1.6 Cell Division

Question 1

How do you form chromosome?

Answer 1

-the DNA molecule winds around proteins called histones to form nucleosomes.​
-they then supercoil to form the dense chromosome structure.​
-this replicates before the cell divides to make a double stranded chromatid.

Question 2

How many chromosomes in a typical human cell?

Answer 2

-In a typical human cell there are 46, but each species has a different chromosome number.

Question 3

What is a karyotype?

Answer 3

a photograph of the chromosomes in a cell

Question 4

3 main phases of the cell cycle

Answer 4

1- interphase
2- mitosis
3- cytokinesis

Question 5

interphase is further split into 3 phases:

Answer 5

-G1 (gap 1)
-S phase (synthesis)
-G2 (gap 2)

Question 6

G1 phase

Answer 6

-The cell increases in size and develops​
-Normal cell processes occur​
-Cell makes proteins needed for cell division​
-ATP is produced for the energy intensive process of cell division about to occur

Question 7

S phase

Answer 7

-The genetic material is copied​
-Chromosomes replicate and become double stranded chromatids

Question 8

G2 phase

Answer 8

-New organelles are made e.g. new mitochondria or chloroplasts​
-More cytoplasm is produced as the cell grows further ready for division​
-DNA is checked for errors

Question 9

Cyclins

Answer 9

-Cyclins are a groups of regulatory proteins that control the progression of the cell cycle​
-Different cyclins control when each next stage of the cell cycle occurs.​
-Cyclins bind to enzymes called cyclin-dependent kinases.​
-These then become active.​
-They cause phosphate groups to attach to proteins. ​
-These proteins then become active and carry out tasks specific to one of the phases of the cell cycle.

Question 10

How many types of cyclins are there?

Answer 10

4 main types = D,E,A,B
-unless these reach a THRESHOLD CONCENTRATION, the cell does not progress to the next stage

Question 11

When does the DNA molecule number change?

Answer 11

-The number of DNA molecules increase only when DNA replicates, i.e. only in S phase
-The number of DNA molecules decrease only when the cell divides – so after cytokinesis, NOT in Telophase

Question 12

Mitosis is split into 4 stages:

Answer 12

Prophase
Metaphase
Anaphase
Telophase

Question 13

Prophase

Answer 13

-In prophase the centrioles begin to move to opposite poles.​
-The nucleolus breaks down followed later by the nuclear envelope.​
-The duplicated chromosomes are clearly visible with the two sister chromatids joined by a centromere.​
-Spindle starts to form

Question 14

Metaphase

Answer 14

-The centrioles form the spindle which is made from microtubules.​
-The microtubules attach to the centromere of each chromatid pair.​
-They push and pull them so that they line up along the equator (metaphase plate) of the cell.

Question 15

Spindle

Answer 15

-The spindle fibres are polymers made from tubulin monomers.​
-The removal of tubulin monomers causes the spindle fibres to shorten

Question 16

Anaphase

Answer 16

-The centromeres split and the sister chromatids become individual chromosomes again.​
-They are pulled centromere first towards opposite poles.​
-This is done by the microtubule fibres of the spindle contracting and shortening which uses up the ATP stored in interphase.

Question 17

Telophase

Answer 17

-The spindle starts to break down.​
-Nuclear envelopes and nucleoli reform.​
-Chromosomes become less dense.

Question 18

Cytokinesis

Answer 18

After PMAT the cell has 2 complete nuclei but is sharing the cytoplasm so has to go through cytokinesis

Question 19

Cytokinesis in animal cells

Answer 19

-In animal cells the spindle degenerates at telophase ​
-In animal cells the plasma membrane is pulled inwards around the equator of the cell to form a cleavage furrow .​
-This is done using a contractile ring of protein made up of actin and myosin fibres.

Question 20

Cytokinesis in plant cells

Answer 20

-Plant cells lack centrioles so although they have a spindle it is not generated by centrioles. ​
-In plant cells the spindle remains as a new cell wall is formed across the equator of the cell.​
-A middle lamella or cell plate forms first and the cellulose is deposited here by both of the cells on either side to form the cell walls.

Question 21

Chemotherapy drugs

Answer 21

Because cancer is uncontrolled cell division – many chemotherapy drugs target a phase in the cell cycle to interfere with it.

Question 22

Preparing a root tip squash

Answer 22

A dye called acetic orcein can be used to stain chromosomes in dividing cells such as the meristem at the tip of a root in an onion.

Question 23

Meristems

Answer 23

-Regions of growth – and therefore rapid cell division (mitosis).​
-They are found at the apices (tips) of the shoot and root​
-There is also another meristematic region that enables the plant to grow outwards in width, this is called the cambium

Question 24

What is mitosis used for?

Answer 24

• asexual reproduction
• growth
• repair

Question 25

Asexual reproduction

Answer 25

-Asexual reproduction involves one parent, there are no males and no females.​ -The one individual can reproduce itself exactly.​ -They are genetically identical and therefore clones.​ -It is very fast and simple as the cells just divide by mitosis to achieve it.​ -However, as all the offspring are identical there is no variation which does make them vulnerable to changes in the environment.

Question 26

Ways of achieving asexual reproduction

Answer 26

- Budding – A small bud develops on the side of parent organism. Once the new nucleus moves into it, it breaks off and grows to normal size. Example: Yeast which are single celled fungi -Binary Fission – Carried out by bacteria, the parent undergoes mitosis and splits down the middle to make two even sized offspring . - Sporulation – Fungi and plants such as moss reproduce by spraying out asexual spores. These travel in the air and wind and will develop into a full organism . - Regeneration – Some organisms like this axolotl can replace whole body parts other organisms like starfish can grow new adults from fragments of their original body this is called fragmentation. - Vegetative reproduction – This occurs in flowering plants in various ways. A new adult plant grows from part of the original plant. Examples include runners like this strawberry plant, bulbs, tubers like in potatoes, suckers and corms

Question 27

Growth

Answer 27

-Growth is the permanent increase in number, size or mass of cells and the best way to do that is by mitosis.​ -You started life as one cell, a zygote. You are now billions of cells all produced from that one first cell by mitosis.​ -Organisms grow in different ways, animals stop at maturity, plants continue throughout life in regions called meristems.​ ​

Question 28

Repair

Answer 28

-Even if you are fully grown your cells are still dividing by mitosis for repair – repeated cell renewal. ​ -You lose cells all the time, in the next 60 seconds for example you will shed about 40,000 skin cells. These need replacing.​ -Tissues become damaged and need healing all the time.​

Question 29

Mitotic index

Answer 29

- represents the proportion of cells undergoing mitosis calculated by: cells in mitosis / total number of cells in field view = mitotic index

Question 30

Unrestricted cell growth

Answer 30

-If a cell goes into unrestricted cell growth – either because of a genetic mutation or environmental mutagenic influence it can lead to a cancerous growth.​ -Proto-oncogenes that normally control growth can mutate to become Oncogenes – which are involved in causing uncontrolled cell division and the growth therefore of a tumour.

Question 31

Sexual reproduction

Answer 31

-involves a cell from the male and a cell from the female fusing together (fertilisation) - produces varied offspring- For a species to survive long term you want the offspring to be varied so that not all of them are vulnerable if the environment changes.

Question 32

Gametes

Answer 32

- are produced in the sex organs of plants and animals.​ - The male tends to produce a lot of small mobile gametes and the female few larger ones - all other cells besides gametes are known as somatic cells

Question 33

Plant gametes

Answer 33

male gamete = pollen male sex organ = anthers female gamete = ovules female sex organ = ovaries

Question 34

Animal gametes

Answer 34

male gamete = sperm male sex organ = testes female gamete = ova (eggs) female sex organ = ovaries

Question 35

Plant organs that carry out mitosis are:

Answer 35

the root and shoot tips

Question 36

plant organs that carry out meiosis are the:

Answer 36

anthers and ovaries

Question 37

Meiosis

Answer 37

-The cell divides twice not once.​ -You end up with 4 haploid cells not 2 diploid.​ -Chromosomes line up in homologous pairs.​ -Independent assortment happens.​ -Crossing over occurs.

Question 38

Homologous chromosomes

Answer 38

-They are termed homologous because they are the same size and shape and carry the same genes in the same order at the same gene locus (position of the gene on the chromosome)​ -The alleles (different forms of a gene) can be the same or can be different.

Question 39

Prophase 1

Answer 39

=In prophase the centrioles begin to move to opposite poles.​ -The nucleolus breaks down followed later by the nuclear envelope.​ -The duplicated chromosomes are clearly visible with the two sister chromatids joined by a centromere.​ different to mitosis = -HOMOLOGOUS CHROMOSOMES PAIR UP WITH EACH OTHER.​ -CROSSING OVER OCCURS.

Question 40

Crossing over

Answer 40

-Crossing over is when homologous chromosomes swap parts of their chromosomes.​ -Chiasmata form where the chromosomes break.​ -This adds further genetic variation to the offspring

Question 41

Metaphase 1

Answer 41

-The centrioles form the spindle which is made from microtubules – polymers of tubulin monomers.​ -The microtubules attach to the centromeres of each homologous pair.​ -They push and pull them so that they line up along the equator (metaphase plate) of the cell IN THEIR HOMOLOGOUS PAIRS!

Question 42

Independent assortment at metaphase 1

Answer 42

- This is because each pair can orient themselves differently. -This creates millions of options as to which (maternal or paternal) chromosomes end up in the gametes .

Question 43

Anaphase 1

Answer 43

-One of EACH PAIR of DOUBLE CHROMATIDS are pulled towards opposite poles.​ -This is done by the microtubule fibres contracting which uses up the ATP stored in interphase.

Question 44

Telophase 1

Answer 44

-Nuclear membrane reforms and cell begins to divide.​ -The two cells formed are now haploid cells with 23 double chromatids in each.

Question 45

Prophase 2

Answer 45

-The second division of meiosis now works exactly as mitosis does for both the cells.​ -So the centrioles begin to move to opposite poles and the nucleolus breaks down followed later by the nuclear envelope.

Question 46

Metaphase 2

Answer 46

-The spindles join to the centromeres of the double chromatids .​ -They line up down the equator of each cell.

Question 47

Anaphase 2

Answer 47

The centromeres divide and the individual chromosomes are pulled to the poles of each cell.

Question 48

Telophase 2

Answer 48

-Nuclear envelopes reform and the chromosomes return to normal state.​ -Cytokinesis occurs to make 4 haploid cells.

Question 49

Importance of meiosis

Answer 49

1- It introduces GENETIC VARIATION to the offspring cells ​(through random assortment, crossing over at the chiasmata of the bivalents etc)​ 2- It produces haploid (n) gametes, which allows the diploid (2n)state to be restored in the zygote at fertilisation. ​ i.e. IT PREVENTS DOUBLING OF THE CHROMOSOME NUMBER AT FERTILSATION.

Question 50

Importance of mitosis

Answer 50

1- It produces daughter cells that are genetically identical to parents so brings genetic stability​ 2- It is important for growth​ 3- It is important for repair and replacement of missing cells/tissue​ 4- Repair to wounds or damaged tissues​ 5- Asexual reproduction by plants eg bulbs/tubers, bacteria and yeast​ 6- Allows large numbers of offspring to be produced so you can quickly colonise an area​ 7- It causes cancers through proliferation of cells

Question 51

Down's syndrome

Answer 51

- Down’s syndrome is an example of polysomy.​ - You end up with three copies of chromosome 21.​ - This affects both mental and physical development .​ - Life expectancy is reduced but most people with Down’s syndrome live to over 30.