chromosomes and diseases genetics mbg-210. how many chromosomes in humans? theophilus painter in...

Chromosomes and Diseases

Genetics MBG-210

How many chromosomes in humans?

• Theophilus Painter in 1921 characterized the number of chromosomes as 24 on spermatocytes. Others repeated the experiment and somehow came up with the same conclusion.

• Until 1955, it was agreed that humans have 24 pairs of chromosomes!!!– With better techniques, the number came down

to 23 pairs.

Chromosome 2: Humans, chimps, gorillas, and orangutans

Hypothesis

• Observation: Chimps, gorillas, and orangutans have 24 pairs of chromosomes, whereas humans have 23 pairs.

• Hypothesis: Common ancestor of all great apes had 24 pairs of chromosomes and that the fusion of two of the ancestor's chromosomes (chromosomes 2p and 2q) created chromosome 2 in humans

http://www.evolutionpages.com/chromosome_2.htm

Telomere: a sequence of DNA at the end of the chromosome whose function is to protect the ends of the chromosomal DNA strands during replication.

Centromere: seen on a condensed chromosome as a pinched region, contains proteins which form kinetochores to attach the sister chromatids.

Evidence 1

• Chromosomes 2p and 2q, when laid end to end, create an identical banding structure with the human chr. 2.

Evidence 2

• The remains of telomeres is found in the middle of human chromosome 2 where the ancestral chromosomes fused.

Evidence 3

• the centromere of human chromosome 2 lines up with the chimp chromosome 2p chromosomal centromere.

Evidence 4

• At the place where we would expect it on the human chromosome we find the remnants of the chimp 2q centromere

Are humans more closely related to chimps than they are to

gorillas?

How related are chimps and humans?

• Apart from the fusion in chr. 2, the 13 other chromosomes no visible differences exist.

• 98% of the chimp and human genome are identical.

• Although we look rather different from chimps when we compare ourselves to amoebae (called an outgroup in phylogenetic analysis) or even worms, chimps and humans look alike.

Gene Gateway• http://www.ornl.gov/sci/techresources/

Human_Genome/posters/chromosome/

http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/chooser.shtml

Chromosome 2

• BRCA1-associate ring domain (breast cancer)

• Liver cancer oncogone• Muscular dystrophy• Orofacial cleft

Select a disease on a chromosome

• Go to the website www.genetests.org– On this page select geneReviews on the bar and

search for the disease name/keyword.

Go to Genes and disease, and select a chromosome

http://www.ncbi.nlm.nih.gov/disease/chr9-12.html