1 DNA and Protein SynthesisBiology 12 Chapter 24

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

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

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

5 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

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

7 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

8 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

9 Steps to DNA ReplicationEnzyme 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

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11 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

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

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

14 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

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

16 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

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

18 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

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

20 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

21 Example: Reading the Genetic CodeWhich 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

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

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

24 b) Translation (step 2) Chain ElongationtRNA 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…

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

26 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)