Cell Division, Cell Diversity And Cellular Organisation

Question 1

what is the cell cycle

Answer 1

regulated cycle of cell division with intermediate growth phases

Question 2

what are the stages of the cell cycle

Answer 2

• interphase
• mitosis or meiosis ( nuclear division )
• cytokinesis ( cytoplasmic division )

Question 3

what happens during interphase

Answer 3

G1 - cell synthesis proteins for replication - tubulin for spindle fibres & cell size doubles, organelles replicate, cell increases in size

S - DNA replicates, chromosomes consist of 2 sister chromatids joined at the centrosome

G2 - cell continues to grow in size, energy stores increases, and duplicated dna is checked for errors

Question 4

purpose of mitosis

Answer 4

produces two genetically identical daughter cells for
- asexual reproduction
- growth
- tissue repair and replacement

Question 5

what are the stages of mitosis

Answer 5

Prophase
Metaphase
Anaphase
Telophase

Question 6

What happens during prophase

Answer 6

chromosomes condense, shorten and thicken to become visible - 2 sister chromatids joined at the centrosome in an X shape
centrioles move to opposite poles of the cell in order to form mitotic spindle fibres
nuclear envelope and nucleolus disintegrate so the chromosomes are free in the cytoplasm

Question 7

What happens during Metaphase

Answer 7

Chromosomes are moves by spindle fibre and align up along the centre of the cell called the metaphase plate and are held in position

Question 8

What happens during Anaphase

Answer 8

Spindle fibres contract and the centromeres start to divide.
Sister chromatids are pulled to opposite ends of the cell.
Distinct V shape formed due to centromeres being dragged through liquid cytosol by the spindle fibres
Spindle fibres disintegrate

Question 9

What happens during Telophase

Answer 9

Chromosomes decondense and become invisible again
Nuclear envelope forms around each set of chromosomes

Question 10

What happens during cytokinesis

Answer 10

cell membrane cleavage furrow forms - the cell surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle forming 2 separate cells

Question 11

How are checkpoints used to control the cell cycle

Answer 11

Ensure damaged cells do not progress to the next stages of the cell cycle. Check for DNA damage, if the cell has grown to the right size, whether it has enough nutrients and if chromosomes are in the right position.

Question 12

G1 Checkpoint

Answer 12

At the end of the G1 Phase, before entering the S phase
Cell checked for: cell size, nutrients, growth factors, and DNA damage

Question 13

G2 Checkpoint

Answer 13

At the end of the G2 Phase, before entering mitosis.
Cell checked for: Cell size, DNA replication and DNA damage

Question 14

Spindle Assembly checkpoint/ Metaphase checkpoint

Answer 14

At the point in mitosis where all the spindle fibres should be attached to the centromeres
Check for: spindle fibre attachment to centromeres

Question 15

What happens during cytokinesis in plants

Answer 15

Plant cells have cell walls so cannot form a cleavage furrow. Vesicles from the golgi apparatus begin to assemble in the same place as where the metaphase plate was formed. Vesicles fuse with eachother and the cell surface membrane, dividing the cell into 2. New section of cell wall then form along the new sections of membrane.

Question 16

What happens if the cell wall was formed before the daughter cells seperated

Answer 16

They would immediately undergo osmotic lysis from the surrounding water

Question 17

What are homologous chromosomes

Answer 17

Each chromosome in a homologous pair has the same genes at the same loci

Question 18

What are Alleles

Answer 18

Different versions of the same gene - gene variants

Question 19

Prophase 1

Answer 19

Chromosomes condense, the nuclear envelope disintegrates, the nucleolus disappears and spindle formation begins, HOWEVER the homologous chromosomes pair up forming bivalents.

Question 20

What is Crossing Over

Answer 20

Chromosomes are large molecules of DNA and moving them through the liquid cytoplasm as they are brought together results in the chromatids entangling

Question 21

Metaphase 1

Answer 21

Same as Metaphase in mitosis except the homologous pairs of chromosomes assemble along the metaphase instead of the individual chromosomes

Question 22

What is independent assortment

Answer 22

The orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair. The maternal or paternal chromosomes can end up facing either pole which results in many different combinations of alleles facing the poles - this results in genetic variation

Question 23

Anaphase 1

Answer 23

The homologous chromosomes are pulled to the opposite poles and the chromatids stay joined to each other. Sections of DNA on ‘sister’ chromatids which became entangled during crossing over, now break off and rejoin - sometimes resulting in an exchange of DNA.

Question 24

What is a chiasmata

Answer 24

The point at which the chromatids break and re-join

Question 25

What happens when the chromatids re-join

Answer 25

Forms recombinant chromatids with genes being exchanges between chromatids. The genes being exchanged might be different alleles of the same gene meaning the combination of alleles on the recombinant chromatids will be different from the allele combination on either the original chromatids. - Genetic variation arises and the sister chromatids are no longer identical

Question 26

Telophase 1

Answer 26

The same as telophase in mitosis - the chromosomes assemble at each pole and the nuclear membrane reforms. Chromosomes uncoil. The cell then undergoes cytokinesis and divides into two cells. Diploid —> Haploid

Question 27

Prophase 2

Answer 27

The chromosomes still consist of two chromatids and condense to become visible again. The nuclear envelope breaks down and spindle formation begins

Question 28

Metaphase 2

Answer 28

The individual chromosomes assemble on the metaphase plate as in mitosis. Due to crossing over, the chromatids are no longer identical so there is independent assortment again and more genetic variation produced in metaphase 2

Question 29

Anaphase 2

Answer 29

Results in chromatids of the individual chromosomes being pulled to opposite poles after the division of the centromeres

Question 30

Telophase 2

Answer 30

The chromatids assemble at the poles. The chromosomes uncoil and form chromatin again. The nuclear envelope reforms and the nucleolus becomes visible. Cytokinesis results in division of the cells forming 4 daughter cells in total

Question 31

Levels of Organisation

Answer 31

Specialised cells –> Tissues –> Organs –> Organ Systems –> Organism

Question 32

What is a specialised cell

Answer 32

The cells within a multicellular organism that are differentiated meaning they are specialised to carry out very specific functions

Question 33

What is an erythrocyte and how are they specialised

Answer 33

Red blood cells - Transport oxygen around the body
Flattened biconcave shape which increases their SA:V ratio
In mammals they dont have nuclei to increase the space availability for haemoglobin
Flexible so that they can squeeze through narrow capillaries