1. Lecture 4 By Ms. Shumaila Azam
Genomics
Lecture 4
By Ms. Shumaila Azam

2. Central Dogma
The expression of the genetic information stored in the DNA molecule occurs in two stages:
(i) transcription, during which DNA is transcribed into mRNA;
(ii) translation, during which mRNA is translated to produce a protein.
DNA mRNA  protein

3. The genetic code
• DNA: sequence of four different nucleotides. • Proteins: sequence of twenty different amino acids. • The correspondence between DNA's four-letter alphabet and a protein's twenty-letter alphabet is specified by the genetic code, which relates nucleotide triplets or codons to amino acids.

4. The genetic code Start codon: initiation of translation (AUG, Met).
Stop codons: termination
of translation.
Mapping between codons and amino acids is many-to-one: 64 codons but only 20 a. a..
Third base in codon is often redundant,
e.g., stop codons.

5. Transcription

6. Transcription
It is the process that involves transcribing genetic information from DNA to RNA.
Analogous to DNA replication: several steps and many enzymes.
The transcribed DNA message or RNA transcript is used to produce protein.
The information in the DNA is not directly converted into protein but must first be copied to RNA.
This ensures that the information contained within the DNA does not become ruined.

7. Transcription
Nucleotide base sequences are the genetic code or instructions for protein synthesis.
RNA polymerase
Promoter sequence
Transcription factors
Terminator

8. RNA Polymerase
RNA polymerase synthesizes an RNA strand complementary to one of the two DNA strands.
The RNA polymerase carries the responsibilities for the creation of all types of RNAs.
RNA polymerase forms a machine with active sites that surrounds the DNA, unwind it, and builds RNA strand.
The active sites are designed to be able to remove the nucleotides as well as add them to the growing strand.

9. RNA polymerase
The RNA polymerase recruits rNTPs ( ribo-nucleotide triphosphate) in the same way that DNA polymerase recruits dNTPs (deoxynucleotide triphospate).
• However, synthesis is single stranded and only proceeds in the 5' to 3' direction of mRNA (no Okazaki fragments).

10. Promoter
All the DNA sequences containing binding sites for RNA polymerase and the transcription factors necessary for normal transcription.
Promoters are located near the genes to be transcribed.
Upstream on the DNA (towards the 3’ end of the anti sense strand ) .
Unidirectional sequence upstream of the coding region (i.e., at 5' end on sense strand) that tells the RNA polymerase both where to start and on which strand to continue synthesis. E.g. TATA box.
It has bp

11. Transcription factors
Any protein other than RNA polymerase that is required to for transcription.
A protein needed to initiate the transcription of a gene, binds either to specific DNA sequences (e.g. promoters) or to other transcription factors.

12. Function of transcription factor
Bind to RNA polymerase
Bind another transcription factor
Bind to cis-acting DNA sequences

13. Terminator
Regulatory DNA region signaling end of transcription, at 3' end .

14. Transcription

15. Steps in Transcription
There are three steps in DNA Transcription.
Initiation
Elongation
Termination

16. Initiation RNA polymerase binding to DNA
DNA is transcribed by an enzyme called RNA polymerase
Specific nucleotide sequences tell RNA polymerase where to begin and where to end.
RNA polymerase attaches to the DNA at a specific area called the promoter region.
RNA polymerase recognizes the promoter sequence by hydrogen bonding.
It is the most important step in gene expression.

17. Elongation
Certain proteins called transcription factors unwind the DNA strand and allow RNA polymerase to transcribe only a single strand of DNA into a single stranded RNA polymer called messenger RNA (mRNA).
Anti-Sense Strand:
The strand that is serve as a template is called the anti-sense strand.
Sense Strand:
The strand that is not transcribed is called sense strand.
The RNA strand newly synthesized from and complementary to the template contains the same information as the coding strand.

18. Elongation

19. Elongation
When RNA polymerase transcribes the DNA.
G-C
A-U

20. Termination
RNA polymerase moves along the DNA until it reaches the terminator sequence.
At that point RNA polymerase releases the mRNA polymer and detaches from the DNA.

21. Transcription

22. Transcription
Since proteins are constructed in the cytoplasm of the cell, mRNA must cross the nuclear membrane to reach the cytoplasm.
Where process of translation have to continue.