Transcript chromosomes

Genetics for Statisticians
MSc Course
Duration: 18Hours, 6 x 3hour sessions
Dates:
Thursday 23rd May: 4:30-8:15
Thursday 30th May: 4:30-8:15
Thursday 6th June: 4:30-8:15
Thursday 13th June: 4:30-8:15
Thursday 20th June: 4:30-8:15
Presenters:
Dr Samuel Murray BSc, Hons , PhD, MRCP
Dr Fotios Siannis PhD
Session Titles:
1. Genetics: The basics
2. Analytical methodologies and global standardisation issues
[measures, cutoffs, ROC, EQA, sens/spec, reproducibility and requirements]
3. Biomarkers and their surrogates: integration into clinical medicine
[prog/pred, designs – old v new v novel, case control, Harvey W Equilb, endpoints-RR, cross over, censorship]
4. Practical application and interpretation of large data sets: ‘Omics’
[design, interpretation, validation, algorithms]
5. Spotting methodological flaws, limitations of study design, multiplicity
[p53, OncoDx, missing data, understanding the question, MA, post hoc]
www.biology-resources.com/genetics
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Cell 1
nuclear
membrane
cytoplasm
In a cell which is not about to divide, the
structures in the nucleus are not distinct
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Cell 2
Just before cell division, thread-like
structures appear in the nucleus
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Cell 3
chromosomes
These structures are called chromosomes
They get shorter and thicker and take up
stains very strongly
7
Cell 4
The shortening and thickening continues.
Now the chromosomes are seen to be
in pairs
Chromosome pairs
It may not be obvious from this illustration, but the
chromosomes are always in pairs. They are distinguished
by their size.
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9
10
The chromosomes are always in pairs because one
of them is derived from the male parent and the
other from the female parent
Different species have different numbers and
shapes of chromosomes
Members of the same species have identical
sets of chromosomes
Chromosome numbers
kangaroo (6 pairs)
chicken (18 pairs)
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a sedge (21 pairs) hawkweed (4 pairs*)
fruit fly (4 pairs)
human (23 pairs)
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Although chromosomes can be seen distinctly
only at the time of cell division, they are present
and active all the time
The chromosomes carry the DNA
DNA controls all the chemical reactions in the cell
DNA also determines the species of organism
and its individual characteristics
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Chromosomes and DNA
DNA
chromosome
protein
double helix
Chromosomes consist of DNA molecules supported
by a ‘scaffold’ of proteins. The diagram illustrates such
an arrangement but it is really more complex than this
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The DNA in the chromosomes carries the genes
Consequently, the genes are spaced out along
the chromosome
The genes consist of distinct stretches of the DNA
By means of their DNA content, the genes control
the activities of the cell, the type of cell it becomes,
the species of the organism and the individual
characteristics of that organism
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Chromosomes and genes
chromosome
genes for hair colour
single gene
genes for tallness
genes for eye colour
The diagram illustrates the relationship between chromosomes
and genes but it does not represent an actual chromosome.
The genes for these characteristics are not necessarily on the same
chromosome and the number of genes shown is arbitrary
Chromosomes and genes
A
Because the chromosomes are in
pairs, the genes they carry are also
in pairs
Each member of a pair of genes comes
from either the male or the female
parent just as the chromosomes do
The individual genes of a pair, control
the same characteristic, e.g. B and b
could control eye colour; G and g could
control hair colour
a
B
b
C
c
D
d
E
e
F
f
G
g
H
h
I
I
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Chromatids
chromatids
Just before cell division, it can be seen that the chromosomes
have replicated; that is each chromosome has made a copy of
itself (including its DNA). These copies and the originals are
now called chromatids
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Centromere
The chromatids
tend to separate but
are held together by a
special region called
the centromere
chromatids
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The next sequence of slides shows the way the
chromosomes are distributed during cell division
For clarity, only two pairs of chromosomes are
represented
The chromosomes coloured blue are derived from
the male parent; the chromosomes shown in red
are from the female parent
Cell division 1
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Two pairs of chromosomes.
Each chromosome has
replicated to form chromatids
Cell division 2
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The nuclear membrane
disappears
Fibres appear in the
cytoplasm and form a
spindle
The chromosomes move
to the ‘equator’ of the
spindle
Cell division 3
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The spindle fibres
shorten and appear
to pull the chromatids
apart by their centromeres
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Cell division 4
The chromatids are now
chromosomes.
The chromosomes
migrate to opposite ends
of the cell as the spindle
fibres shorten
Cell division 5
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The cell begins to divide
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Cell division 6
The nuclear membrane
forms again
The cell constricts
The chromosomes
become less distinct
Cell division 7
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Two cells formed
The chromosomes revert to
their elongated thin shape
and eventually cannot be
seen
Each cell now has a full set
of chromosomes identical to
the parent cell
Mitosis
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This process of cell division, which produces cells
containing identical sets and numbers of
chromosomes, is called
MITOSIS
Mitosis ensures that every cell of an organism
carries an identical set of genes
Can you see a problem with this?
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The problem is that if every cell carries the same
set of genes, how do cells become specialised in
shape and function to do different jobs?
For example, what can a gene for brown eyes do
in a stomach cell?
The answer is that the genes which are not relevant
to the cell’s function are not activated.
We say the gene for brown eyes is not expressed
in a stomach cell
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Mitosis
1
2
3
7
4
6
5
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The next slide shows a photomicrograph of onion root cells.
In a root tip, a great many cells are dividing by
mitosis, leading to rapid growth.
The preparation is made by softening the root tip tissue,
squashing it on a microscope slide and staining the
chromosomes and nuclei.
The ‘squash’ technique spreads the cells out.
See if you can associate the various stages of cell division
with the stages 1-6 in the previous slide.
Bear in mind that the previous slide is a purely
diagrammatic representation.
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Root tip squash
4
1
7
4
2
5
© McLeish & Snoad
Macmillan
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Question 1
Staining techniques show up the nucleus because
(a) The nucleus is enclosed in a nuclear
membrane
(b) Nucleus and cytoplasm have different
chemical properties
(c) The cytoplasm is less concentrated than
the nucleoplasm
(d) The nucleus is in the middle of the cell
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Question 2
When are chromosomes present in the nucleus ?
(a) Only just before cell division
(b) Only during cell division
(c) Only after cell division
(d) All the time
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Question 3
Chromosomes are in pairs because
(a) They have replicated
(b) Each is derived from either the male
or female parent
(c) They are joined at the centromere
(d) They have to be shared at cell division
Question 4
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How many chromosomes are there in a human cell?
(a) 100+
(b) 92
(c) 46
(d) 23
Question 5
Which of these statements are correct ?
DNA controls
(a) The species of the organism
(b) The function of the cell
(c) Features of the organism such as size
(d) Chemical reactions in the cell
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Question 6
Which of these statements are correct?
A chromosome contains
(a) Protein
(b) Cellulose
(c) DNA
(d) Genes
Question 7
The process by which a chromosome makes a
copy of itself is called
(a) Reproduction
(b) Recombination
(c) Relocation
(d) Replication
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Question 8
a
b
c
d
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e
f
What is the correct sequence of events in mitosis ?
(a) c, d, a, b, e, f
(b) b, c, a, d, e, f
(c) c, b, a, e, d, f
(d) c, b, d, a, e, f