Unit 2 Topic 3Bi Mitosis, Meiosis

Question 1

What are chromosomes

Answer 1

Made up of a mass of coiled threads of DNA and proteins

Question 2

How does DNA molecules get packaged when condensing

Answer 2

With the help of positively-charged basic proteins: histones

Question 3

How does DNA form chromosomes

Answer 3

DNA winds around histones to form dense clusters: nucleosomes -> interact to form more coilings and supercoiling –> form dense chromosomes

In the supercoiled area, genes cannot be transcribed to make proteins

Question 4

What are the three main phases in cell cycle?

Answer 4

1.Interphase
- non-division
- growth of cell
- carries out normal cellular activities
- synthesis of proteins and DNA, organelles and other materials to prepare the cell for division

2.Mitosis
- active division

3.Cytokinesis
- new cells separate

Question 5

Give two roles of the cell cycle

Answer 5

1. growth
2. asexual reproduction

Question 6

What are the three stages of interphase

Answer 6

G1, S, G2

Question 7

What happens in G1 phase

Answer 7

• Organelles replicate
• Transcription, translation to produce proteins for replication
• Size of cell increases (ensures that daughter cells are in correct size)

(longest phase in interphase)

Question 8

What happens in S phase

Answer 8

• Chromosomes duplicate (DNA semi conservative replication)

Question 9

What happens in G2 phase

Answer 9

• Replaces the energy stores used in the S phase
• ## Size of cell increases (forming cytoplasm)

Question 10

Explain why mitochondria replicate during the early stages of cell cycle?

Answer 10

1. as the number of mitochondria will later by halved by the end of the cell divisions
2. to provide energy for mitosis / growth

Question 11

Define chromatid

Answer 11

one of the 2 identical DNA molecules that make up a chromosome

Question 12

Explain why are cells with damaged DNA not replicated

Answer 12

checkpoint mechanism stop cells with damaged DNA from dividing as they will produce genetically identical cells by mitosis with damaged DNA => faulty transcription or no protein transcribed

Question 13

Importance of mitosis

Answer 13

• mitosis is the process which somatic cells divide to produce new cells so that organisms can grow and repair
• asexual reproduction of some organisms
• repair of damaged tissues

Question 14

Describe prophase in mitosis

Answer 14

Prophase
- chromosomes condense (shorter / thicker) ==> 2 sister chromatids joined by centromere
- nuclear envelope breaks down
- centrioles move to opposite poles, forming spindle network

Question 15

(past paper) Prophase is a stage in mitosis. Describe the events that occur during prophase (3)

Answer 15

1. chromosomes / chromatids condense and become visible
2. nuclear envelope breaks down
3. nucleolus breaks down
4. spindle fibres begins to form
5. centrioles migrate to opposite poles

Question 16

(past paper) Describe the appearance of cell in metaphase (3)

Answer 16

1. no nucleus
2. chromosomes aligned along the equator
3. spindle fibres attached to centromeres
4. centrioles of animal cells at poles of the cell

Question 17

(past paper) Describe what is happening inside the cell during the anaphase stage

Answer 17

1. spindle fibres are contracting
2. centromeres split (due to 1)
3. chromatids are pulled aparts to the opposite ends of the cell

do not accept chromosomes

Question 18

(past paper) Describe the events that take place during late telophase and cytokinesis in a plant cell (3)

Answer 18

1. formation of nuclear envelope
2. formation of nucleoli
3. formation of cell wall
4. cell division

Question 19

Functions of meiosis

Answer 19

1. give rise to cells that are genetically different from each other
2. used to produce haploid gametes (chromosome number is not doubled with each fertilisation)

Question 20

State the two features of meiosis that contribute to genetic variation

Answer 20

1. independent assortment
2. crossing over

Question 21

Describe prophase 1 in meiosis 1

Answer 21

Prophase 1
- each chromosome duplicates, remains closely assoicated (sister chromatids)
- crossing over

Question 22

Describe prophase 2 in meiosis 2

Answer 22

Prophase 2
- DNA des not replicate
- nuclear membrane disintegrates
- new spindle fibres formed

Question 23

Define cytokinesis

Answer 23

Ring of contratile fibres contract to constrict cell membrane around the middle of the cell, cytoplasm divide into two parts

Question 24

Cytokinesis of plant cells

Answer 24

• formation of new cell membrane / cell wall
• mainly in meristems

Question 25

Explain the control of cell cycle

Answer 25

Checkpoint mechanism Protein kinase - phosphorylation of proteins (catalyzes transfer of phosphate group from ATP to certain proteins) Cyclin - small proteins that acts as checkpoints Cyclin-dependent kinase - active only when bound cyclins - involves sevearl kinds of Cdk molecules and cyclins binding ==> variety of Cdk-cyclin complexes

Question 26

State four difference between mitosis and meiosis

Answer 26

1. **mitosis**: asexual reproduction, growth; **meiosis**: sexual reproduction, gametes 2. **mitosis**: one cell division; **meiosis**: 2 cell divisions 3. **mitosis**: 2 daughter cells; **meiosis**: 4 haploid daughter cells 4. **mitosis**: gentically identical; **meiosis**: genetically different, crossing over, independent assortment

Question 27

Describe and explain independent assortment

Answer 27

- occurs during **metaphase 1** - the order the chromosomes line up in their pairs is random ==> combinations of chromosomes going into duaghter cells is **random**

Question 28

Describe and explain crossing over

Answer 28

- occurs during prophase 1 - relatively rare process - homologous chromosomes **swap portion of their chromatids** ==> **mixing** of parental genetic informtion in offspring chromosomes / **new allele combinations** - structure formed: **bivalent**

Question 29

(past paper) Compare and constrast metaphase in mitosis and meiosis (3)

Answer 29

Similarities - lining up of chromosomes / chromatids on equator of cells - centromere bonds to spindle fibres Differences - two metaphase stages in meiosis, one in mitosis - independent assortment occurs in meiosis only - meiosis: homologous pairs of chromosomes (1) and sister chromatids (2); mitosis: involves sister chromatids

Question 30

(past paper) Flowering plants such as silver trumpet tree contain several ovules. Explain how the silver trumpet tree produces seed that are genetically different from each other (4)

Answer 30

Meiosis 1. each male nucleus and egg cell nucleus is **genetically different**(each ovule may have been fertilised by pollen from **different trees**) 2. due to **crossing over** of alleles/DNA between chromatids 3. due to **independent assortment** of chromosomes

Question 31

What are the simliarities betwen mammalian gametes and plant gametes

Answer 31

haploid nuclei formed by meiosis

Question 32

Describe metaphase in mitosis

Answer 32

***Metaphase*** - chromosomes align along **equator** - spindle fibres attach to chromosomes at centromeres

Question 33

Describe anaphase in mitosis

Answer 33

***Anaphase*** - spindle fibres contact + **pull** chromatids to opposite poles of cell - **centromere divides**

Question 34

Describe telophase in mitosis

Answer 34

***Telophase*** - chromsomes **uncoil** (larger / thinner) - nuclear envelope **reforms** (2 nuclei) - spindle fibres and centrioles break down

Question 35

Describe metaphase 1

Answer 35

Metaphase 1 - chromosomes align at the center of the cell, guided by spindle fibres - spindle fibres atach to centromeres - independent assortment

Question 36

Describe anaphase 1

Answer 36

Anaphase 1 - chromosome pairs separate with sister chromatids remaining together

Question 37

Describe telophase 1

Answer 37

Telophase 1 - two daughter cells formed (each daughter cell containing only one chromosomes of the chromosome pair)

Question 38

Describe metaphase 2

Answer 38

Metaphase 2 - chromosomes line up along equator of cell, guided by spindle fibres

Question 39

Describe anaphase 2

Answer 39

Anaphase 2 - centromere split to separate 2 chromatids of each chromosomes - move to the opposite poles cell - guided by spindle fibres - cytokinesis begins

Question 40

Describe telophase 2

Answer 40

Telophase 2 - new nuclear membrane forms around each group of chromosomes