topic 2.3 - cell cycle & division

Question 1

what is the cell cycle & its 3 main stages

Answer 1

• cell cycle is a process in which cells divide into two identical daughter cells
• interphase
• mitosis
• cytokinesis

Question 2

interphase

Answer 2

• interphase - the period between active cell divisions when cells increase size and mass and replicate their DNA
• new DNA is produced, chromosomes replicate, new proteins, cytoplasm, cell organelles are synthesised
• ATP production increases at times to provide extra energy needed for cell to divide

~ 90% of the cell cycle

Question 3

G1, S, G2

Answer 3

• G1 - time between end of previous round of mitotic cell division and start of chromosome duplication.
  grows and develops
  very short
• S - stage where chromosomes replicate and become double stranded chromatids.
  CHROMATIDS - one strand of the replicated chromosomes pair that’s joined to other chromatid at the centromere.
• G2 - time that organelles + materials needed for cell division are synthesised - need 2 for everything before cell divides.

Question 4

prophase

Answer 4

• chromosomes coil and condense - can take up stain and become visible
• each chromosome consists of 2 chromatids joined at centromere
• nuclear envelope breaks down and centrioles move to opposite poles and pull apart to from spindle fibres between them

Question 5

metaphase

Answer 5

• chromosomes line up along the equator
• each centromere pair attaches to the spindle fibres

Question 6

anaphase

Answer 6

• the centromere splits, separating the 2 chromatids into two separate ‘chromosomes’
• the chromatids from each old pair are drawn to opposite poles of the cell
• they cannot move - spindle moves them
• contraction of overlapping spindles cause them to move to opposite ends

Question 7

telophase

Answer 7

• spindle fibres break down and nuclear envelopes form around the 2 sets of chromosomes
• chromosomes uncoil & become less dense (long + thin) & harder to see

Question 8

cytokinesis

Answer 8

• happens after mitosis; division of cytoplasm
• plasma membrane folds inwards
• cell splits into 2 new identical diploid daughter cells

Question 9

what are cyclins

Answer 9

small proteins that build up during interphase and are involved in the control of the cell cycle by their attachment to cyclin-depending kinases

Question 10

what are cyclin-depending kinases - CDK’s

Answer 10

• cyclin-dependent Kinases (CDKs) are enzymes involved in the control of the cell cycle
• cyclin attach to CDKs and this cyclin/CDKs complex adds phosphate to other proteins, changing their shape and bringing about the next stage in the cycle

Question 11

what are the roles of mitosis

Answer 11

• growth
• repair
• reproduction

Question 12

what are some measure of growth?

Answer 12

• height
• head circumference
• mass (dry mass for fungi/plants not animals as they’ll die with no water)

Question 13

continuous growth

Answer 13

rapid period of growth until maturity is reached - growth slows and may stop.
(land animals, marine animals grow for life)

Question 14

discontinuous growth

Answer 14

• grow, stop, repeat.
• insects grow in series of steps/bursts.
• they shed one exoskeleton whilst new one is soft they expand their body by taking in air and water = ‘grow’
• then new skeleton hardens & water is released & there’s room for tissues to increase in mass/size

Question 15

asexual reproduction

Answer 15

• asexual reproduction involves only one parent
• results in genetically identical clones to each other & parent

Question 16

advantages vs disadvantages of asexual reproduction

Answer 16

ADV:
- don’t need to find mate so it’s easier
- gives rise to large numbers of offspring rapidly
- can replicate desired traits

DISADV:
- no variation
- if conditions change/disease comes = can cause total destruction to entire species

Question 17

asexual reproduction methods

Answer 17

• binary fission
• sporulation
• regeneration
• Budding
• Vegetative propagation

Question 18

binary fission

Answer 18

• mitosis followed by splitting of an individual
• 2 new individuals formed.
• most common in bacteria

Question 19

sporulation

Answer 19

• production of spores through mitosis
• spores land on suitable substrate
• spores grow into new organism
• spores are well suited to survive in adverse conditions
• most common in fungi and plants such as mosses

Question 20

regenration

Answer 20

• organisms replace lost body parts by producing new cells by mitosis
• lizard’s tails

Question 21

fragmentation

Answer 21

• can reproduce themselves asexually through fragments of their original body.
• starfish

Question 22

budding

Answer 22

• organisms produce a growth of ‘bud’ from their bodies through mitosis
• the bud eventually breaks of & forms a new independent organism genetically identical to parent.
• yeast / hydra animal

Question 23

vegetative propogation

Answer 23

• when parts of a plant extend out under soil (propagate from stem/leaf/root/bud) and give rise to a new plant
• e.g. farmers use it with plants/strawberries

Question 24

what are gametes and where are they produced?

Answer 24

• Gametes are sex cells
• haploid (23 chromosomes)
• contain half the number of chromosomes as the rest of the cells which make up our body
• when two gametes fuse during sexual reproduction, the fertilised egg ( zygote) contains the full number of chromosomes
• it is diploid = 46

Question 25

difference between egg & sperm

Answer 25

- sperm = many, mini, mobile (can move) - egg = few, fat/big, fixed/can’t move.

Question 26

meiosis 1

Answer 26

- Prophase 1 - chromosomes condense with 2 chromatids joined at centromere; (crossing over occurs) - Metaphase 1 - spindle fibres form and pair of chromosomes line up at equator/middle plate. (independent assortment occurs) - Anaphase 1 - centromeres DO NOT divide. one chromosome from each homologous pair move to opposite ends of cell = chromosome number in each cell will be half the original due to this. - Telophase 1 - nuclear membrane re-forms and the cells begin to divide

Question 27

meiosis 2

Answer 27

- Metaphase 2 - new spindles formed and chromosomes line up in metaphase plate/equator - Anaphase 2 - centromere NOW divides and chromatids split & go to opposite ends of cell. - Telophase 2 - nuclear envelope re-forms & cytokinesis occurs. results in 4 genetically different haploid daughter cells ( half number chromosomes as original diploid cell) - haploid gametes formed

Question 28

how does meiosis bring variation? 1. independent/random assortment

Answer 28

- chromosomes from parents are distributed to gametes completely randomly - so new gamete can have nay amount of chromosomes from either parent - this guarantees great variety in gametes

Question 29

how does meiosis bring variation? 2. crossing over/recombination

Answer 29

- a large enzyme ‘cuts & joins’ bits of maternal and paternal chromatids together - the chiasmata (points where chromatids break) are important in 2 ways: 1. exchange of genetic material = genetic variation 2. errors in the process leads to mutations - introduces new combinations into the genetic make-up of a species.

Question 30

what is a chromosome mutation?

Answer 30

- changes to the number or structure of chromosomes during meiosis

Question 31

types of chromosome mutations

Answer 31

1. translocation 2. deletion 3. inversion 4. non disjunction

Question 32

mutation through translocation

Answer 32

- where a part of one chromosome breaks off and reattaches to another completely different chromosome - can be balanced or unbalanced - balanced -a piece is swapped between 2 different chromosomes - unbalanced - one chromosome loses a part and doesn’t get another one in exchange whilst the other chromosome gains it.

Question 33

mutations through deletion

Answer 33

deletion - a section of chromosome is removed resulting in loss of a large number of genes

Question 34

mutations through insertion

Answer 34

inversion - a part of a chromosome flips its orientation with respect to the rest of the chromosome

Question 35

non disjunction

Answer 35

- causes abnormal number chromosomes in all the cells - 3 chromosome 21 - results in Down's syndrome

Question 36

why do we need meiosis

Answer 36

- results in genetic variation - because crossing over occurs and there is independent assortment - it produces new combinations of alleles - genetic variation allows organism to adapt to environmental changes

Question 37

describe the process of crossing over

Answer 37

- homologous chromosomes line up - chiasmata forms - chromosomes break - genetic information exchanged between chromatids

Question 38

what is an allele

Answer 38

different version of a gene

Question 39

how does meiosis result in genetic variation

Answer 39

- results in recombination of alleles - chromosomes cross over - independent assortment occurs