CHROMOSOMES 2016 Paul Billiet ODWS Chromosomes in eukaryotes
CHROMOSOMES © 2016 Paul Billiet ODWS
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 © 2016 Paul Billiet ODWS
Chromosomes in eukaryotes n n n 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. © 2016 Paul Billiet ODWS
Numbers of chromosomes n n n 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. © 2016 Paul Billiet ODWS
Mouse © 2016 Paul Billiet ODWS Maize
Organism Homo sapiens Human Chromosome numbers (2 n) 46 Pan trogolodytes Chimpanzee 48 Canis familiaris Domestic dog 78 Oryza sativa Domesticated rice Horse roundworm 24 Parascaris equorum © 2016 Paul Billiet ODWS 4
Human chromosomes © 2016 Paul Billiet ODWS
The chromosomes of a human female © 2016 Paul Billiet ODWS
Identifying chromosomes Chromosomes can be identified by: n Their size n Their shape (the position of the centromere) NB Chromosomes are flexible n Banding patterns produced by specific stains (Giemsa) Chromosomes are analysed by organising them into a KARYOGRAM © 2016 Paul Billiet ODWS
Female © 2016 Paul Billiet ODWS Male
Down syndrome © 2016 Paul Billiet ODWS
Development and Differences in chromosomes are associated with difference chromosomes in the way we grow. n n The karyograms of males and females are not the same Females have two large X chromosomes Males have a large X and a small Y chromosome The X and the Y chromosomes are called sex chromosomes The sex chromosomes are placed at the end of the karyotype n Unusual growth can be associated with chromosome abnormalities e. g. People who develop Down syndrome have trisomy 21 © 2016 Paul Billiet ODWS
Chromosomal abnormalities: Aneuploidy Trysomy-21 Down’s syndrome Trysomy-18 Edward’s syndrome © 2016 Paul Billiet ODWS
Downs syndrome v age of mother © 2016 Paul Billiet ODWS
Therefore genetic information is found on our chromosomes © 2016 Paul Billiet ODWS
Chromosomes and cell division n n Multicellular organisms copy their chromosomes before cell division. The cells 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. © 2016 Paul Billiet ODWS Interphase
The cell cycle Cytokinesis Division of the cytoplasm M G 0 Some cells may stay in this stage for over a year G 1 First growth phase. Varies in length G 2 Second growth phase S Copying of chromosomes G 1 + S + G 2 = INTERPHASE © 2016 Paul Billiet ODWS
Cell cycles in different cells Cell type Cell cycle / hours Bean root tip 19 Mouse fibroblast 22 Chinese hamster fibroblast 11 Mouse small intestine epithelium Mouse oesophagus epithelium 17 © 2016 Paul Billiet ODWS 181
Chromosomes and reproduction n Chromosomes come in pairs (diploid sets) n One of the pair is maternal the other is paternal n When parents make sex cells the number of chromosomes must be halved One of each type of chromosome is taken. © 2016 Paul Billiet ODWS
Meiosis and fertilisation Mother Father 23 pairs of chromosomes Meiosis Sex cells 23 unpaired chromosomes Fertilisation 23 pairs of chromosomes Child © 2016 Paul Billiet ODWS 23 unpaired chromosomes
Meiosis (From the Greek: meioun to diminish) n A special type of cell division n Used to make sex cells n Meiosis halves the numbers of chromosomes (haploid sets) n Meiosis picks one chromosome from each pair at random and places them in a sex cell. This results in enormous variation amongst the sex cells. © 2016 Paul Billiet ODWS
The inheritance of gender Is it going to be a boy or a girl? © 2016 Paul Billiet ODWS
The inheritance of gender Mother Father XX XY Meiosis Sex cells X Fertilisation © 2016 Paul Billiet ODWS X X XX Y XY X XX XY Y Possible children Chance of a girl 50% Chance of a boy 50%
Sex chromosomes n n The sex of many animals is determined by genes but on chromosomes called sex chromosomes The other chromosomes are called autosomes One sex is homogametic The other sex is heterogametic. © 2016 Paul Billiet ODWS
Sex determination in different animals HOMOGAMETIC SEX HETEROGAMETIC SEX Female XX Male XY Male ZZ Female ZW Female XX Male Xo © 2016 Paul Billiet ODWS SEX DETERMINATION Presence of Y-chromosome = maleness (mammals and fish) Presence of second Xchromosome = femaleness (Drosophila, the fruit fly) Birds, amphibians, reptiles, butterflies, moths. Grasshoppers
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