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

Identifying chromosomesChromosomes 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

© 2007 Paul Billiet ODWS

© Biologyreference.com

• 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

KARYOTYPE

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

• 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

• 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)

Chromosomes in eukaryotes and prokaryotes are different

PROKARYOTES EUKARYOTES

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

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

© 2007 Paul Billiet ODWSImage believed to be in the Public Domain

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 ODWSImage believed to be in the Public Domain

The cell cycle

First growth phase.Varies in length

Copying of chromosomes

Some cells may stay in this stage for over a year

Second growth phase

M

G1

S

G2

G0

Cytokinesis Division of the cytoplasm

G1 + S + G2 = INTERPHASE

© 2007 Paul Billiet ODWS

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

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

What is the human haploid chromosome number?

Remember…Haploid = half number of chromosomesAnswer = 23 chromosomes

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

What is the human diploid chromosome number?

Remember…Diploid = total number of chromosomesAnswer = 46

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

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

chromosomesAnswer = 46