DNA and Protein Synthesis Biology 12 Chapter 24

Overview/Objectives Part 1: DNA DNA structure Functions of DNA DNA Replication Part 2: Protein Synthesis (Gene Expression) RNA Protein Synthesis Transcription Translation Mutations

Part 1: DNA DNA = Deoxyribonucleic Acid = The genetic material of all living things = Contains all of the “instructions” needed for life

I. DNA structure Determined by Watson and Crick in the early 1950’s Made up of nucleotides  each nucleotide has 3 subunits: Phosphate group Pentose sugar (deoxyribose) Nitrogen-containing base

4 possible bases: Purines (Double Ring) Pyrimidines (Single Ring) Adenine (A) pairs with  Thymine (T) Guanine (G) pairs with  Cytosine (C) Complementary base pairing Done by hydrogen bonds

DNA is a double stranded molecule Strands are held together by hydrogen bonding Alternate Sugar-Phosphate backbone Strands twist to form a double helix Two strands run antiparallel (in opposite directions)

II. Functions of DNA Store information: control cell activity via. Genes Have mutations: allows for evolution Replicate: make copies of self before cell division to ensure daughter cells get equal amount of DNA

III. DNA Replication Needed when a cell divides  each new cell needs a copy of the DNA Semi-conservative replication  each new double helix has one old strand and one new strand

Steps to DNA Replication Enzyme DNA helicase unwinds and unzips the double-stranded DNA, breaking the hydrogen bonds New nucleotides base-pair to parent strands helped out by enzyme DNA polymerase Enzyme DNA ligase seals the sugar-phosphate backbone 2 double helix molecules identical to each other, having one parent and one new strand

Part 2: Protein Synthesis (Gene Expression) Gene: A segment of DNA that specifies the amino acid sequence of a protein Genes pass their info to RNA  RNA is involved in protein synthesis

I. RNA RNA = Ribonucleic Acid Type of nucleic acid consisting of: Ribose sugar Phosphate Base: adenine, guanine, uracil, cytosine Single stranded

RNA vs. DNA Sugar Ribose Deoxyribose Bases A G U C A G T C Strand single double Helix no yes

Types of RNA 3 types: Messenger RNA (mRNA): Takes a message from DNA to ribose Ribosomal RNA (rRNA): Along with proteins, make up the ribosomes (location of protein synthesis) Transfer RNA (tRNA): Transfers amino acids to the ribosome

II. Protein Synthesis Two steps involved with protein synthesis a) Transcription  genetic information is transferred from DNA to RNA b) Translation  RNA transcript directs the sequence of amino acids in the polypeptide (protein)

a) Transcription (step 1) Process of creating mRNA from a DNA template Enzyme RNA polymerase binds to a promoter (special nucleotide sequence) on DNA, and opens up the DNA helix

a) Transcription (step 2) Complementary base pairing occurs and RNA polymerase joins the RNA nucleotides, forming an RNA molecule

a) Transcription (step 3) 3. Processing of RNA One end is “capped”(addition of altered guanine nucleotide) Addition of poly-A-tail to the other end Introns removed, exons joined

a) Transcription (step 4) 4. Mature mRNA leaves the nucleus through a nuclear pore and enters cytoplasm

The Genetic Code Triplet code  every 3 bases of mRNA codes for 1 amino acid Each 3 letter (nucleotide) unit of mRNA is called a codon tRNA brings amino acids to ribosomes: one end holds the amino acid, the other end contains an anticodon  a group of 3 bases complementary to a codon on mRNA

Example: Reading the Genetic Code Which amino acid sequence is formed from the DNA strand: GGT ACA TAG CAC DNA CCA UGU AUG GUG mRNA proline-cysteine-isoleucine-valine amino acids Use table 24.8 p.492

b) Translation The step by which gene expression leads to protein synthesis 3 steps: Chain initiation Chain elongation Chain termination

b) Translation (step 1) Chain Initiation Smaller ribosomal subunit binds mRNA Initiator tRNA binds Larger ribosomal subunit binds, completing the ribosome

b) Translation (step 2) Chain Elongation tRNA binds in 2nd binding site of ribosome Peptide bond forms between amino acids tRNA in 1st binding site leaves, ribosome moves down mRNA (translocation) tRNA in 2nd binding site has moved to 1st binding site… cycle continues…

b) Translation (step 3) Chain Termination Stop codon on mRNA reached Polypeptide (amino acid chain) is cleaved from the last tRNA Ribosome separates and releases mRNA

III. Mutations Mutation= any alteration in the code or expression of the code (DNA) Causes of Mutations: Errors in Replication Mutagens (environmental influences) Transposons (DNA sequences that move between chromosomes) Types of Mutations: Frame shift mutation (nucleotides are inserted or deleted) Point mutation (substitution of one nucleotide for another)