Chromosome Tightly coiled DNA Form when cells divide

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 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 © 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

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 ODWS Image 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. Interphase • The nucleus divides, distributing the chromosomes into two equal groups (mitosis). • The cytoplasm then divides (cytokinesis) each part taking a nucleus. © 2007 Paul Billiet ODWS Image believed to be in the Public Domain

The cell cycle Some cells may stay in this stage for over a year Cytokinesis Division of the cytoplasm M G 0 G 1 G 2 First growth phase. Varies in length Second growth phase S G 1 + S + G 2 = INTERPHASE © 2007 Paul Billiet ODWS Copying of chromosomes

The cell cycles in different cells Cell type Bean root tip Cell cycle / hours 19. 3 Mouse fibroblast 22 Chinese hamster fibroblast Mouse small intestine epithelium 11 Mouse oesophagus epithelium © 2007 Paul Billiet ODWS 17 181

How many chromosomes are in brain cells? Remember…Brain 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 chromosomes Answer = 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 chromosomes Answer = 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 chromosomes Answer = 46