Cell Division

Question 1

Describe chromosome structure

Answer 1

• When a cell is not dividing, the DNA exists as chromatin within the nucleus. Chromatin consists of DNA and histone proteins and is found in the nucleus of eukaryotic cells
• When cell division begins, chromatin condenses to form chromosomes
• Chromosomes consist of DNA, protein and a little RNA. The DNA is made up of many small sections called genes
• In diploid cells, chromosomes are found in matching pairs called homologous chromosomes
• At the start of cell division, the DNA replicates (during interphase) and the chromosomes are visible as 2 identical sister chromatids joined at the centromere

Question 2

How many pairs of homologous chromosomes do humans have?

Answer 2

23 pairs

Question 3

What does each homologous pair consist of?

Answer 3

One chromosome inherited form the mother (maternal) and one chromosome inherited from the father (paternal)

Question 4

Finish the sentence: Gametes (sex cells) have one of each type of chromosome and are __________

Answer 4

Haploid (n)

Question 5

What activities occur in a cell during interphase?

Answer 5

• DNA replicates
• Organelles are produced
• Protein synthesis occurs
• ATP synthesis occurs

Question 6

State (in order) the 4 phases of mitosis

Answer 6

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Question 7

Describe prophase

Answer 7

• Chromatin condenses into visible sister chromatids joined at the centromere
• Centrioles move to opposite poles, forming protein microtubules and developing the spindle
• The nuclear envelope disintegrates, the nucleolus disappears and chromosomes move towards the cell equator

Question 8

Which is the longest stage of mitosis?

Answer 8

Prophase

Question 9

Describe metaphase

Answer 9

Chromosomes align along the cell equator and attach to spindle fibres at the centromere

Question 10

Describe anaphase

Answer 10

• Rapid stage in which spindle fibres contract and shorten, the centromere splits and sister chromatids are pulled apart
• Chromatids are drawn to opposite poles with the centromere leading

Question 11

Describe telophase

Answer 11

• Chromosomes uncoil and lengthen
• Spindle fibres break down
• Nucleolus and nuclear envelope reform

Question 12

Describe cytokinesis in animals

Answer 12

Constriction of the parent cell from the outside inwards, forming a cleavage furrow

Question 13

Describe cytokinesis in plants

Answer 13

A cell plate forms across the equator of the parent cell from the inside outwards and a new cellulose wall is laid down

Question 14

State 5 differences in mitosis between plants and animals
(Shape, centrioles, cytokinesis, spindle, occurrence)

Answer 14

1. Animals: cells become rounded before mitosis, Plants: no shape change
2. Animals: centrioles are present, Plants: centrioles absent
3. Animals: during cytokinesis, cleavage furrow develops from the outside inwards, pinching the cytoplasm, Plants: cell plate develops from the centre outwards, forming a new cell wall
4. Animals: spindle degenerates at telophase, Plants: spindle remains throughout new cell wall formation
5. Animals: in epithelial tissue, bone marrow and other sites, Plants: in meristems (tips of roots and shoots)

Question 15

Finish the sentence: Mitosis leads to ________ of an organism, _______ of damaged tissues and ____________ of dead cells
Give an example form plants and animals

Answer 15

Growth, repair, replacement
Plants: mitosis occurs in root tips and growing points of shoots
Animals: in human skin, mitosis replaces dead surface cells with genetically identical cells from below

Question 16

Finish the sentence: Most animals and plants also produce _______ _________, which are not identical to the parent cell, allowing for ________ ___________

Answer 16

Haploid gametes, sexual reproduction

Question 17

State 3 advantages of asexual reproduction

Answer 17

• Offspring are genetically identical to the parent, so they are able to survive successfully if the environmental conditions are stable
• Less time and energy are required as there is no need to produce gametes and find a reproductive partner
• The population can rapidly increase in number if environmental conditions are stable

Question 18

Give a disadvantage of asexual reproduction

Answer 18

• Lack of genetic variation means offspring cannot adapt to changing environmental conditions

Question 19

_____________ such as radiation, certain chemicals and some viruses can mutate DNA and affect the genes that control the cell cycle

Answer 19

Carcinogens

Question 20

What regulates the cell cycle by signalling when cells can divide? What happens if these genes mutate?

Answer 20

Proto-oncogenes. If these genes mutate, uncontrollable cell division can occur, leading to tumour formation. These mutated genes are called oncogenes.

Question 21

State and describe the 2 types of tumour that can form

Answer 21

• Benign - non cancerous and won’t invade nearby tissues
• Malignant - cancerous and can spread to other tissues

Question 22

Meiosis produces __________ in ___________ reproducing organisms, resulting in __ genetically different daughter cells

Answer 22

Gametes, sexually, 4

Question 23

Describe prophase I

Answer 23

• Chromosomes condense into visible sister chromatids joined at the centromere
• Homologous chromosomes pair up in a bivalent
• Chromatids from homologous pairs wrap around each other and partially repel each other but remained joined at the chiasmata. A chiasmata is where homologous chromosomes can exchange equivalent sections of DNA. This swapping is called crossing over.
• Nuclear envelope disintegrates, nucleolus disappears, centrioles move to opposite poles and spindle microtubules form

Question 24

Describe metaphase I

Answer 24

• Chromosomes align themselves either side of the equator in their homologous pairs, attached to spindle fibres at their centromeres
• Homologous pairs of chromosomes (bivalents) align randomly along the equator, leading to independent assortment
• This results in a random combination of paternal and maternal chromosomes in each daughter cell

Question 25

Describe anaphase I

Answer 25

- Spindle fibres contract and shorten, separating bivalents and chromosomes are pulled to opposite poles - Each pole receives one chromosome from each homologous pair. This results in a random mix of maternal and paternal chromosomes due to independent assortment in metaphase I

Question 26

Describe telophase I

Answer 26

- In some species, the nuclear envelope reforms around the haploid chromosomes, which uncoil and lengthen. In many species, chromosomes remain condensed - Cytokinesis then occurs - when the cytoplasm is divided, producing 2 haploid gametes

Question 27

Describe prophase II

Answer 27

Centrioles migrate to the poles and the new spindle develops at right angles to the previous one

Question 28

Describe metaphase II

Answer 28

- Chromosomes align on the equator, attached to spindle fibres at their centromere - independent assortment occurs with chromatids randomly facing either pole

Question 29

Describe anaphase II

Answer 29

- Spindle fibres contract and shorten - Centromeres split and sister chromatids are pulled to opposite poles

Question 30

Describe telophase II and cytokinesis

Answer 30

- Chromosomes uncoil and lengthen and are no longer visible under a microscope - Spindle breaks down - Nuclear envelope and nucleolus reform Cytokinesis produces 4 haploid daughter cells

Question 31

In what 3 ways does meiosis introduce genetic variation?

Answer 31

1. Crossing over 2. Independent assortment 3. Random fusion of haploid gametes

Question 32

Describe how meiosis introduces genetic variation through crossing over

Answer 32

- During prophase I, chromosomes condense, and homologous pairs form a bivalent, this process is called **synapsis** - Chromatids in the bivalent join at chiasmata, where parts of the chromosome may be exchanged (crossing over)

Question 33

Describe how meiosis introduces genetic variation through independent assortment

Answer 33

- During metaphase I and II, chromosomes randomly distribute themselves at the equator of the cell - This random distribution leads to independent assortment of chromosomes. With 3 pairs of chromosomes there are 2^3 = 8 possible combinations of maternal and paternal chromosomes

Question 34

Describe how meiosis introduces genetic variation through random fusion of haploid gametes

Answer 34

- At fertilisation, the diploid chromosome number is restored when two random haploid gametes fuse to form a zygote - Half of the chromosomes in the new cell come from the maternal gamete and half come from the paternal gamete

Question 35

State 2 advantages of **sexual reproduction**

Answer 35

1. Allows offspring to adapt to a changing environment, this allows natural selection to take place 2. Spores produced by fungi and seeds by plants are dispersed widely to help reduce intraspecific competition

Question 36

Name 6 differences between mitosis and meiosis

Answer 36

**Mitosis** 1. Only 1 cell division 2. Two daughter cells are produced 3. Chromosomes so not associated with in homologous pairs (bivalents) 4. The chromosome number in each daughter cell is diploid 5. Crossing over does not take place 6. Daughter cells are genetically identical to each other and to the mother **Meiosis** 1. 2 cell divisions 2. 4 daughter cells produced 3. Chromosomes associated with in homologous pairs (bivalents) 4. The chromosome number in each daughter cell is haploid 5. Crossing over does take place a prophase I 6. Daughter cells are genetically different