1. DNA and Protein Synthesis
Biology 12
Chapter 24

2. Overview/Objectives Part 1: DNA
DNA 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’s
Made 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 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)

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 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

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 Strand
single
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 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)

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 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

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 Initiation
Smaller ribosomal subunit binds mRNA
Initiator tRNA binds
Larger ribosomal subunit binds, completing the ribosome

24. 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…

25. 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

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)