1. M.B.Ch.B, MSC, DCH (UK), MRCPCH
Molecular Biology
Lec.3
Dr. Mohammed Hussein
M.B.Ch.B, MSC, DCH (UK), MRCPCH

2. Translation

3. The Second Stage in gene expression is translating the nucleotide sequence of a mRNA molecule into the amino acid sequence of a protein.

4. Genetic Code
The genetic code is defined as the relationship between the sequence of nucleotides in DNA (or its RNA transcripts) and the sequence of amino acids in a protein.

5. Codon
Each amino acid is specified by one or more nucleotide triplets (codons) in the DNA.

6. Number of codons There are 4 nitrogenous bases in DNA & RNA
Adenine, Guanine, Thymine (Uracil), Cytosine
Each codon = 3 bases, so we have:
43 (4x4x4) = 64 codons

7. 20 Amino Acids

8. AGC

10. AGC

11. Stop Codons 61 codons code for amino acids.
The other 3 (UAA, UAG,UGA) are stop codons (or nonsense codons) that terminate translation.

13. Start (Initiation) Codon
There is one start codon (initiation codon)
AUG, coding for methionine.
Protein synthesis begins with methionine (Met) in eukaryotes

16. Important features of the genetic code include
The code is unambiguous. Each codon specifies no more than one amino acid.
The code is degenerate. More than one codon can specify a single amino acid. All amino acids, except Met and tryptophan (Trp), have more than one codon.
The code is universal (the same in all organisms). Some minor exceptions to this occur in mitochondria.
The code is commaless (contiguous). There are no spacers or “commas” between codons on an mRNA.
Neighboring codons on a message are nonoverlapping.

17. Mutation
A mutation is any permanent, heritable change in the DNA base sequence of an organism.
This altered DNA sequence can be reflected by changes in the base sequence of mRNA, and, sometimes, by changes in the amino acid sequence of a protein.

18. Types of Mutations Silent Missense Nonsense Frameshift mutation
Large segment deletion
Splice site mutation
Trinucleotide repeat expansion

24. Large Segment Deletions
Large segments of DNA can be deleted from a chromosome during an unequal crossover in meiosis
β-thalassemia
Cri- du-chat

25. Mutations in Splice Sites
Mutations in splice sites affect the accuracy of intron removal from pre-mRNA during posttranscriptional processing.
If a splice site is lost through mutation, spliceosomes may:
Delete nucleotides from the adjacent exon.
Leave nucleotides of the intron in the processed mRNA.
Use the next normal upstream or downstream splice site, deleting an exon from the processed mRNA.

27. Mutations in Splice Sites
Mutations in splice sites have now been documented in many different diseases, including
ß-thalassemia
Gaucher disease
Tay-Sachs.

28. Trinucleotide Repeat Expansion

29. Translation (Protein Synthesis)
The process of protein synthesis occurs in 3 stages:
Initiation
Elongation
Termination

31. Elongation

32. Termination