1. Lecture 10 DNA Translation and Control

2. Translation
Translation converts the order of the nucleotides of a gene into the order of amino acids in a protein
The rules that govern translation are called the genetic code
mRNAs are the “blueprint” copies of nuclear genes
mRNAs are “read” by a ribosome in three-nucleotide units, termed codons
Each three-nucleotide sequence codes for an amino acid or stop signal

3. The Genetic Code The genetic code is (almost) universal
Only a few exceptions have been found

4. Sites play key roles in translation
Ribosomes
The protein-making factories of cells
They use mRNA to direct the assembly of a protein
A ribosome is made up of two subunits
Each of which is composed of proteins and rRNA
Sites play key roles in translation

5. Hydrogen bonding causes hairpin loops
Transfer RNA
Hydrogen bonding causes hairpin loops
tRNAs bring amino acids to the ribosome
They have two business ends
Anticodon which is complementary to the codon on mRNA
3’–OH end to which the amino acid attaches
3-D shape

6. Making the Protein mRNA binds to the small ribosomal subunit
The large subunit joins the complex, forming the complete ribosome
mRNA threads through the ribosome producing the polypeptide

7. How translation works
The process continues until a stop codon enters the A site
The ribosome complex falls apart and the protein is released
Play
Protein Synthesis

8. Architecture of the Gene
In eukaryotes, genes are fragmented
They are composed of
Exons – Sequences that code for amino acids
Introns – Sequences that don’t
Eukaryotic cells transcribe the entire gene, producing a primary RNA transcript
This transcript is then heavily processed to produce the mature mRNA transcript
This leaves the nucleus for the cytoplasm

9. Protect from degradation and facilitate translation
Processing eukaryotic mRNA
Protect from degradation and facilitate translation
Different combinations of exons can generate different polypeptides via alternative splicing
Play
How Spliceosomes Process RNA

10. How protein synthesis works in eukaryotes
Cytoplasm
Nuclear membrane
tRNA
Amino acid
4. tRNA molecules become attached to specific amino acids with the help of activating enzymes. Amino acids are brought to the ribosome in the order dictated by the mRNA.
6. The polypeptide chain grows until the protetin is completed.
Completed polypeptide
7. Phosphorylation or other chemical modifications can alter the activity of a protein after it is translated.
5. tRNAs bring their amino acids in at the A site of the ribosome. Peptide bonds form between amino acids at the P site, and tRNAs exit the ribosome from the E site. 5’ 3’
Ribosome
Ribosome moves toward 3’ end
DNA
RNA polymerase
Primary
RNA transcript 5’ 3’
1. In the cell nucleus, RNA polymerase transcribes RNA from DNA 5’
mRNA
Cap
Nuclear pore
Poly-A tail
3. mRNA is transported out of the nucleus. In the cytoplasm, ribosomal subunits bind to the mRNA
Small ribosomal subunit
Large ribosomal subunit 5’ 3’
Cap
Poly-A tail
2. Introns are excised from the RNA transcript, and the remaining exons are spliced together, producing mRNA
Introns
mRNA
Exons
Play
Control of Gene Expression

11. Architecture of the Gene
Most eukaryotic genes exist in multiple copies
Clusters of almost identical sequences called multigene families
As few as three and as many as several hundred genes
Transposable sequences or transposons are DNA sequences that can move about in the genome
They are repeated thousands of times, scattered randomly about the chromosomes

12. Turning Genes Off and On
Genes are typically controlled at the level of transcription
In prokaryotes, proteins either block or allow the RNA polymerase access to the promoter
Repressors block the promoter
Activators make the promoter more accessible
Most genes are turned off except when needed

13. The lac Operon
An operon is a segment of DNA that contains a cluster of genes that are transcribed as a unit
The lac operon contains
Three structural genes
Encode enzymes involved in lactose metabolism
Two adjacent DNA elements
Promoter
Site where RNA polymerase binds
Operator
Site where the lac repressor binds

14. The lac Operon
In the absence of lactose, the lac repressor binds to the operator
RNA polymerase cannot access the promoter
Therefore, the lac operon is shut down

15. The lac Operon
In the presence of lactose, a metabolite of lactose called allolactose binds to the repressor
This induces a change in the shape of the repressor which makes it fall off the operator
RNA polymerase can now bind to the promoter
Transcription of the lac operon is ON

16. The lac Operon

17. The lac Operon
What if the cell encounters lactose, and it already has glucose?
The bacterial cell actually prefers glucose!
The lac operon is also regulated by an activator
The activator is a protein called CAP
It binds to the CAP-binding site and gives the RNA polymerase more access to the promoter
However, a “low glucose” signal molecule has to bind to CAP before CAP can bind to the DNA
Play
Combination of Switches

18. Activators and repressors of the lac operon

19. Enhancers
DNA sequences that make the promoters of genes more accessible to many regulatory proteins at the same time
Usually located far away from the gene they regulate
Common in eukaryotes; rare in prokaryotes

20. Mutation The genetic material can be altered in two ways Recombination
Change in the positioning of the genetic material
Mutation
Change in the content of the genetic material
Bithorax mutant

21. Mutation
Mutation and recombination provide the raw material for evolution
Evolution can be viewed as the selection of particular combinations of alleles from a pool of alternatives
The rate of evolution is ultimately limited by the rate at which these alternatives are generated
Mutations in germ-line tissues can be inherited
Mutations in somatic tissues are not inherited
They can be passed from one cell to all its descendants

22. Kinds of Mutation Mutations are caused in one of two ways
Errors in DNA replication
Mispairing of bases by DNA polymerase
Mutagens
Agents that damage DNA

23. Kinds of Mutation
The sequence of DNA can be altered in one of two main ways
Point mutations
Alteration of one or a few bases
Base substitutions, insertion or deletion
Frame-shift mutations
Insertions or deletions that throw off the reading frame

24. Kinds of Mutation

25. Kinds of Mutation
The position of genes can be altered in one of two main ways
Transposition
Movement of genes from one part of the genome to another
Occurs in both eukaryotes and prokaryotes
Chromosomal rearrangements
Changes in position and/or number of large segments of chromosomes in eukaryotes

26. Kinds of Mutation

27. Kinds of Mutation

28. Mutation, Smoking and Lung Cancer
Agents that cause cancer are called carcinogens
These are typically mutagens
The hypothesis that chemicals cause cancer was first advanced in the 18th century
Many investigations since then have determined that chemicals can cause cancer in both animals and humans
For example, tars and other chemicals in cigarette smoke can cause cancer of the lung