1. Chapter 11.4-11.6 From DNA to Proteins
College Prep Biology
Mr. Martino

2. Introduction Protein synthesis is the process of making proteins
2 stages:
1. Transcription – process of transferring the DNA gene onto an mRNA “transcript”
2. Translation – process of tRNA and rRNA assembling the amino acids in the proper sequence according to mRNA
Involves RNA

3. 11.4 Transcription 3 main classes of RNA:
mRNA: messenger RNA carries the instructions from DNA
tRNA: transfer RNA delivers amino acids to ribosome to form protein
rRNA: ribosomal RNA a major component of ribosomes

4. More Transcription RNA is similar yet different from DNA
Single stranded
4 types of nucleotides (uracil substitutes for thymine – pairing with adenine)
Ribose substitutes for deoxyribose
Different types of RNA
Found in nucleus (like DNA) but also in cytoplasm

5. Uracil is only found in RNA

6. Transcription again
Transcription is similar to replication – but differs in 3 important ways:
1. Only a small section of DNA (gene) serves as template
Promoter: a base sequence on DNA that signals the beginning of a gene
2. RNA polymerase attaches nucleotides instead of DNA polymerase
3. Results in a single strand of RNA – not double helix

7. 3 Phases of transcription:
1. Initiation: when RNA polymerase attaches to the promotor DNA
2. Elongation: growing of the RNA strand
As it continues, RNA strand pulls away from DNA & DNA rejoins
3. Termination: RNA reaches a specific sequence that codes for stop
Polymerase detaches from RNA and gene

9. Not all of the new mRNA is used
Introns: base sequences that are removed before translation
Exons: parts of the mRNA that gets translated into a protein

10. 11.5 Genetic Code
Gobind Khorana and Marshall Nirenberg realized ribsomes read nucleotides 3 at a time
Codon: set of 3 nucleotides = amino acid
64 codons code for 20 amino acids
AUG – codes for methionine /“start”
3 codons for “stop”
Several codons code for some amino acids
Codons are universal for all living organisms

11. Structure and Function of tRNA & rRNA
tRNA: (transfer RNA) carry amino acids
One end has anticodon while other carries a.a.
Anticodon: nucleotide triplet that base pairs with codon
Specific for 1 amino acid
rRNA: component of ribosomes

12. 11.5 Translation
Translation has 3 stages – initiation, elongation, and termination:
1. Initiation:
Step 1: mRNA binds to small subunit initiator tRNA locates & binds to start codon (AUG)
Step 2: large subunit binds to small one

13. 2. Elongation: After initiation, A.A.’s are added one by one
Step 1: Codon recognition
tRNA anticodon matches mRNA codon
Step 2: Peptide Bond Formation
A.A. separates from tRNA and attaches to polypeptide (P to A)
Step 3: Translocation
A site tRNA moves to P site

14. Termination: stop codon on mRNA is reached
Triggers a release factor to bind to ribosome
Enzyme detaches mRNA and polypeptide
Many polypeptides then float freely in cytoplasm – others enter rough ER before being sent to their ultimate destinations

15. 11.6 Mutations
Gene mutations: change in nucleotide sequence of DNA genes
Substitution: one amino acid is substituted for another
Ex. Sickle-cell anemia
Frame shift mutations: change “down-stream” codons
Insertion: an extra base is put into a gene
Deletion: loss of a base
Transposable Elements: (translocation) DNA segments that move spontaneously

16. Causes of Mutations
Many mutations occur spontaneously during replication
Small number of mistakes do sneak by proofreaders
Mutagen: mutation causing agents in environment
UV light, gamma rays, X-rays
Carcinogen: cancer causing agents – disrupt base pairing
Only spontaneous mutations of germ cells or gametes are passed on to offspring