Topic 9 - Cell division

Question 1

Explain the process that divides the sister chromatids from their centromere in mitosis

Answer 1

the two sister chromatids divide by the spindle fibres sent out by the centriole, then attach themselves to the cell membrane, drawing back.

Question 2

Compare the process of cell division in prokaryotic and eukaryotic cells stating three similarities and three differences

Answer 2

Similarities:
Result in two identical daughter cells that are genetically identical to each other and the parent cell

Require DNA replication of chromosome(s) before division by DNA polymerase

Involve cytokinesis
Require energy
Can be a form of asexual reproduction

Differences:
In eukaryotes there are spindle fibres that extend from the centrioles to the centromeres of the chromosomes to separate the chromosomes and then to contract to move the chromosomes to the poles but they are absent in prokaryotes

In prokaryotes the cell division involves binary fission which has few stages and quick whereas in eukaryotes it is mitosis that has many more stages and a slower process

In prokaryotes the chromosomes separate by the cell membrane expanding but in eukaryotes the spindle fibres contract separating the chromosomes

Question 3

The chromosomes in the second cell diagram are double stranded and visible. State when in the cell cycle semi-conservative DNA replication occurs

Answer 3

Interphase

Question 4

What is the name of the structure (not shown in the diagram) which is formed to ensure that the duplicated chromosomes are separated into two equal groups?

Answer 4

(mitotic) spindle fibres

Question 5

The first cell drawn is not an accurate representation of what the DNA looks like at this stage of the cell cycle (pre-replication).
Draw a more accurate representation of what the DNA looks like

Question 6

State how many chromosomes are present in each of the cells

Answer 6

4

Question 7

Complete this sentence: One daughter cell is genetcially identical to…

Answer 7

….to both the other daughter cell AND the original parent cell.

Question 8

Explain the difference between asexual and sexual reproduction .

Answer 8

Asexual reproduction is mitosis in eukaryotes and binary fission in prokaryotes (1). It produces two daughter cells that are genetically identical to each other and the parent cell (diploid to diploid or haploid to haploid) (1). The only source of variation is mutation (1).

Sexual reproduction involves meiosis where 4 genetically non identical haploid cells are produced from a diploid cell (1). Fertilisation occurs when there is the union of gametes (haploid cells) that are produced by meiosis and this restores the diploid number of the organism (1). The sources of variation include crossing over, independent assortment, mutation and random union of gametes (1).

Question 9

What is a diploid cell? Explain using human cells as an example.

Answer 9

A Diploid cell contains homologous pairs (twice as much DNA as a haploid cell) and is a somatic or body cell (1). Ie: two copies of each chromosome in humans this is 46 (1)

Question 10

What is a haploid cell? Explain using human cells as an example.

Answer 10

A haploid cell is a sex cell e.g. sperm or egg that does not contain homologous pairs. (1) Ie: one copy of each chromosome – in humans this is 23 (1)

Question 11

Gametes are sex cells. Explain the difference between a gamete and a normal (somatic) cells using human examples.

Answer 11

Gametes are haploid cells where no homologous pairs of chromosomes are present eg 23 chromosomes (1). Somatic cells are diploid and homologous pairs of chromosomes are present eg 46 (1).

Question 12

Describe the term homologous chromosomes

Answer 12

A chromosome that contains the same genes that makes the pair of chromosomes in a diploid cell (one maternal and one paternal).

Question 13

Describe how each of the following can add to genetic variability of the offspring of sexual reproduction.

1) crossing over
2) independent assortment of chromosomes
3) fertilisation

Answer 13

1) Swapping of genes on non sister chromatids between the maternal and paternal chromosomes that make a homologous pair (1). So each chromosome has a unique combination of maternal and paternal genes (1). They exchange at points called synapsis (1)

2) Each gamete gets a mix of maternal and paternal chromosomes (1) and thus genes as the maternal and paternal chromosomes of the homologous pair line up independently on either side of the metaphase plate during metaphase 1 (1).

3) chromosomes of the homologous pair line up independently on either side of the metaphase plate during metaphase 1 (1).
Words: 38
Random union of gametes (1) and hence genes that make the homologous pair in the new offspring (1)

Question 14

There are 2 main types of regulatory proteins that make up the cell cycle control mechanism regulating the cell cycle. Why are these referred to as gene products?

Answer 14

Proteins are coded for by genes and produced by the cell that is using it

Question 15

Describe the rise and fall of cyclin, Cdk and MPF as you move through the cell cycle

Answer 15

Throughout interphase cyclin levels increase (1)
This binds to Cdk, activating it ti form MPF (1)
MPF id required in high concentrations to pass through checkpoint G2 (1) MPF levels will then reduce (as cyclin is broken down) through prophase and metaphase resulting in low levels of MPF (1)
[Low levels of MPF are required to pass through checkpoint into anaphase]

Question 16

Describe how signal cells releasing growth factors can stimulate many cells to divide even though the growth factors themselves do not enter cells

Answer 16

Growth factors (hormones) have a specific shape which is complementary to their specific membrane receptor.
By ensuring that the specific receptor for this hormone is found on the many specific target cells, when the hormone is released into the blood it will circulate the body and bind to any cells containing this specific complementary receptor on its cell membrane.
Once it binds to the receptor it triggers a cascade of relay proteins, which eventually stimulate gene expression and trigger cell division.

Question 17

A cell will normally only divide when environmental conditions are favourable.
There are 3 main types of environmental conditions that influence cell division. Describe them in detail:

1)nutrient dependance
2) density dependance
3) anchorage dependance
4) size dependance

Answer 17

1) Cells require certain/sufficient nutrients in the environment. Eg: glucose, nucleotides, amino acids etc

2) Usually cells which are closely packed and have membranes touching will not divide. If cells are growing in a dish in tissue culture and some cells are removed, the cells that remain will divide again to fill the space. When the space is filled and the newly produced cells touch other cells, they will stop dividing again- this is called contact inhibition.
It appears that when a certain density of cells is reached the amount of growth factor and nutrients available to each cell becomes insufficient to allow further division.

3) Some cells only divide when they are attached to a surface such as a culture jar (in tissue culture) or the extracellular matrix of tissue in the body.
The signal that the cell is attached is signalled to the cell cycle control mechanism by the plasma membrane and the cytoskeleton.

4) If cells are too large and have a small SA:V they will divide to reduce their size and increase their SA:V and thus their efficiency of exchange

Question 18

What do you call a factor that can cause cancer?

Answer 18

Carcinogen (a specific mutagen that causes cancer)

Question 19

List 3 factors that can cause cancer?

Answer 19

Any of
UV Radiation, Nuclear Radiation, Carcinogenic chemicals, Asbestos, Chemicals in cigarettes

Question 20

Describe what an oncogene is

Answer 20

An oncogene is a gene, which if mutated, causes cancer. These are the genes affected by carcinogens, or those that control the cell cycle.

Question 21

Describe the role of at least one internal and at least one external factor in controlling the cell cycle

Answer 21

Internal Factor:
Mitosis Promoting Factor (MPF) (1) …which is produced when the protein cyclin binds to a cyclin dependent kinase (Cdk) to activate it (1)
External factor:
Hormones/Growth Factors (1) …which bind to receptors on target cells, triggering division within those cells (1)
Density dependence (Contact inhibition) (1)
…cells will be inhibited form division when surrounded on all sides (no space to divide into) (1)
Anchorage dependence (1)
…cells must be attached to other cells or extracellular matrix to divide
Nutrient dependence (1)
…cells will not divide unless correct and sufficeint nutrients are present (1)

Question 22

Describe the consequences of loss of control of the cell cycle

Answer 22

Loss of control of the cell cycle leads to the development of diseases called cancer.
(1) Cancer comes from mutations of the genes producing the proteins that regulate it (oncogenes). (1)
Cells will then divide when not required to by the cell (uncontrolled cell division) (1) or will not go through programmed cell death when they are meant to (1)
One of the following: This can result in
Tumours developing which are large growths which can be localised or throughout the body. (1)
Tumours can be benign and restricted to one location or malignant, meaning they can metastasise and spread throughout the body. (1)
Cancer can also result in a proliferation of immature or under-developed cells., such as leukaemia (1)

Question 23

Explain the process that divides the replicated chromosome from the original chromosome in binary fission

Answer 23

The chromosomes are attached to the cell membrane (1) and as the cell gets larger so does the membrane and the chromosomes separate and move away from each other (1).