Cell division

Question 1

What is mitosis?

Answer 1

A form of cell division where 2 genetically identical daughter cells are formed.
Growth and tissue repair.

Question 2

What are the 4 stages of mitosis?

Answer 2

Prophase
Metaphase
Anaphase
Telophase

Question 3

What happens to chromosomes in interphase?

Answer 3

DNA is replicated
Before, each chromosome is a single chromatid.
After replication, each chromosome is made up of 2 sister chromatids joined by a centromere.

Question 4

What happens in prophase?

Answer 4

DNA supercoils into shorter and condensed structures.
Nuclear envelope breaks down.
Centrioles divide and move to opposite poles of the cell.
Spindle fibres form from the centrioles and make up a network.

Question 5

What happens in metaphase?

Answer 5

Spindle fibres attach to the centromere of each chromatid pair.
The chromatid pairs are pulled along the spindle fibres where they line up on the cell’s equator.

Question 6

What happens in anaphase?

Answer 6

Centromeres divide into 2 - splitting sister chromatids.
2 sister chromatids of each pair are pulled apart to the poles by the motor proteins on the tubule fibres.
When single chromatids reach opposite poles = chromosomes.
Each pole has a full set of chromosomes - identical to the opposite pole.

Question 7

What happens in telophase?

Answer 7

Spindle fibres begin to disintegrate.
Chromosomes lengthen and uncurl (not seen by microscope)
Nuclear envelope reforms.

Question 8

What happens in cytokinesis in animals?

Answer 8

Plasma membrane folds inwards - separating 2 daughter cells.

Question 9

What happens in cytokinesis in plants?

Answer 9

Cellulose starts to build up at the equator = end plate. Plasma membrane forms in the middle - separates cells.

Question 10

What is the difference between a haploid and diploid cell?

Answer 10

Diploids have 2 sets of chromosomes, haploid have 1.
Gametes are haploid and the zygote is diploid.
Meiosis occurs in the gonads.

Question 11

What are homologous chromosomes?

Answer 11

A pair of chromosomes that have their genes in the same locations, but may have different alleles for same genes.

Question 12

What is meiosis?

Answer 12

The process of creating haploid cells via 2 divisions to form 4 genetically different daughter cells.

Question 13

What happens during meiosis 1?

Answer 13

Just after chromosomes duplicated. Homologous chromosomes are separated with 1 chromosomes from each pair going into a daughter cell.

Question 14

What happens during meiosis 2?

Answer 14

The sister chromatids are separated. 4 haploid cells have been formed.

Question 15

What happens in prophase 1?

Answer 15

Chromatin condenses and chromosomes coil.
Nuclear envelope breaks down.
Spindle fibres form
Homologous pairs form and undergo crossing over.

Question 16

What happens during crossing over?

Answer 16

Sections of DNA are exchanged between homologous chromosomes.
Line up and twist around each other - sections can break off and be swapped.
Crossed over chromosomes have the same genes as before.
Genetic variation is introduced.

Question 17

What happens in metaphase 1?

Answer 17

Spindle fibres attach to chromosomes at their centromeres.
Within a homologous pair, the chromosomes are randomly organised on either side of the equator = independent assortment.
How they line up determines how they will segregate during anaphase.

Question 18

What happens in anaphase 1?

Answer 18

Chromosomes in homologous pairs are pulled apart by motor proteins.
Each chromosome in a pair will be on opposite sides of the cell. Allele shuffling - swapped areas separate.
Each chromosome made up of 2 chromatids.

Question 19

What happens in telophase 1 and cytokinesis?

Answer 19

A new nuclear envelope forms.
Cells divide by cytokinesis and may have a short interphase.
Each nucleus has half the original number of chromosomes.
In animals, cytoplasm divides. In plants, directly into prophase 2.

Question 20

What happens in prophase 2?

Answer 20

Chromosomes coil and condense.
Nuclear envelope disintegrates.
Spindle fibres from from centrioles to poles.

Question 21

What happens in metaphase 2?

Answer 21

Chromosomes line up at cell equator by spindle fibres.
Orientation of chromosomes is random - determines how they separate in anaphase 2.

Question 22

What happens in anaphase 2?

Answer 22

Centromeres divide, chromatids of each chromosome are pulled apart.
Motor proteins move chromatids to opposite poles.
Chromatids now randomly segregated.

Question 23

What happens in telophase 2?

Answer 23

Nuclear envelope starts to reform and cell splits in cytokinesis.
In plants, nuclei not separated by plasma membrane.

Question 24

How is there variation in mitosis and meiosis?

Answer 24

Mitosis - mutation.
Meiosis - crossing over and independent assortment introduce diversity with new genetic combinations.

Question 25

How is there genetic variation in meiosis?

Answer 25

Crossing over creates different allele combinations on sister chromatids.
Independent assortment in anaphase 1 - random distribution between 2 daughter cells.
Independent assortment in anaphase 2 - increases random distribution of genetic material.

Question 26

Why is random fertilisation important?

Answer 26

Increase genetic variation further - 2 haploid gametes randomly fuse to form a diploid cell.
Many possible gamete combinations.