The Cell Cycles

Question 1

What is a centromere?

Answer 1

Where two chromatids join together

Question 2

What are chromatids?

Answer 2

When chromosomes are converted into two identical dna molecules

Question 3

Why must chromatids stay together during mitosis?

Answer 3

So that they can be precisely manoeuvred and and segregated equally, one into each of the two new daughter cells

Question 4

What are the 4 stages of mitosis?

Answer 4

Prophase

Metaphase

Anaphase

Telophase

Question 5

How to prepare slides showing mitosis?

Answer 5

Obtain cells from growing root tips of plants.

Root tips treated with a chemical to allow the cells to be separated.

They can be squashed to form single layers of cells on a microscope slide.

Stains that bind DNA are used to make the chromosomes clearly visible.

Question 6

What happens during prophase?

Answer 6

During prophase, chromatin fibres begin to coil and condense to form chromosomes that will take up stain and become visible.

Nucleolus disappears

Nuclear membrane begins to break down.

Two centrioles migrate to opposite poles of the cell

Spindle fibres attach to specific areas on centromeres and begin to move chromosomes to the centre of the cell.

Question 7

What happens during metaphase?

Answer 7

During metaphase the chromosomes are moved by the spindle fibres to form a place in the centre of the cell, called the metaphase plate and are then held in position.

Question 8

What happens during anaphase?

Answer 8

The centromeres holding together the pairs in chromatids in each chromosome divide during anaphase.

Chromatids are separated and pulled to opposite poles of the cell by shortening spindle fibres.

V shape chromatids moving towards the poles as a result of them being dragged by their centromeres through the liquid cytosol.

Question 9

What happens during telophase?

Answer 9

The chromatids have reached the poles and are now called chromosomes.

The 2 new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them. The chromosomes start to uncoil and the nucleolus of formed

Question 10

What happens during cytokinesis?

Answer 10

The actual devision of the cell into 2 separate cells which begins during telophase

Question 11

How does cytokinesis happen in animal cell?

Answer 11

A cleavage furrow forms around the middle of the cell. The cell surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming 2 cells.

Question 12

How does cytokinesis occur in plant cells?

Answer 12

Vesicles from the Golgi apparatus begin to assemble in the same place as where to metaphase plate was formed. The vesicles dose with each-other and the cell surface membrane dividing the cell into 2z

Question 13

What does meiosis form?

Answer 13

4 genetically different haploid gametes

Question 14

What does haploid mean?

Answer 14

Contains half of the chromosome number for the parent cell

Question 15

What are homologous chromosomes?

Answer 15

Matching sets of chromosomes

Question 16

What is an allele?

Answer 16

Different versions of the same genes

Question 17

What happens during meiosis 1?

Answer 17

The first division is the reduction division when the pairs of homologous chromosomes are separated into two cells.

Each intermediate cell will only contain one full set of genes instead of two so the cells are haploid.

Question 18

What happens during meiosis 2?

Answer 18

The second division is is similar to mitosis, and the pairs of chromatids present in each daughter cell are separated, forming 2 cells. Four haploid daughter cells are produced in total.

Question 19

What happens during prophase 1?

Answer 19

The chromosomes condense, nuclear envelope disintegrates, the nucleolus disappears and spindle formation begins.

Homologous chromosomes pair up, forming bivalents. Chromosomes crossing the cytoplasm causes the chromatids entangling. This is called crossing over.

Question 20

What happens in metaphase 1?

Answer 20

Same as metaphase on mitosis except the homologous pairs of chromosomes assemble across the metaphase plate not just individual chromosomes.

Independent assortment happens here.

Question 21

What is independent assortment?

Answer 21

Orientation of the homologous pairs is random and independent of any other homologous pairs. The maternal / paternal chromosomes can end up facing either pole.

This results in many different combinations of alleles facing the poles. This results in genetic variation.

Question 22

What happens during anaphase 1?

Answer 22

Homologous chromosomes pulled to opposite poles and the chromatids stay joined to eachother.

Sections of DNA on sister chromatids become entangled during crossing over and break off and rejoin - exchanging DNA, forming recombinant chromatids leading to genetic variation.

Question 23

What is a chiasmata?

Answer 23

The points at which the chromatids break and rejoin during anaphase 1

Question 24

What happens during telophase 1?

Answer 24

Chromosomes assemble at each pole and the nuclear membrane reforms. The chromosomes uncoil.

The cell undergoes cytokinesis and divided into 2 cells, Reduction in cell number from diploid to haploid is complete.

Question 25

What happens during prophase 2?

Answer 25

The chromosomes, that still consist of two chromatids, condense and become visible again. The nuclear envelope breaks down and spindle formation begins.

Question 26

What happens in metaphase 2?

Answer 26

The individual chromosomes assemble on the metaphase plate.

Due to crossing over, the chromatids are no longer identical so there is independent assortment again and therefore even more genetic variation.

Question 27

What happens in anaphase 2?

Answer 27

Anaphase 2 results in the chromatids of the individual chromosomes being pulled to opposite poles after division of the centromeres - same as mitosis.

Question 28

What happens during telophase 2?

Answer 28

Chromatids assemble at the poles. The chromosomes uncoil and form chromatin again. The nuclear envelope reforms and the nucleolus becomes visible.

Cytokinesis results in division of cells forming 4 daughter cells in total, that are 4 genetically different haploid gametes.