Cell division

Question 1

What happens in interphase?

Answer 1

The DNA replicates.

Question 2

What happens in prophase of mitosis?

Answer 2

The chromosomes supercoil & become visible under a light microscope.
The nuclear envelope breaks down.
The centriole divides in two and move to opposite ends of the cell to form a spindle.

Question 3

What happens in metaphase of mitosis?

Answer 3

The chromosomes line up along the middle of the cell.
They attach to a spindle thread by their centromere.

Question 4

What happens in anaphase of mitosis?

Answer 4

The replicated sister chromatids are separated when the centromere splits.
The spindle fibres shorten, pulling the chromatids apart.

Question 5

What happens in telophase of mitosis?

Answer 5

As the separated sister chromatids reach the poles of the cells.
A new nuclear envelope forms around each set.
The spindle breaks down.
The chromosomes uncoil so they are no longer visible under a light microscope

Question 6

What happens in cytokinesis?

Answer 6

The whole cell splits to down two new cells, each one genetically identical to each other and to the parent cell. They are clones.

Question 7

What is the significance of mitosis in growth?

Answer 7

multicellular organisms produce new extra cells to grow. Each new cell is genetically identical to the parents cell, and so can perform the same function

Question 8

What is the significance of mitosis in repair?

Answer 8

damaged cells need to be replaced by new ones that perform the same functions and so need to the genetically identical to the parent cell, as with growth.

Question 9

What is the significance of mitosis in asexual reproduction?

Answer 9

single celled organisms divide to produce two daughter cells that are separate organisms (they are clones). Some multicellular organisms produce offspring from parts of the parent (spider plants, strawberry runners).
Mitosis maintains the chromosome number in all cells

Question 10

How does meiosis lead to variation?

Answer 10

cells produced as a result of meiosis are not genetically identical
They are haploid (contain half the genetic information) and 4 cells produced rather than 2. They are also gametes rather than somatic cells and Crossing over ‘shuffles’ alleles
Random distribution and subsequent segregation of maternal and paternal chromosomes in the homologous pairs during meiosis I leads to genetic assortment
Random distribution and segregation of the chromatids at meiosis II leads to genetic assortment
Random mutations

Question 11

What happens in prophase I?

Answer 11

1. The chromatin condenses and supercoils
2. The chromosomes come together in their homologous pairs to form a bivalent. Each member of the pair has the same genes at the same loci. Each pair consists of one maternal and one paternal chromosome
3. The non sister chromatids wrap around each other and attach at points called chiasmata
4. They may cross over and swap sections of chromatids with each other
5. The nucleolus disappears and the nuclear envelope breaks down
6. A spindle forms

Question 12

What happens in metaphase I?

Answer 12

1. Bivalents line up across the equator of the spindle, attached to spindle fibres at the centromeres
2. The bivalents are arranged randomly (random assortment) with each member of the homologous pair facing opposite poles

Question 13

What happens in anaphase I?

Answer 13

1. The homologous chromosomes in each bivalent are pulled by the spindle fibres to opposite poles
2. The centromeres do not divide
3. The chiasmata separate and the lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached

Question 14

What happens in telophase I?

Answer 14

1. In most animal cells two new nuclear envelopes form- one around each set of chromosomes at each pole and the cell divides by cytokenesis. There is a brief interphase and the chromosomes uncoil
2. In most plant cells the cell goes straight from Anaphase I to Meiosis II

Question 15

What happens in prophase II?

Answer 15

1. If a nuclear envelope has reformed, it breaks down again
2. The nucleolus disappears, chromosomes condense and spindles form

Question 16

What happens in metaphase II?

Answer 16

1. The chromosomes arrange themselves on the equator of the spindle. They are attached to spindle fibres at the centromeres
2. The chromatids of each chromosome are randomly assorted

Question 17

What happens in anaphase II?

Answer 17

1. The centromeres divide and the chromatids are pulled to opposite poles by the spindle fibres. The chromatids randomly segregate

Question 18

What happens in telophase II?

Answer 18

1. Nuclear envelopes reform around the haploid daughter nuclei
2. In animals, the two cells now divide to give four daughter cells
3. In plants, a tetrad of four haploid cells if formed

Question 19

What are homologous chromosomes?

Answer 19

Chromosomes that have the same genes at the same loci. Members of a homologous pair of chromosomes pair up during meiosis. Diploid organisms produced by sexual reproduction have homologous pairs of chromosomes- one member of each pair from the mother and one from the father.