The document discusses cell division and inheritance, including how staining reveals nuclei, chromosomes are always present but only visible during cell division, and mitosis duplicates chromosomes to produce identical cells through nuclear envelope changes and chromatid separation.

1. 1

2. In a living cell seen under
the microscope, it is often
difficult to see the nucleus
When certain stains are applied, the
nucleus takes up the stain more
strongly than the cytoplasm does,
making the nucleus easily visible
Staining 2

3. By means of some special techniques of
illumination, the nucleus and other structures
may be seen in the living cell.
But in this presentation and in most textbooks, the
nucleus is shown dark, as if it had been stained
It is the differences in chemical composition
between nucleus and cytoplasm that make one take
up a particular stain more strongly than the other
3

4. INHERITANCE
• Is the transmission of genetic information
from generation to generation.

5. Terms you should know
• CHROMOSOME: thread of DNA, made up of a string of
genes.
• GENE: a length of DNA that is the unit of heredity and
codes for a specific protein. A gene may be copied and
passed on to the next generation.
• ALLELE: any of two or more alternative forms of a gene.
• HAPLOID NUCLEUS: a nucleus containing a single set
of unpaired chromosomes (e.g. sperm and egg)
• DIPLOID NUCLEUS: a nucleus containing two sets of
chromosomes (e.g. in body cells)

6. In a cell which is not about to divide, the
structures in the nucleus are not distinct
nuclear
membrane
cytoplasm
Cell 1 4

7. Just before cell division, thread-like
structures appear in the nucleus
Cell 2 5

8. These structures are called chromosomes
They get shorter and thicker and take up
stains very strongly
chromosomes
Cell 3
6

9. The shortening and thickening continues.
Now the chromosomes are seen to be
in pairs
To continue the sequence for cell division click here
Cell 4
7

10. It may not be obvious from this illustration, but the
chromosomes are always in pairs. They are distinguished
by their size.
Chromosome pairs
8

11. 9

12. 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
10

13. kangaroo (6 pairs) a sedge (21 pairs) hawkweed (4 pairs*)
chicken (18 pairs) fruit fly (4 pairs) human (23 pairs)
Chromosome numbers 11

14. 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 (See the
presentation on ‘DNA’ for more detail)
12

15. 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
Chromosomes and DNA
chromosome
DNA
protein
double helix
13

16. 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
14

17. genes for eye colour
genes for hair colour
single gene
chromosome
Chromosomes and genes
genes for tallness
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
15

18. A
B
C
D
E
F
G
H
I
a
b
c
d
e
f
g
h
I
Because the chromosomes are in
pairs, the genes they carry are also
in pairs
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
Each member of a pair of genes comes
from either the male or the female
parent just as the chromosomes do
Chromosomes and genes 16

19. 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
chromatids
Chromatids 17

20. The chromatids
tend to separate but
are held together by a
special region called
the centromere
chromatids
Centromere 18

21. CELL DIVISION:MITOSIS
• Is a nuclear division giving rise to
genetically identical cells in which the
chromosome number is maintained by the
exact duplication of chromosomes.

22. 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
19

23. Two pairs of chromosomes.
Each chromosome has
replicated to form chromatids
Cell division 1
20

24. The nuclear membrane
disappears
Fibres appear in the
cytoplasm and form a
spindle
The chromosomes move
to the ‘equator’ of the
spindle
Cell division 2
21

25. The spindle fibres
shorten and appear
to pull the chromatids
apart by their centromeres
Cell division 3 22

26. The chromatids are now
chromosomes.
The chromosomes
migrate to opposite ends
of the cell as the spindle
fibres shorten
Cell division 4
23

27. The cell begins to divide
Cell division 5 24

28. The nuclear membrane
forms again
The cell constricts
The chromosomes
become less distinct
Cell division 6
25

29. Two cells formed
Each cell now has a full set
of chromosomes identical to
the parent cell
The chromosomes revert to
their elongated thin shape
and eventually cannot be
seen
Cell division 7 26

30. Plant cells divide by
building a new cell wall
Animal cells divide by
a constriction of their
cytoplasm
Plant and animal cells 27

31. This process of cell division, which produces cells
containing identical sets and numbers of
chromosomes, is called
MITOSIS
Mitosis 28
Mitosis ensures that every cell of an organism
carries an identical set of genes
Can you see a problem with this?

32. For example, what can a gene for brown eyes do
in a stomach cell?
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?
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
29

33. 1
2
3
4
56
7
Mitosis 30

34. 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.
31

35. 2
4
4
5
1
7
Root tip squash
32
© McLeish & Snoad
Macmillan

36. Mitosis is involved in growth
and development

37. GROWTH
• Permanent increase in size and dry mass by
an increase in cell number or cell size or
growth.

38. DEVELOPMENT
• Increase in complexity.

39. 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
33

40. 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
34

41. 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
35

42. Question 4
How many chromosomes are there in a human cell?
(a) 100+
(b) 92
(c) 46
(d) 23
36

43. 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
37

44. Question 6
Which of these statements are correct?
A chromosome contains
(a) Protein
(b) Cellulose
(c) DNA
(d) Genes
38

45. Question 7
The process by which a chromosome makes a
copy of itself is called
(a) Reproduction
(b) Recombination
(c) Relocation
(d) Replication
39

46. 40
What is the correct sequence of events in mitosis ?
a b c d e f
(c) c, b, a, e, d, f
(a) c, d, a, b, e, f
(b) b, c, a, d, e, f
(d) c, b, d, a, e, f
Question 8

47. Answer
Correct

48. Answer
Incorrect