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Mitosis Produces 2 cells – called daughter cells – from one parent cell Daughter cells are identical to each another and to the original parent cell Most body cells reproduce this way

Mitosis Required for: 1.GROWTH OR MULTICELLULAR ORGANISMS 2.REPLACEMENT OF OLD CELLS 3.ASEXUAL REPRODUCTION IN SINGLE- CELL ORGANISMS LIKE BACTERIA.

Mitosis Chromosomes are made of DNA and are located in the nucleus.

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

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

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

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

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

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

On the next slide: How many homologous pairs can you see in the last frame?

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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

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

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

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

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

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

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 on a Homologous Pair 16

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 sister chromatids chromatids Chromatids 17

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

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

Two homologous pairs of Chromosomes are shown. Each chromosome has replicated to form chromatids Cell division 1 20 sister chromatids homologous pair

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

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

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

The cell begins to divide Cell division 5 24

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

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

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

ell_cycle.htm Cell Cycle Animation:

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?

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

Mitosis 30

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. See if you can associate the various stages of cell division with the stages 1-6 in the previous slide. 31

Root tip squash 32 © McLeish & Snoad Macmillan

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

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

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

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

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

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

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

40 What is the correct sequence of events in mitosis ? abcde 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

Answer Correct

Answer Incorrect