Cell Division

Question 1

What are the 3 phases within interphase?

Answer 1

• first growth - the cell grows and proteins from which cell organelles are produced are synthesised
• synthesis - when DNA replicates (not the same as DNA replication - period when this happens)
• second growth - when organelles e.g. mitochondria grow and divide and energy stores are increased, the cell increases further in size

Question 2

What is interphase?

Answer 2

• the cell is engaged in metabolic activity in preparation for cell division
• chromosomes are not visible under a light microscope

Question 3

What are the stages of the cell cycle?

Answer 3

• interphase
• nuclear division
• cytokenesis

Question 4

What is nuclear division?

Answer 4

• division of the nucleus, either by mitosis or meiosis

Question 5

What is cytokenisis?

Answer 5

• the process by which the whole cell divides, resulting in an even distribution of organelles and cytoplasm into each daughter cell

Question 6

How long is the cell cycle?

Answer 6

• varies between organisms
• typically a mammalian cell takes 24 hours to complete a cell cycle
• of which 90% is interphase

Question 7

What is the definition of mitosis?

Answer 7

• the division of the nucleus of a cell that results in each of the 2 daughter cells having an exact copy of the DNA of the parent cell

Question 8

Are daughter cells genetically identical in mitosis?

Answer 8

• the daughter cells are genetically identical to the parent cells and each other
• may not be identical due to mutations

Question 9

Describe interphase

Answer 9

• DNA widely dispersed in the nucleus
• each molecule replicated to produce a genetically identical molecule
• metabolic activity to prepare for mitosis (second growth)

Question 10

Describe prophase

Answer 10

• DNA condenses to form visible chromosomes
• centrioles migrate to opposite poles of the cell and spindle fibres are formed
• the nuclear envelope disintegrates
• chromosomes are randomly arranged

Question 11

Describe metaphase

Answer 11

• spindle fibres develop fully to form spindle apparatus
• spindle apparatus stretches from pole to pole
• chromosomes line up along the EQUATOR of the cell, attaching to spindle fibres by their CENTROMERES

Question 12

Describe anaphase

Answer 12

• centromeres divide into 2
• the spindle fibres contract, pulling the chromatids to opposite poles
• mitochondria gather near the spindle to supply the energy needed
• (can now be referred to as chromosomes)

Question 13

Describe telophase

Answer 13

• chromosomes reach opposite ends of the cell and disperse
• the chromosomes are no longer visible
• nuclear envelope reforms and the spindle disintegrates
• new membranes form around daughter nuclei

Question 14

Describe cytokenesis

Answer 14

• cell divides into 2, producing 2 genetically identical daughter cells

Question 15

What is the key difference between telophase and cytokenesis?

Answer 15

• telophase - 2 daughter nuclei
• cytokenesis - 2 daughter cells

Question 16

What is the definition of meiosis?

Answer 16

• nuclear division that results in the production of 4 daughter cells which all contain half the chromosome number of the parent cell

Question 17

When does meiosis occur?

Answer 17

• during the formation of gametes in the sex organs
• ovary/testes/stamens/carpel

Question 18

Why is meiosis necessary?

Answer 18

• in sexual reproduction, 2 gametes fuse to give rise to a new offspring
• require 2 haploid cells to join together to make 1 diploid cell
• to maintain the number of chromosomes in adults, chromosomes need to be halved in meiosis

Question 19

What does n mean when looking at life cycles?

Answer 19

• n is the number of chromosomes
• 2n is the full set
• meiosis happens when 2n halves to n

Question 20

Why is meiosis important?

Answer 20

• produces haploid gametes (egg, sperm, pollen and ovule)
• introduces genetic variation

Question 21

Describe the first meiotic division (meiosis 1)

Answer 21

• homologous chromosomes pair up and their chromatids wrap around each other
• equivalent portions of these chromatids may be exchanged in a process called crossing over
• by the end of this stage, homologous pairs have separated with one chromosome from each pair going at random into one of the 2 daughter cells
• this is known as independent segregation

Question 22

Describe the second meiotic division (mitosis 2)

Answer 22

• the chromatids move apart
• by the end of meiosis 2, 4 cells have been formed each containing 23 chromatids (now called chromosomes)

Question 23

How does independent segregation increase genetic diversity?

Answer 23

• during meiosis 1 each chromosome lines up alongside its homologous partner
• they line up randomly
• one of each pair will pass into the daughter cell
• which one goes into which depends on how the homologous pairs line up on the equator/spindle
• since pairs line up at random, combination of chromosomes that go into daughter cells in meiosis1 are random

Question 24

How does crossing over increase genetic variation?

Answer 24

• during meiosis 1, the homologous pairs of chromosomes associate with each other and line up
• their chromatids wrap around each other crossing over many times
• tensions are created and portions of the chromatids break off, these broken portions recombine with the chromatids of the homologous partner
• homologous pair have effectively swapped portions of their DNA
• new genetic combinations of paternal and maternal alleles
• process is known as recombination and happens as a result of crossing over