DNA and Chromosomes Nucleotides are composed of a base sugar and phosphate group In deoxyribonucleic acid DNA the sugar is deoxyribose NH2 phosphate OH HO P O CH2 O H O N C C N HC base N C N CH sugar H H OH H H Nucleotides are linked via phosphodiester bonds to form nucleic acids phosphodiester bonds form via condensation reactions DNA nucleotides can be purines G A or pyrimidines T C DNA strands have a polarity 5 versus 3 DNA strands are negatively charged Double stranded DNA consists of two complementary chains of nucleotides in an antiparallel conformation The DNA stands are held together by hydrogen bonds The DNA double helix contains 10 base pairs bp per turn Most genes contain information to make proteins A genome is all the information contained within an organism s DNA Eukaryotic DNA is divided into chromosomes homologous chromosomes homologues one pair of chromosomes one maternal and one paternal Karyotype display of the mitotic chromosomes Human Genome draft sequence 2001 finished 2004 22 autosomes and 2 sex chromosomes 3 2 billion base pairs approximately 25 000 genes 1 5 of the genome encodes exons The duplication and segregation of chromosomes occurs through an ordered cell cycle Three DNA sequence elements ensure chromosomes can be replicated replication origin and telomere and segregated centromere Chromosomes in a cell are condensed mitotic chromosomes most condensed interphase chromosomes less condensed Chromatin DNA and its associated proteins Nucleosomes are the basic unit of chromatin packing consist of 4 pairs of histones 8 total Histones are highly conserved small positively charged proteins 147 bp piece of DNA Nucleosomes are further packed into 30 nm fibers via 1 interactions between histone tails 2 binding of histone H1 Nucleosome wrapped DNA is accessible to other DNA binding proteins via 1 nucleosome structure is fluid and dynamic 2 chromatin remodeling complexes slide DNA in nucleosomes in an ATP dependent manner Nucleosome wrapped DNA is accessible to other DNA binding proteins via 3 Modification of histone tails by modifying proteins disrupts stability of the 30 nm fibers and higher order packing recruits proteins that condense or decondense chromatin The pattern of histone modification indicates the status of the nearby chromatin There are two levels of interphase chromatin condensation 1 heterochromatin most condensed induced by tail modifications including methylation of lysine 9 on the histone H3 tail gene poor regions centromeres and telomeres genes are not expressed Heterochromatin example X inactivation There are two levels of interphase chromatin condensation 2 euchromatin less condensed gene rich regions Genes undergoing transcription are found in areas of chromatin with the least amount of condensation Chromatin structure is inherited epigenetic inheritance DNA is contained within the nucleus of eukaryotic cells nuclear envelope two co centric lipid bi layers nuclear pores allow exchange between the cytoplasm and nucleus nuclear lamina mesh of intermediate filaments that support the nucleus Chromosomes occupy discrete territories within the nucleus There are biochemical neighborhoods within a nucleus Nucleolus example of a biochemical neighborhood Nucleolus contains genes that encode ribosomal RNAs
View Full Document
Unlocking...