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BIOLOGICAL CHEMISTRY Prof. J.H.P. Bayley, Dr. R.M. Adlington and Dr. L. Smith Trinity Term 2007 - First Year Lecture 8 Hagan Bayley Introduction to genetics and the machinery of protein synthesis. Genotype and phenotype, Mendelian genetics, dominant and recessive genes, chromosomes, genetic maps, genome sequencing. [HB] Hartl, D. L. and E. W. Jones, 2006 Essential Genetics. 4th ed. Jones & Bartlett, Sudbury, MA. 600 pp. 3rd ed Lecture notes are at: www.chem.ox.ac.uk/bayleygroup/ on weblearn soon Nucleus Genes arranged on chromosomes Site of chromosome replication Site of synthesis of mRNA, which codes for proteins Ribosomes are made in the nucleolus Regulatory factors & gene products transported in and out via nuclear pores The space between the two nuclear membranes (the "perinuclear space") is continuous with the lumen of the rough endoplasmic reticulum (RER) Genotype and phenotype ... Genotype- the genetic constitution of an organism (or cell), i.e. the nature of the organism's genome. Phenotype- The observable properties of an organism (or cell), which result from the interactions of the genotype and the environment Chromosomes • genes are organized in chromosomes- 46 in humans • in eukaryotes, chromosomes are very long linear sequences of double-stranded DNA (encased in proteins including histones) • 23 pairs (sets) of homologous chromosomes in diploid cells- one in each pair from father, one from mother • germ cells (gametes: sperm cells and eggs) are haploid - 23 chromosomes, one from each set Genetic maps and genome sequencing • until recently genes were painstakingly mapped to specific chromosomes and ordered within each chromosome (genetic map) • now the entire sequence of every chromosome of the human genome is available- rough sequence 2001, by now highly refined Human genome Each position 2 bits: 22 = 4 bases Human diploid genome: 6 X 109 bases 12 X 109 bits = 1.5 X 109 bytes = 1.5 Gb … or the number of letters in 1000 long novels; there are 660,000 words in War and Peace … Storage space of CD 700 Mb Would fit, with error correction, on a single DVD (4.7 Gb) Genetic variation • We are all "mutants"- variations in our genes have arisen through the generations allele- any one of the alternative forms of a gene • gene chips and rapid re-sequencing for mutational analysis, e.g. for forensic or medical applications cell cycle and somatic (non-germ) cell division mitosis- all 46 chromosomes replicated in each of the two progeny cells. DNA is replicated (chromosomes are duplicated) before nuclear division starts this organism has two pairs of homologous chromosomes red/ orange and green/ blue meiosis- DNA is replicated (chromosomes are duplicated). There follow two successive cell divisions with no further replication to yield four haploid cells (each contains one copy of each of the 23 chromosomes). The chromosomes (and their associated variations- alleles) are randomly assorted during this process. This is the basis of Mendelian inheritance illustrated for just one pair of homologous chromosomes crossing over/ recombination occurs during meiosis leading to the occasional re-assortment of genes between members of homologous pairs of chromosomes- an additional source of genetic variation Types of mutation produces allelic variation • molecular basis of mutation base substitution base deletion base insertion • effects of mutation silent (degenerate code) UCCUCG, SerSer missense GAGGUG, GluVal (sickle cell disease) nonsense (termination) UACUAG, TyrSTOP frameshift Genetic code- three base codons … at least one for each amino acid (degenerate code) Genetic diversity • mutations are redistributed by: chromosome re-assortment- in meiosis crossing over (recombination) • fertilization brings together 23 chromosomes from each gamete- sperm and egg to produce a 46 chromosome zygote Gregor Mendel- genes come in pairs, separate in gametes, and randomly come back together again as pairs during fertilization. This is explained in modern terms by reassortment during meiosis allele- any one of the alternative forms of a gene homozygous organism- organism having the same allele of a gene on the two homologous chromosomes heterozygous organism- carrying dissimilar alleles of a gene(s); not homozygous dominant trait- if an allele is present in a heterozygous genotype, it results in a phenotype characteristic of the allele recessive trait- refers to an allele that results in a characteristic phenotype only in the homozygous genotype XY sex chromosomes are an exception to rule of homologous pairs Technologies DNA microarrays • spotted • in situ synthesis DNA sequencing ddCTP ddTTP analog