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

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 © Biologyreference.com © 2007 Paul Billiet ODWS

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

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 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 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 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 Image believed to be in the Public Domain © 2007 Paul Billiet ODWS

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

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

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 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