1. Chromosomes Chromosome: Tightly coiled DNA Form when cells divide
Chromosomes created for the new cells
2 Parts:
1) Chromatids: two identical parts of a chromosome
2) Centromere: Joins chromatids together
Chromosomes

2. Identifying chromosomes
Chromosomes can be identified by:
Their size
Their shape (the position of the centromere) NB Chromosomes are flexible
Banding patterns produced by specific stains (Giemsa)
Chromosomes are analysed by organising them into a KARYOTYPE
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© 2007 Paul Billiet ODWS

3. Karyotype Female Male Defined: Picture of an individuals chromosomes
Help identify sex & chromosome defects
First 22 pairs = autosomes
Size of final pair identifies sex
Same size: XX = female
Different size: XY = male
Normal human will have 46 chromosomes
Female
Male

4. Numbers of chromosomes
Constant for each cell in the body except sex cells which only have half sets
Constant throughout the life of an individual (you don’t lose or gain chromosomes)
Constant for all members of a species
Humans have 46 chromosomes
Chimpanzees have 48
Maize (corn) has 20
© 2007 Paul Billiet ODWS

5. Diploid Cells Diploid Cells = Cells with the full set of chromosomes
Paired chromosomes
Half of our chromosomes come from each parent (23 from each parent)
Somatic (non-sex) cells are diploid
Created by mitosis
Ex: Skin, Muscle, Nerve, Blood Cells

6. Haploid Cells
Haploid Cells = Cells with ½ the total number of chromosomes
Gametes (sex cells) are the only haploid cells
Ex: Sperm, Egg, Pollen
Created by meiosis
Chromosome number reduced by 1/2
How do humans get 46 chromosomes?
Haploid sperm cell (23)
+ Haploid egg cell (23)
= Diploid zygote (46)

7. Chromosomes in eukaryotes and prokaryotes are different
single chromosome plus plasmids
many chromosomes
circular chromosome
linear chromosomes
made only of DNA
made of chromatin, a nucleoprotein (DNA coiled around histone proteins)
found in cytoplasm
found in a nucleus
copies its chromosome and divides immediately afterwards
copies chromosomes, then the cell grows, then goes through mitosis to organise chromosomes in two equal groups
© 2007 Paul Billiet ODWS

8. Chromosomes in eukaryotes
Found in the nucleus
Condensed and visible during cell division
At the beginning of mitosis they can be seen to consist of two threads (sister chromatids) joined by a centromere
The sister chromatids are identical copies
During mitosis the sister chromatids separate and are placed into two nuclei
Image believed to be in the Public Domain
© 2007 Paul Billiet ODWS

9. Chromosomes and cell division
Multicellular organisms copy their chromosomes before cell division.
They must grow to a mature size.
The nucleus divides, distributing the chromosomes into two equal groups (mitosis).
The cytoplasm then divides (cytokinesis) each part taking a nucleus.
Interphase
© 2007 Paul Billiet ODWS
Image believed to be in the Public Domain

10. The cell cycle INTERPHASE Mitosis
Cytokinesis
Division of the cytoplasm
Some cells may stay in this stage for over a year G0 M
Mitosis
First growth phase.
Varies in length G1 G2
Second growth phase
INTERPHASE S
Copying of chromosomes
G1 + S + G2 = INTERPHASE
© 2007 Paul Billiet ODWS

11. The cell cycles in different cells
Cell type
Cell cycle / hours
Bean root tip
19.3
Mouse fibroblast 22
Chinese hamster fibroblast 11
Mouse small intestine epithelium 17
Mouse oesophagus epithelium
181
© 2007 Paul Billiet ODWS

12. How many chromosomes are in brain cells?
Remember…Brain cells are non-sex cells… …Somatic cells are non-sex cells… …Somatic cells are diploid… …Diploid cells have the full set of chromosomes Answer = 46

13. What is the human haploid chromosome number?
Remember…Haploid = half number of chromosomes Answer = 23 chromosomes

14. How many chromosomes are in female egg cells?
Remember…Egg cells are sex cells… …Sex cells are gametes… …Gamete cells are haploid… …Haploid cells have ½ the set of chromosomes Answer = 23

15. What is the human diploid chromosome number?
Remember…Diploid = total number of chromosomes Answer = 46

16. What is the human gamete chromosome number?
Remember…Gamete cells are sex cells… …Sex cells are haploid… …Haploid cells have ½ the set of chromosomes Answer = 23

17. What is the human somatic chromosome number?
Remember…Somatic cells are non-sex cells… …Somatic cells are diploid… …Diploid cells have the full set of chromosomes Answer = 46