1
Q
gene
A
- a base sequence of DNA that codes for the
amino acid sequence of a polypeptide. - Each gene occupies a fixed position on
the chromosome called the locus.
2
Q
homologous chromosomes
A
- homologous chromosomes are the same
size and shape and carry the same genes but these may be different versions called
alleles. - One chromosome of each pair comes from each parent.
3
Q
allele
A
a different form of
the same gene.
4
Q
polyploidy
A
where an
organism has more than
two complete sets of
chromosomes.
5
Q
What do chromosomes consist of
A
- DNA and a protein called histrone
6
Q
When are chromosomes visible
A
after they condense at the onset of cell division
7
Q
What happens to chromosomes after DNA replication
A
- a chromosome exists as two identical ‘sister’ chromatids joined by the centromere.
8
Q
what is a property of sister chromatids
A
- genetically identical
9
Q
what does each chromosome consist of
A
genes that code for specific polypeptides
9
Q
describe the number of chromosomes for different species
A
- Different species have different numbers of chromosomes
- e.g. humans have
46, whilst a potato has 48.
9
Q
Describe the arrangement of chromosomes in humans
A
- 23 pairs
- one from each parent
- pairs are homologous Ie contain same genes but may be different versions or alleles
10
Q
What is the term diploid
A
organism is said to have two complete sets of chromosomes
11
Q
what is the diploid number in humans
A
46
12
Q
where are haploid numbers found
A
- human gametes
- some organisms eg mosses, male worker bees
13
Q
interphase
A
a period of
synthesis and growth
during the cell cycle.
14
Q
mitosis
A
- involves four stages where chromosomes
are arranged and separated prior to cell division. - It results in two genetically identical cells that contain the same number of chromosomes as the parent cell.
15
Q
cytokenesis
A
the division of the cytoplasm to form
two daughter cells following mitosis.
16
Q
describe the cell cycle in eukaryotes
A
- The majority of the cell cycle in eukaryotes involves interphase, where DNA,
protein and organelles are synthesised - This is followed by four stages of mitosis
where chromosomes are arranged and separated, - prior to the formation oftwo genetically identical cells via cytokinesis.
17
Q
What is interphase and describe what happens in it
A
- longest phase in the cell cycle
- very metabolically active
- quantity of DNA doubles (number of chromosomes remains the same as chromosomes exist as two sister chromatids joined together by the centromere)
- protein synthesis and organelle replication occur
- requiring much ATP
- cell metabolically very active
18
Q
what does mitosis result in
A
production of two genetically identical cells
19
Q
why is mitosis important
A
- growth and repair when differentiated cells replicate
20
Q
Describe what happens in prophase
A
- Chromosomes condense to become shorter and thicker
- Chromosomes become visible as two sister
chromatids attached by a centromere - Centrioles move to opposite poles (not higher plants)
- Nuclear envelope disintegrates
- Nucleolus disappears
21
Q
Describe what happens in metaphase
A
- Spindle forms
- Chromosomes align at the equator of the cell attached to spindle microtubules via
centromere
22
Q
Describe what happens in anaphase
A
- Spindle fibres shorten
- Centromeres divide, and chromatids are pulled towards opposite poles
23
Describe what happens in telophase
* Chromatids reach poles and become indistinct by uncoiling
* Nuclear envelope reforms
* Nucleolus reforms
* Spindle disintegrates
24
What is cytokenesis and describe the difference in cytokenesis between animal and plant cells
* The division of the cytoplasm into two distinct cells is different in plants and
animal cells.
* In animal cells, the membrane infolds via a cleavage furrow, until the two cells become separated.
* In plant cells, the presence of the cellulose
cell wall prevents this from happening, so instead a cell plate develops from
the centre outwards, until the cell is divided into two.
25
what is the mitotic index
* ration of the number of cells in a population undergoing mitosis to the number of cells not undergoing mitosis
* measure of growth
26
What is the significance of mitosis in animal and plant cells
* By producing new cells, an organism can grow, repair tissues and replace
dead cells.
* In animals, skin and blood cells are constantly being replaced
as they wear out.
* in plants, cells at the root and shoot tip meristems are
constantly undergoing mitosis.
27
Why is mitosis important in asexual reproduction
* reproduction where genetically identical
offspring can be produced resulting in a rapid increase in numbers during
favourable conditions,
* e.g. yeast cells and bacteria. It is also used by some plants, e.g. strawberries, when runners are produced.
28
How is mitosis controlled
* number of genes including proto-oncogenes
29
why is it important mitosis is controlled
* . A utation in one of these genes from chemicals, e.g. benzene, or radiation,
e.g. UV light, will cause them to turn them into oncogenes:
* this results in
uncontrollable cell division, which leads to the formation of tumours and
cancers.
30
What is the role of vincristine
* Vincristine has been a successful drug in the treatment of cancers
* as it prevents the formation of the spindle so arresting mitosis at metaphase,
slowing the rate of cell division.
31
How do you calculate the lenght of a stage in the cell cycle
1. First calculate the proportion of cells in that stage by counting using a
microscope, e.g. 20 cells seen, with 16 in interphase (16/20 = 80%).
2. Apply the proportion to the length of the cell cycle, e.g. 24-hour cycle ×
80% = 19.2 hours (19 hours and 12 mins) is the length of interphase.
32
What is meiosis
* two consecutive cell divisions
* four genetically different haploid cells produced
33
why is meiosis important
* It occurs in the reproductive organs
of animals, plants and some protoctistans prior to sexual reproduction.
* Interphase only occurs before prophase 1, and is responsible for DNA
replication and protein synthesis.
34
What happens in meiosis 1
* following interphase meiosis 1 is similar to mitosis
* except in prophase I homologous pairs come together to form bivalents and crossing over may occur
* metaphase I bivalents align randomly at the equator - independent assortment
35
What is the result of crossing over
increases genetic variation
36
What are the variants from independednt assortment
* 8388608
* from 23 bivalents 2^23
37
What happens in interphase in meiosis
* occure before meiosis
* DNA replicated
38
What happens in prophase I in meiosis
* Chromosomes condense to become shorter and thicker
* Centrioles move to opposite poles (not higher plants)
* Chromosomes come together in homologous pairs (bivalent)
* Crossing over occurs – part of one chromatid is exchanged with another
* Nucleolus and nuclear membrane disappear
39
What happens in metaphase I in meiosis
* Spindle forms Homologous chromosome
pairs (bivalents) align at the equator of the cell attached to spindle microtubules via
centromere.
* This alignment is random and called
independent assortment
40
What happens in anaphase I in meiosis
* Spindle fibres shorten
* Bivalents separate and chromosomes are pulled towards opposite poles
41
What happens in telophase I in meiosis
* The chromosomes reach poles
* in some cases:
* Nuclear envelope reforms
* Nucleolus reforms
* Spindle disintegrates
42
What occurs in cytokenesis in meiosis
division of cytoplasm occurs creating two haploid cells
43
What is meiosis 2
similar to mitosis no pairing of homologous chromosomes
44
What happens in prophase 2 of meiosis
Centrioles separate, and arrange themselves at 90o to the previous spindle
45
What happens at metaphase 2 in meiosis
Chromosomes align at the equator of the
cell attached to spindle microtubules via
centromere
46
What happen at anaphase 2 of meiosis
Spindle fibres shorten Centromeres divide, and chromatids are pulled towards opposite poles
47
What happens in telophase 2 of meiosis
* Chromatids reach poles and become indistinct
* Nuclear envelope reforms
* Nucleolus reforms
* Spindle disintegrates
48
What happens in cytokenesis in meiosis 2
four haploid daughter cells produced
49
What is the significance of meiosis
▪ Generates genetic variation through crossing over (prophase 1) and
independent assortment (metaphase 1 and 2).
▪ Keeps the chromosome number constant: by producing haploid gametes that recombine during fertilisation, restoring the diploid number in the zygote.
50
Compare mitosis and meiosis
mitosis
* one cell division
* produces genetically identical cells
* cells are diploid
* no crossing over
* no independent assortment
meiosis
* two cell divisions
* produces genetically different cells
* cells are haploid
* crossing over occurs at prophase I
* independent assortment occurs in metaphse I and II
Biology component 2
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