTER CHROMATIDS may cross over at a point called a CHIASMA.

2 (this point is important). At a chiasma, non-sister chromatids in the same chromosome pair break and rejoin, exchanging alleles between them.

3. This RECOMBINES ALLELES, so produces {recombinants/RECOMBINANT CHROMATIDS} and gametes which contain new combinations of alleles from both parents.
4. This produces genetic variation in gametes.

NOTE: A non-sister chromatids are chromatids on different chromosomes.