Cell division

Question 1

what are the three stages of the cell cycle?

Answer 1

Interphase (resting phase), Mitosis (nuclear division), cytokinesis

Question 2

what is cytokinesis?

Answer 2

division of cytoplasm

Question 3

what happens during DNA replication? (2)

Answer 3

1. The chromatin thread replicates to produce two identical chromatin threads. (interphase)
2. These chromatin threads coil and shorten to become chromosomes. (prophase)

Question 4

what happens during interphase? (3)

Answer 4

Chromosomes appear as long thin threads called chromatin.
Just before the cell enters the nuclear division phase,
1. the chromatin threads replicate
2. the centrioles replicate and divide

Question 5

After DNA replication, what does the chromosome consist of?

Answer 5

Each chromosome consist of two identical DNA molecules known as sister chromatids.
The sister chromatids are joined at a point known as the centromere.

Question 6

Why must DNA replication and mitosis be precisely controlled?

Answer 6

To ensure that all daughter cells are genetically stable
If an error occurs during replication, the new DNA strand formed would be different from the original.
This is known as gene mutation and is passed on to the daughter cell. This may lead to harmful changes to the genes and affect how the daughter cell functions.
Some forms of gene mutation may lead to uncontrolled division of cells (cancer), which may be fatal.

Question 7

what makes a daughter cell genetically identical?

Answer 7

When the genetic information of a daughter cell is exactly the same as the parent cell, it is genetically stable.

Question 8

What are the stages of mitosis?

Answer 8

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

Question 9

What is the structure of a chromosome?

Answer 9

Structure of one chromosome differs depending on whether DNA replication has occurred.
After DNA rep: each chromosome has two sister chromatids joined at the centromere, each with identical and replicated copy of a DNA molecule
Count the no. of chromosomes by the no. of centromeres.

Question 10

Prophase of mitosis

Answer 10

1. condensation of chromatin into chromosomes
2. movement of centrioles to opp poles of nucleus
3. formation of aster, made up of microtubules, around centrioles (cenrtioles are absent in plant cells)
4. disintergration of nuclear envelope and nucleolus
5. spindle fibres form

Question 11

Metaphase of mitosis

Answer 11

1. spindle fibres attach to centromeres

2. chromosomes line up along the equatorial plate

Question 12

Anaphase of mitosis

Answer 12

1. Division of each centromere into two
2. Separation of sister chromatids, each chromatid is considered a chromosome
3. Spindle fibres contract and pull the sister chromatids apart
4. sister chromatids move towards opposite poles of the cell
5. once the chromatids are separated, they are called daughter chromosomes

Question 13

Telophase of mitosis

Answer 13

1. Chromosomes arrive at opp poles of the cell and spindle fibres break down
2. formation of nuclear envelope around the chromosomes at each pole of the cell, nucleolus reforms in each nucleus
3. chromosomes uncoil and lengthen to become thin chromatin threads

Question 14

Why is it important to produce genetically identical daughter cells?

Answer 14

to ensure that the daughter cells contains all the DNA needed for subsequent cell division and differentiation.

Question 15

What else happens during interphase (apart from replication of DNA)?

Answer 15

cells still carry out activities such as absorbing nutrients, building up protoplasm, synthesizing new organelles

Question 16

How many chromosomes does a human cell have?

Answer 16

23 pairs of chromosomes or a total of 46 chromosomes

Question 17

What is a homologous chromosome?

Answer 17

23 chromosomes are inherited from each parent in the form of a gamete when two gametes fuse during fertilization to form a zygote. One of each pair of a homologous chromosome comes from each parent, one from the mother, the other from the father. Hence a cell contains 23 pairs of homologous chromosomes

Question 18

Characteristics of homologous chromosome?

Answer 18

same length and centromere position

same genes but may take alternative forms

Question 19

What is a diploid and haploid?

Answer 19

normal cell= 46 chromosomes
diploid= 46 chromosomes= complete set
haploid=23=half of complete set

Question 20

What are the types of chromosomes?

Answer 20

in a somatic/human cell, the first 22 homologous pairs are autosomes, which the last pair is known as sex chromosome.

Question 21

What happens during cytokinesis after mitosis?

Answer 21

animal cells: cleavage furrow forms in the cell membrane, furrows deepen and cell membrane in the furrow joins together to completely separate the daughter nuclei. 2 identical daughter cells are formed
plant cells: Deposition of Golgi vesicles along the equatorial plate. vesicles merge to form a cell plate which completely separates the two daughter nuclei.

Question 22

Why is mitosis important?

Answer 22

1. Growth, mitosis poduing new cells that enables growth of an organsim
2. Cell replacement, dead or worn out cells are replaced by new cells produced by mitosis
3. regeneration, allow animals to regenerate whole body parts like skin
4. asexual reproduction, ensures that offsprings are genetically identical to their parent
5. genetic stability, daughter cells contains same type of genetic material as the parent, no genetic variation, desired trait can be passed down to offsrping without any genetic variation

Question 23

What are gametes?

Answer 23

reproductive cells with half the number of chromosomesas normal human cells. This is because during meiosis(aka reduction division), each of the daughter cells produced contains half the number of chromosomes as the parent cell. Daughter chromosome=gamete. in humans, sperm is male gamete, ovary is female gamete

Question 24

Why are gametes produced?

Answer 24

because sexual reproduction requires two gametes to fuse for fertilisation to occur, whuch is when the nucleus of the male gamete fuses with the nucleus of the femlae gamete to produce a zygote that divides by mitosis to form an embryo

Question 25

What is meiosis?

Answer 25

it is a form of nuclear division which produces daughter nuclei that contains half the number of chromosomes as the parent nucleus

Question 26

What happens in meiosis 1?

Answer 26

Prophase 1, Metaphase 1, Anaphase 1 and telophase 1

Question 27

prophase 1

Answer 27

1. condensation of chromotin threads into chromosomes 2. synapsis occurs: homologous chromosomes pair along their whole length 3. chromatids of homologous chromosomes may cross and twist around each other. the point where they cross over is called a chiasma 4. crossing over may occur when the strength of their coiling is enough to break and exchange parts. 5. crossing over produces new combinations of genes along the chromosomes 6. formation of asters around the centrioles, centrioles move to opposite ends of the nucleus 7. nuclear envelope and nucleolus disintegrates, spindle fibres form

Question 28

metaphase 1

Answer 28

1. spindle fibres attach to the centromeres of the chromosomes 2. homologous chromosomes line up in pairs along the equatorial plate and are pulled into place by the spindle fibres 3. chromosomes of each pair randomly face opposite ends of the cell, resulting in independent assortment of genes when the cell divides independent assortment of chromosomes= one chromosome from each pair can combine with either chromosome of each pair.

Question 29

anaphase 1

Answer 29

1. Homologous chromosomes separate and are pulled to opposite ends of the cell as the spindle fibres contract 2. each chromosome still consists of two chromatids

Question 30

telophase 1

Answer 30

spindle fibres disintegrates, nucleolus and nuclear envelope reforms at each pole to produce two daughter nuclei chromosome uncoils and uncondenses to form chromatin threads

Question 31

cytokinesis after meiosis 1

Answer 31

1. cleavage furrow forms in the cell memberane, cleaves the cytoplasm to form 2 daughter cells 2. centrioles divide

Question 32

cytokinesis 2

Answer 32

cleavage of cytoplasm results in 4 daughter cells being produced, each with half the number of chromosomes as the parent

Question 33

why is meiosis important?

Answer 33

1. to produce haploid gametes which will fuse together during fertilization to restore diploid number in the zygote, thereby maintaining the normal diploid number of chromosomes in the species 2. for variation in gametes produced, variations occurs due to independent assortment of chromosomes and crossing over, variation in gametes produces variation in offspring. these variations may increase the survival chances of the offsprings when there are changes in the environment

Question 34

where and when do mitosis and meiosis occur?

Answer 34

mitosis: normal body cell during growth or repair of body parts meiosis: in gonads during a gamete formation

Question 35

Cancer definition

Answer 35

initiated when cells begin to divide repeatedly by mitosis without control or regulation. results in tumour which is the irregular mass of cells produced by uncontrolled cell division. Malignant/ harmful tumours which grow rapidly and spreads to other parts of the body, benign/harmless tumours do not.

Question 36

causes of cancer

Answer 36

1. genetic factors (skin cancer, breast cancer etc) 2. carcinogens whcih are chemicals that affect genetic activity resulting in abnormal cell division 3. age eg leukaemia in young people, colon cacers in older adults 4. environment, exposure to certain types of chemicals or radiation 5. viruses

Question 37

what is non-disjunction?

Answer 37

it is the failure of one one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division which may result in abnormal distribution of chromosomes in each daughter nuclei

Question 38

examples of non-disjunction

Answer 38

1. Down's syndrome 2. Turner's syndrome, chromosomal condition which affect development in females, causing them to have early loss of ovarian function and result in short stature 3. klinefelter's syndrome, male have X extra chromosome in most of their cells 4. polyploidy in plants