Unit 1. 6 mitosis and meiosis

Question 1

what are chromosomes?

Answer 1

long sections of DNA, proteins and small amount of RNA

Question 2

chromatids?

Answer 2

the single thread of DNA that becomes 2 identical threads

Question 3

how many chromosomes do humans have?

Answer 3

46 chromosomes = 23 pairs of homologous chromosomes

Question 4

what is the diploid number?

Answer 4

total number of chromosomes

Question 5

what is the haploid number?

Answer 5

half the diploid number - human gametes = 23 chromosomes

Question 6

Mitosis?

Answer 6

nuclear division that produces 2 genetially identical daughter cells that are genetically identical to the parent cell

Question 7

What are the 4stages of mitosis?

Answer 7

Interphase (not part but very important part of cell cycle)

prophase
metaphase
anaphase
telophase

Question 8

Interphase?

Answer 8

Replication of DNA
Replication of organelles
synthesis of ATP
synthesis of proteins
increase in cell size

Question 9

Prophase? - longest stage of mitosis

Answer 9

The chromosomes condense and
become visible as two sister
chromatids joined by a centromere.

Centrioles migrate to the opposite sides
of the cell (the poles).

The spindle begins to form in animal cells
only, made by the centrioles.

The nuclear membrane disintegrates and
the nucleolus disappears.

Question 10

Metaphase

Answer 10

The spindle completes

The chromosomes migrate to the
equator of the cell

Spindle fibres attach to the chromosome
at the centromere

Each chromosome is attached to one
spindle fibre

Question 11

Anaphase? fastest stage of mitosis

Answer 11

The centromere divides.

The spindle fibres contract and shorten pulling
the chromatids to opposite poles, centromere
first.

Question 12

cytokinesis?

Answer 12

The cytoplasm divides

In animal cells, the cell constricts around the
equator

In plant cells, a cell plate forms across the
equator to form a new cell wall

2 genetically identical cells are formed

Question 13

what is the purpose of mitosis?

Answer 13

essential for growth, the repair of tissues and the replacement of dead or worn out cells.

Question 14

what happens by mitosis?

Answer 14

asexual reproduction - offspring = genetically identical to the parent
advantage = ability to increase in numbers quickly to take advantage of an ideal environment
disadvantage = lack of genetic variation , leading to an inability to adapt if the environment changes.

Question 15

mitotic index?

Answer 15

number of cells

Question 16

cancer?

Answer 16

result of uncontrolled mitosis . cancerous cells divide repeatedly out of control with the formation of a tumour.
tumour = irregular mass of cells that prevent the normal function of body organs. cancers thought to be initiated when mutations occur in the genes that control cell division.

Question 17

what is meiosis?

Answer 17

type of nuclear division that gives rise to cells that are genetically different. used to produce gametes.

Question 18

what happens to the number of chromosomes?

Answer 18

must be halved when gametes = formed, otherwise there would be double the number of chromsomes after they join at fertilisation in the zygote

Question 19

what are the phases?

Answer 19

meiosis starts exactly the same as in mitosis but there are 2 divisions . meiosis 1 and meiosis 2

Question 20

interphase?

Answer 20

Synthesis of ATP
Synthesis of proteins
Replication of organelles
Replication of DNA.
Increase in size

Question 21

Prophase 1?

Answer 21

DNA condenses and becomes visible as chromosomes

The chromosomes are arranged side by side in homologous pairs

A pair of homologous chromosomes is called a bivalent

As the homologous chromosomes are very close together the crossing over of
non-sister chromatids may occur. The point at which the crossing over occurs is
called the chiasma (chiasmata; plural)

In this stage centrioles migrate to opposite poles and the spindle is formed

The nuclear envelope breaks down and the nucleolus disintegrates

Question 22

metaphase 1 ?

Answer 22

The bivalents line up along the equator of the spindle, with the spindle fibres
attached to the centromeres

The maternal and paternal chromosomes in each pair position themselves
independently of the others; this is known as independent assortment

This means that the proportion of paternal or maternal chromosomes that end up on
each side of the equator is due to chance

Question 23

Anaphase 1?

Answer 23

The homologous pairs of chromosomes are separated as the spindle fibres pull
whole chromosomes to opposite poles of the cell

The centromeres do not divide

Question 24

Telophase 1?

Answer 24

The chromosomes arrive at opposite poles

Spindle fibres start to break down

Nuclear envelopes form around the two groups of chromosomes and nucleoli reform

Some plant cells go straight into meiosis II without reformation of the nucleus in
telophase I

Question 25

Prophase 2?

Answer 25

There is no interphase between meiosis I and meiosis II so the DNA is
not replicated. The second division of meiosis is almost identical to
the stages of mitosis.

In prophase II:

The nuclear envelope breaks down and chromosomes
condense

Centrioles migrate to opposite poles but this time,

A spindle forms at a right angle to the old one

Question 26

Metaphase 2?

Answer 26

Chromosomes line up in a single file along the equator of the spindle

Question 27

Anaphase 2?

Answer 27

Centromeres divide and individual chromatids are pulled to
opposite poles

This creates four groups of chromosomes that have half the
number of chromosomes compared to the original parent
cell

Question 28

Telophase 2?

Answer 28

Nuclear membranes form around each group of
chromosomes

In cytokinesis:

Cytoplasm divides as new cell surface membranes are formed creating four haploid
cells – they have half the number of chromosomes as the parent cell.

The 4 cells are genetically different from each other

Question 29

what happens at the end of meiosis 2?

Answer 29

4 haploid genetically different daughter cells.

Question 30

Differences in mitosis and meiosis?

Answer 30

mitosis

One division resulting in 2 daughter
cells
Number of chromosomes is unchanged
Homologous chromosomes do not
associate in pairs