DNA Structure and Function

Figure 16.1 How was the structure of DNA determined?

Overview: Life’s Operating Instructions In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA DNA, the substance of inheritance, is the most celebrated molecule of our time Hereditary information is encoded in DNA and reproduced in all cells of the body This DNA program directs the development of biochemical, anatomical, physiological, and (to some extent) behavioral traits

Building a Structural Model of DNA: Scientific Inquiry After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredity Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure Franklin produced a picture of the DNA molecule using this technique

Franklin’s X-ray diffraction photograph of DNA Figure 16.6 Rosalind Franklin and her X-ray diffraction photo of DNA. (a) Rosalind Franklin (b) Franklin’s X-ray diffraction photograph of DNA

Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases The pattern in the photo suggested that the DNA molecule was made up of two strands, forming a double helix

5 end 3 end 3 end 5 end Hydrogen bond 3.4 nm 1 nm 0.34 nm (a) Figure 16.7 5 end C G Hydrogen bond C G 3 end G C G C T A 3.4 nm T A G C G C C G A T 1 nm C G T A C G G C C G A T Figure 16.7 The double helix. A T 3 end A T 0.34 nm 5 end T A (a) Key features of DNA structure (b) Partial chemical structure Space-filling model (c)

Purine  purine: too wide Pyrimidine  pyrimidine: too narrow Figure 16.UN01 In-text figure, p. 310 Purine  pyrimidine: width consistent with X-ray data

Nucleotide Structure Each nucleotide consists of: phosphate group Sugar group ribose (RNA) deoxyribose (DNA) nitrogenous base in this picture = adenine

Nucleotide Structure

Nitrogenous Bases Guanine (G) Adenine (A) Cytosine (C) Thymine (T) Uracil (U)  replaces thymine in RNA

DNA Structure Consists of a pair of nucleotide chains held together by hydrogen bonding between complimentary base pairs  creates double helical shape A - T C - G

Complementary Base Pairing Nitrogenous bases united by hydrogen bonds DNA base pairings A-T and C-G Law of complementary base pairing one strand determines base sequence of other Segment of DNA

DNA Replication DNA Helicase Complimentary base pairing DNA polymerase Breaks hydrogen bonds between bases Complimentary base pairing DNA polymerase Joins complimentary nucleotides forming new strand

The Basic Principle: Base Pairing to a Template Strand Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules

(a) Parent molecule A T C G T A A T G C Figure 16.9 A model for DNA replication: the basic concept.

(a) Parent molecule (b) Separation of strands A T A T C G C G T A T A Figure 16.9 A model for DNA replication: the basic concept.

(a) Parent molecule (b) Separation of strands (c) G C G C G C G T A T A T A T A A T A T A T A T G C G C G C G C (a) Parent molecule (b) Separation of strands (c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand Figure 16.9 A model for DNA replication: the basic concept.

Semiconservative Replication One parental DNA strand is conserved and one newly synthesized strand is produced

Protein Synthesis

DNA Contains the instructions for protein synthesis Sequence of bases codes for one protein Specifically, a sequence of three bases codes for a particular amino acid (“triplet code”)

RNA: Structure and Function Consists of a single chain of nucleotides Ribose replaces deoxyribose as the sugar Uracil replaces thymine as a nitrogenous base Forms of RNA: messenger RNA (mRNA) transfer RNA (tRNA) ribosomsal RNA (rRNA) Function: interpret DNA code and direct protein synthesis in the cytosol (ribosome)

Genetic Code Method of information storage along DNA strands Codon: Stored in the sequence of bases Genetic code called “triplet code” because three base sequence codes for a single amino acid Codon: “mirror-image” sequence of bases found in mRNA Complimentary base pairing

Preview of Protein Synthesis Transcription messenger RNA (mRNA) is formed next to an “activated” gene  copy of instructions is created mRNA migrates to cytosol Translation mRNA code is “read” by ribosomal RNA as amino acids are assembled into a protein molecule transfer RNA delivers the amino acids to the ribosome

Transcription Instructions are transcribed or copied from DNA to RNA RNA polymerase binds to DNA opens DNA helix and transcribes bases Bases pair in complimentary manner mRNA moves through nuclear envelope into cytosol and binds to ribosome

Translation of mRNA Formation of linear chain of amino acids by using instructions mRNA Codon on mRNA designates a particular amino acid

Transfer RNA (tRNA) Binds specific amino acid and brings to ribosome to join amino acid to growing protein molecule Anticodon binds to complementary codon on mRNA

DNA and Peptide Formation