Lecture 37 Prof Duncan Shaw. Textbooks in the library Emery's Elements of Medical Genetics by RF...
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Transcript of Lecture 37 Prof Duncan Shaw. Textbooks in the library Emery's Elements of Medical Genetics by RF...
Lecture 37
Prof Duncan Shaw
Textbooks in the library
• Emery's Elements of Medical Genetics by RF Mueller & ID Young, or PD Turnpenny for latest edition
• A general genetics textbook, such as Genetics (Principles and Analysis) by DL Hartl and EW Jones
Introduction• Previous lectures covered genes, proteins,
cells etc. as isolated things
• Genes vary between individuals and between populations
• Genetic variation is crucial for differences between individuals: – In normal characteristics (e.g. height, weight,
mental ability) – In susceptibility to disease
Cells divide, DNA replicates
• Cells must divide. Obvious, because 1 cell (fertilised embryo) becomes 3x1012 cells in human body
• DNA and genes must be accurately copied - errors would stop genes functioning
• The Cell cycle is crucial - this is where DNA is replicated - if it goes wrong, can cause trouble e.g. cancer
Replication of DNA• When not replicating, DNA in chromosomes
is highly organised
• Replication starts at several sites in chromosome (origins of replication) where DNA starts to unwind
• Replication is catalysed by a complex of enzymes at the Replication Forks
• Continues outwards from origins until all DNA is replicated
Chromosome structure - packing ratio
• Packing ratio is the length of the DNA divided by the length into which it’s packaged
• Smallest human chromosome (21) has 4x107 bp of DNA, 10 times size of E. coli genome
• Equivalent to 14mm of extended DNA• In most condensed state the chromosome is about
2m long• Packing ratio = 14000/2 = 7000• So, there must be an efficient packaging
mechanism
Error correction in DNA replication
• Overall error rate is about 10-10 per division
• About 1 mistake per cell per division in humans
DNA mismatch repair
• When DNA gets damaged e.g. by chemicals, radiation
The human genome
• Its size is 3 x 109 bp (3000Mb) for a single copy
• Contains 23 pairs of chromosomes
• About 35,000 genes
• Other mammals have about the same amount of DNA and number of genes, but often have different number of chromosomes