lectue4 dna,gentics - Biology 213 11.1 DNA and RNA...

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Biology 213 DNA-GENETICS 11.1 DNA and RNA Structure and Function • The work of Mendel and others provided information about the nature of deoxyribonucleic acid (DNA). – Genes occur on chromosomes. – Mutations in genes caused metabolic errors. – DNA is composed of nucleotides Structure of DNA • The research of several scientists helped determine the structure of DNA. – Erwin Chargaff – Rosalind Franklin – James Watson and Frances Crick. Chargaff’s Rules • DNA contains four types of nucleotides, differing in the nitrogen-containing base each contains. – The purine base adenine (A) – The pyrimidine base cytosine (C) – The purine base guanine (G) – The pyrimidine base thymine (T) • A nucleotide from DNA contains one base, one phosphate group, and the sugar deoxyribose. Chargaff’s Rules (cont.) Chargaff’s Rules (cont.)
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Chargaff’s Rules (cont.) • In all organisms, the nucleotide bases are found in specific proportions. • Chargaff’s rules are based on two observations. – The amount of A, C, G, and T varies from species to species. – In each species, the amount of A is equal to T and the amount of G is equal to C. Chargaff’s Rules (cont.) • Equal proportions between two bases indicated that the bases were paired in the structure of DNA. • The order in which these nucleotides occur differs, producing great variability in the DNA. • With four nucleotides, there are 4 140,000000 possible nucleotide sequences. Franklin’s X-Ray Diffraction Studies • Rosalind Franklin used X-ray crystallography to study the structure of DNA. • Franklin took an X-ray of fibers present in a concentrated solution of DNA. • The X-ray diffraction pattern suggested that DNA had a helical shape. Franklin’s X-Ray Diffraction Studies (cont.) The Watson and Crick Model • Watson and Crick developed the definitive model of DNA structure. – The sugar and phosphate groups are bonded in alternating sequences to form the sides of a twisted ladder. – Bases are joined by hydrogen bonds to form the rungs of the ladder. – Complementary base pairing occurs, meaning A only bonds with T and G with C. The Watson and Crick Model (cont.)
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The Replication of DNA • DNA replication is the process of copying a DNA molecule. • During replication, the original strands serve as templates for the new DNA. • DNA replication is considered semiconservative because one of the original strands is present in each new DNA helix. The Replication of DNA (cont.) • DNA replication requires several steps. – The enzyme helicase unwinds the old strands of the DNA helix. – New nucleotides are positioned by complementary base pairing and the enzyme DNA polymerase. – DNA ligase repairs breaks in the backbone. The Replication of DNA (cont.) The Replication of DNA (cont.) • In eukaryotic cells, DNA replication is initiated at multiple points along the DNA strand, or origins of replication.
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