1. Molecular Biology DNA Expression
Chapter 9
Part B

2. DNA Expression: The Central Dogma
Genes contain specific sequences of bases coding the instructions for proteins
In general one gene codes for one protein
The basic process starts in the nucleus where enzymes transcribe the gene to make a strand of RNA. The RNA exits the nucleus through the nuclear pores. In the cytoplasm the RNA is translated into a sequence of amino acids (the building blocks of proteins)
DNA RNA Protein
Transcription Translation
(Nucleus) (Cytoplasm)

3. Assembly of RNA on unwound regions of DNA molecule
Transcription
Assembly of RNA on unwound regions of DNA molecule
mRNA
rRNA
tRNA
mRNA
processing
proteins
mature mRNA
transcripts
ribosomal
subunits
mature
tRNA
Convergence
of RNAs
Translation
cytoplasmic
pools of
amino acids,
ribosomal
subunits,
and tRNAs
At an intact
ribosome,
synthesis of
a polypeptide
chain at the
binding sites
for mRNA
and tRNAs
Final protein

4. Transcription: from DNA to mRNA
Transcription is the first step in gene expression
Transcription occurs in the nucleus in eukaryotes
The base sequence of a gene in DNA is copied to make a single strand of RNA

5. Transcription: from DNA to mRNA
DNA is unwound
Only the part of the chromosome containing the gene is unwound
Makes a transcription bubble

6. Transcription: from DNA to mRNA
RNA polymerase attaches to a binding site called a promoter
The promoter is just in front of the gene (“up stream”)

7. Transcription: from DNA to mRNA
RNA polymerase adds nucleotides one at a time using the DNA sequence as a template
A-U, T-A, G-C, C-G

8. Transcription: from DNA to mRNA
Transcription ends when the polymerase passes the end of the gene

9. Eukaryotic mRNA Processing
Once RNA is transcribed it will be modified before leaving the nucleus for the cytoplasm
Introns are spliced out
Eukaryotic genes contain
Exons which are coding regions
Introns which are non-coding regions
Before the RNA leaves the nucleus the introns are snipped out leaving just the exons

10. Eukaryotic mRNA Processing
Once RNA is transcribed it will be modified before leaving the nucleus for the cytoplasm
Protection from cytoplasmic RNA enzymes
Poly A tail
adenines are added to the “tail” end (3’ end)
5’ “cap”
A modified guanine is added to the “front” end (5’ end)

11. Translation Translation is the second step in gene expression
Translation occurs in the cytoplasm
The base sequence in the mRNA is decoded into a sequence of amino acids in a protein

12. Translation The Genetic Code
The sequence of nucleotides/bases in the mRNA “spell” out the instructions for what protein the cell should make
Different proteins determine the traits of cells and organisms
Each set of three bases is one “word” or codon
A codon indicates which amino acid to add to the protein
Amino acids are protein monomers/subunits

13. Translation The Genetic Code There are 64 different codons
61 codons code for 20 amino acids
Thus most amino acids are encoded by more than one codon
1 codon is used as a start signal
AUG (codes for methionine)
3 codons are used as a stop signal
UAA, UGA, UAG

14. Translation The Genetic Code 5’ ATG TCG GAC CTA 3’
3’ TAC AGC CTG GAT 5’
5’ AUG UCG GAC CUA 3’ (mRNA)
met
(start)
ser
asp
leu

15. Translation tRNA Each tRNA has two attachment sites
One is an anticodon
A triplet of bases complimentary to mRNA codons
The other attaches to the amino acid specified by the codon

16. Translation Initiation
Translation is initiated when the initiator tRNA binds the first AUG (start codon) of the mRNA and a small ribosomal subunit, then a large ribosomal subunit joins them
Initiator tRNA’s anticodon base pairs with AUG
Amino acid is methionine (met)
Initiator tRNA + mRNA + small and large ribosomal subunits = initiation complex

17. Translation Elongation
Initiator tRNA+met lines up in the P site of the ribosome
Anti-codon matches AUG
Carries methionine
Second tRNA+aa2 is placed in the A site of the ribosome
The second tRNA’s anticodon matches the next three bases (codon) on the mRNA
Carries the appropriate amino acid
called aa2 to indicate it is the second amino acid
Peptide bond forms between met and aa2
Initiator tRNA is released from the P site and from its amino acid, met

18. Translation Elongation
The second tRNA+aa2-met complex moves to the P site of the ribosome
Third tRNA+aa3 moves into the now vacant A site of the ribosome
Peptide bond forms between aa2 and aa3
Forms a polypeptide consisting of three amino acids
Met-aa2-aa3
This process continues adding an amino acid for each codon
The polypeptide elongates

19. Translation Termination
The ribosome encounters a stop codon in the mRNA
There are no matching tRNAs for stop codons
A release factor binds
All the parts of the complex dissociate
mRNA (can be used again)
Small and large ribosomal subunits (can be used again)
New polypeptide chain
The new polypeptide will either stay in the cytoplasm or enter the rough endoplasmic reticulum of the endomembrane system

20. Mutations Mutations are changes in the nucleotide sequence of DNA
These can affect the sequence of amino acids in the encoded polypeptide thus affecting the function of the protein

21. Mutations Common gene mutations Base-pair substitution
A single base pair changes
Affects one codon and thus one amino acid

22. Mutations Common gene mutations Insertions
One or more base pairs are added to the original DNA sequence
Results in a reading frame shift

23. Mutations Common gene mutations Deletions
One or more base pairs are removed from the original DNA sequence
Results in a reading frame shift

24. Mutations Common gene mutations
Frameshift (due to insertion or deletion)
Shifts the 3-bases-at-a-time reading frame
Changes the amino acid sequence from the point of mutation on.
The cat ate the rat
Tec ata tet her at (deletion)
Thr eca tat eth era t (insertion)

25. Mutations Mutation example: Sickle-cell anemia
In the DNA sequence coding for the beta chain in hemoglobin there is a single base-pair substitution
Hemoglobin carries oxygen through the blood
When that DNA is transcribed to mRNA the mistake is copied
When the mRNA is translated into a polypeptide one different amino acid is used
Normal sequence: val-his-leu-thr-pro-glu-glu-
Mutated sequence: val-his-leu-thr-pro-val-glu-
This one change disrupts the normal 3D folding of the protein
Mis-shaped hemoglobin causes red blood cells to distort into a sickle shape under low oxygen conditions
Causes clotting and disrupts blood circulation

26. Summary DNA expression Transcription Translation Gene Mutations
Genes “code” for proteins and thus traits
Transcription
Translation
Genetic code
Gene Mutations