Cell division

Question 1

chromosome structure

Answer 1

DNA as chromatin is coiled into very tightly compact structures which are chromosomes

rod-shaped structures made of protein and DNA - visible only during nuclear division

DNA is coiled around proteins called histones - help in the tight packing of DNA

During interphase, DNA exists as chromatin

Question 2

what is a diploid?

Answer 2

the chromosome number is considered to be diploid when there are two identical sets of chromosomes in each cell

Question 3

what is a haploid?

Answer 3

considered to be a haploid when there is only one set of each pair of chromosomes

Question 4

how do prokaryotes divide?

Answer 4

prokaryotes divide by binary fission

Question 5

what is binary fission?

Answer 5

DNA replicates and the cell divides in two

Question 6

how do eukaryotes divide?

Answer 6

they divide through 3 stages:
- interphase
- mitosis
- cytokinesis

Question 7

what happens in interphase?

Answer 7

it is the longest phase as it has three stages:
G1 phase
S phase
G2 phase

Question 8

what is G1 phase?

Answer 8

it is the first growth phase and is a period of normal metabolic activity as the number of cell organelles increase to normal levels and the volume of cytoplasm increases too

cell performs it’s normal functions - beginning growth and development

Question 9

What is S phase?

Answer 9

synthesis phase - this is where DNA and chromosomes replicate and the cell is now committed to division

Question 10

what is G2 phase?

Answer 10

second growth phase - structures directly involved in mitosis is formed

new DNA is checked for errors and substances needed during mitosis is synthesised

Question 11

what is a checkpoint?

Answer 11

DNA will be replicated and so checking for errors is needed and this happens at checkpoints

a suite of enzymes proofread and repair any damages that happens throughout the course of the cell cycle

if any errors are found and cannot be repaired, the cell may destroy itself to prevent mutations from being passed on

Question 12

when do the checkpoints happen?

Answer 12

there are four checkpoints during the cell cycle:
- during G1: chromosomes checked for damage, if this is detected the cell does not proceed to S phase until the DNA is repaired
- during S phase: a check is made that all the chromosomes have been replicated, if they haven’t the cell cycle will be stopped
- during G2: check is made for DNA damage that may have occurred during DNA replication
- during metaphase: this checks whether the chromosomes have attached to the spindle fibres correctly before anaphase proceeds

Question 13

what is the order of the four steps in mitosis?

Answer 13

1. prophase
2. metaphase
3. anaphase
4. telophase

Question 14

what happens in prophase?

Answer 14

the chromatin condenses into chromosomes by dehydrating and coiling

the chromosomes consists of 2 identical sister chromatids, joined together by a centromere

nuclear envelope and nucleolus disappear and microtubules arrange in a barrel-shaped structure called a spindle in the centre, the microtubules are referred to as spindle fibres

Question 15

what happens in metaphase?

Answer 15

chromosomes move to the equator and the centromeres attach to the spindle fibres so that the sister chromatids line up in the centre of the cell

Question 16

what happens in anaphase?

Answer 16

centromeres of each chromosome are pulled apart by the spindle fibres, moving the chromosomes to opposite poles of the cell

Question 17

what happens in telophase?

Answer 17

as the chromosomes reach the poles, the spindle disappears and a new nuclear envelope begins to from around the chromosomes at each end of the cell, each with it’s own nucleolus

Question 18

what is cytokinesis?

Answer 18

cytokinesis is the final stage of cell division and the two cells formed are roughly equal size

in animal cells:
the cytoplasm divides by the pinching of the cell in the middle

in plants:
a new cell wall begins to form across the centre of the cell and a new cellulose cell wall is laid down on both sides of this and a new cell membrane forms inside that, leaving gaps through which the cytoplasm of one cell links to that of it’s sister, these gaps are known as plasmodesmata

Question 19

what happens in meiosis?

Answer 19

meiosis produces daughter cells with half the number of chromosomes of the parent cell, going from a diploid to a haploid

it consists of two nuclear divisions:
- meiosis 1
- meiosis 2

this results in four haploid daughter cells

Question 20

what happens in meiosis 1?

Answer 20

it goes through 4 stages again:
- prophase 1
- metaphase 1
- anaphase 1
- telophase 1

Question 21

what happens in prophase 1?

Answer 21

nucleolus and nuclear envelope disappears and the centriole divides and forms the spindle

the chromosomes dehydrate and become visible, however there are four chromatids in each pair of chromosomes

portions of each chromatid may break off and reattach to an adjacent chromatid on the homologous chromosome, the places where this happens is called chiasma - the whole process is called crossing-over and results in a unique combination of alleles on each chromatid

Question 22

what happens in metaphase 1?

Answer 22

homologous pairs are still together and arrange on the equator of the cell

Question 23

what happens in anaphase 1?

Answer 23

homologous pair of chromosomes separate from each other and the spindle fibres pull one of each pair to opposite poles of the cell - the random separation of the homologous chromosomes that this produces is called independent assortment

Question 24

what happens in telophase 1?

Answer 24

cytokinesis takes place and each new cell is a haploid, containing one chromosome from each pair

Question 25

what happens in meiosis 2?

Answer 25

it is basically the exact same as what happens in mitosis

Question 26

what are the differences between meiosis and mitosis?

Answer 26

• meiosis has 2 nuclear divisions when mitosis only has one
• meiosis produces 4 nuclei when mitosis only produces 2
• meiosis produces haploid nuclei and mitosis produces diploid nuclei
• daughter nuclei is genetically different to parent nuclei in meiosis whereas they are genetically identical in mitosis