1. Transcription & Translation
Protein Synthesis
Transcription & Translation

2. Protein Synthesis: An Overview
“The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins.”
Proteins are the link between genotype (AAC) and phenotype (eye color)
Protein synthesis is the process of transferring the information encoded in DNA into proteins
Step 1: Transcription
Step 2: Translation

3. Transcription
Transcription is the synthesis of messenger RNA (mRNA) from DNA
Occurs in the nucleus
DNA does not leave the nucleus
Analogy: blueprints do not get used at the job site when constructing a house

4. DNA vs. RNA DNA RNA Contains: Adenine, guanine, cytosine, thymine
Adenine, guanine, cytosine, uracil
5-carbon sugar is deoxyribose
5-carbon sugar is ribose
Double-stranded
Single-stranded

5. Transcription: Initiation
Promoter = a specific sequence of DNA (starting point); consists of initiation site and TATA box)
RNA polymerase binds to the promoter on the DNA molecule
Transcription factors (proteins) bind to promoter region to help RNA polymerase find the starting point.
RNA polymerase then separates the 2 DNA strands

6. Transcription: Elongation
As RNA polymerase moves along the DNA, it untwists the double helix and separates the strands
RNA polymerase adds nucleotides to the 3’ end of the mRNA molecule
Follows the base-pairing rules

7. Transcription: Termination
Termination signal = sequence of bases in DNA molecule that tell RNA polymerase to stop transcription
“finish line” in a race
mRNA is released from the DNA
Most common termination signal = AATAAAA

9. RNA Modifications
In eukaryotes, mRNA is edited before it’s sent out of the nucleus
2 major types of modifications:
Alteration of mRNA ends
RNA splicing

10. Alteration of mRNA Ends
5’ end receives a 5’ cap
Modified form of a guanine nucleotide
3’ end receives a poly-A tail
30 – 200 adenine nucleotides
Protects the ends from being degraded by enzymes in the cytoplasm
Shoelace analogy

11. RNA Splicing
Genes have stretches of nucleotides that don’t code for anything
“junk DNA”
Noncoding sequence = introns
“intervening sequences”
Coding regions = exons
During RNA splicing, the introns are removed by enzymes and the exons are joined together by SLICEOSOMES (made of snRNPs + proteins)

12. Translation Big Picture: Occurs in the cytoplasm, on a ribosome
The building of a polypeptide (protein) from the information encoded in mRNA
Occurs in the cytoplasm, on a ribosome

13. tRNA Transfer RNA (tRNA): Cloverleaf shape
Contains anticodon (triplet of bases, complementary to mRNA)
Carries specific amino acid to ribosome during translation

14. Ribosomes Made up of 2 subunits composed of rRNA & proteins:
Large subunit
Small subunit
Have special binding sites:
P site: holds tRNA with amino acid chain
A site: holds “next” tRNA
The “green room”

15. Translation Consists of 3 stages: Chain Initiation Chain Elongation
Chain Termination

16. Translation: Chain Initiation
mRNA and tRNA join
Codon + complementary anticodon
mRNA is AUG (start codon)
tRNA is UAC
Large and small ribosomal subunits join, forming a functional ribosome

17. Translation: Chain Elongation
Amino acids are added one by one by the following process:
mRNA codon bonds with anticodon on tRNA
Peptide bond is formed between new amino acid and the last one
tRNA moves over from A site to P site

18. Translation: Chain Termination
Elongation continues until a stop codon is reached (UAA, UAG, UGA)
Water is added instead of an amino acid
Polypeptide (protein) is released from ribosome

20. Point Mutations (aka gene mutations)
Just one change in a nucleotide in a single gene
Can seriously affect phenotype  genetic disorder
Sickle cell anemia results from one nucleotide substitution.

21. Point Mutations Normal Hemoglobin sequence: Base-pair substitution
CTT
GAA
Glutamic Acid
Sickle Cell Sequence:
CAT
GUA
Valine
Base-pair substitution
Replacing one nucleotide with another
Example: instead of G, it’s C
Can cause a noticeable change in protein structure
Can be a silent mutation with no actual change in amino acid sequence.
Can change only one a. acid
Non sense mutations change sequence to a stop codon.

22. Point Mutations Base-pair insertions or deletions Normal Sequence:
AAG GCG TAG 
UUC CGC AUC 
Phenylalanine-arginine-isolecine
Insertion:
AAG GGC GTA G 
UUC CCG CAU C 
Phenylalanine-proline-histidine
Base-pair insertions or deletions
Adding or losing a base-pair
Changes the whole “reading frame”
Where, within a gene, would this be the biggest problem?

23. Frame Shift Mutations
Insertions- extra nucleotide is added; moves reading frame up one nucleotide & changes every amino acid after that point.
Deletions- one nucleotide is removed; moves reading frame down one base.
Both are more harmful if at the beginning of a gene.

24. Chromosomal Mutations
A chunk of a chromosome is changed.
Addition or duplication: A piece of a chromosome is added so genes appear twice.
Deletion: part of the chromosome is lost (genes lost)
Inversion: piece of the chromosome breaks twice and broken piece inserts upside down (reverses order of genes)
Translocation: two nonhomologous chromosomes break & exchange genes.