Lecture 18: Transcription

Question 1

The central dogma that explains the preservation and transmission of genetic information
Three processes are involved in the preservation and transmission of genetic information. What are they?

Answer 1

DNA (transcription) -> RNA (translation) -> protein
1. Transcription: nuclear DNA directs the synthesis of specific mRNA molecules
2. mRNA export: mRNA exists through nuclear pores and bind to cytoplasmic ribosomes
3. Translation: a ribosome synthesises the specific protein encoded by the mRNA
Three processes are involved in the preservation and transmission of genetic information:
1. Replication: copying of DNA to form 2 identical daughter molecules, which must take place prior to cell division
2. Transcription: the genetic message is transcribed into mRNA which carries the code to the ribosomes
-also produces ribosomal RNA (rRNA) and transfer RNA (tRNA)
3. Translation: the genetic message in mRNA is decoded on the ribosomes, directing the specific amino acid sequence during protein synthesis
Refer to page for for visual image of it

Question 2

What are some supporting factors to show that RNA is an intermediate

Answer 2

1. Proteins can be synthesised in the absence of DNA but require RNA
   - in vitro protein synthesis
2. RNA is synthesised in the nucleus and subsequently migrates to the cytoplasm
3. Rate of protein synthesis is proportional to the cellular content of RNA but not DNA

Question 3

Chemistry and structure of RNA

How does it differ from DNA

Answer 3

Like DNA, RNA is a polynucleotide. That is it is a polymer made up of nucleotides
It differs from DNA In a number of respects:
1. It contains ribose rather than deoxyribose
2. It contains the pyrimidine URACIL in place of thymine.
3. RNA is a single stranded polyribonucleotide.
Therefore A doesn’t equal U, and G isn’t equal to C

Question 4

The sugar of RNA

Answer 4

Nucleic acids consist of 5-carbon sugars

• general name is pentose sugar
• the pentose sugar in RNA is RIBOSE
• unlike deoxyribose, the oxygen at the 2-carbon position is present

RNA goes from 5’ to 3’

Question 5

What bases are purines, what bases are pyrimidines?

Answer 5

Purine: adenine and guanine
Pyrimidines: thymine, cytosine and uracil

Question 6

The synthesis of RNA

Answer 6

The synthesis of RNA is directed by DNA in a process called transcription
The fact that RNA is a polynucleotide consisting if 4 different bases immediately suggests that the information with the structure if DNA is transferred to RNA by process if complementary base pairing
Transcription:
DNA template strand first then RNA strand
3’ AGCCCT 5’
5’ UCGGGA 3’

Question 7

RNA polymerase. What is it responsible for?

What reaction does it catalyse, and what does it require?

Answer 7

RNA polymerase is the enzyme responsible for forming the phosphodiester bonds that bind the nucleotides together to each other
DNA template
ATP —-> (RNA polymerase) RNA + nPPi
1. The reaction is completely dependant on the presence if DNA- if not there, the enzyme is deactivated
2. The action requires all 4 nucleoside triphosphates
3. The base sequence if the RNA reflects that of the added DNA

Question 8

Properties of RNA polymerase

Answer 8

-It binds to DNA
-It synthesises RNA
-Upon binding, it causes localised unwinding of the DNA double helix
-one of the 2 DNA strands act as a template (template strand)
-it works exclusively in the 5’–> 3’ direction (with respect to newly formed RNA strand)
-since base pairing requires the two strand to be anti parallel it is reading the template in a 3’ –> 5’ direction
Refer to page 12

Question 9

Template DNA

Answer 9

• only one of the two DNA strands act as a template for replication
• the other is the non-template strand
• a gene corresponds only to a defined portion of DNA molecule (not entire molecule)
• the DNA strand that acts as the template for gene 1 may be the non-template strand for gene 2
• promoter regions are critically important in transcription
• promoters are the region of a DNA strand where RNA polymerase binds
• promoters determines whether a DNA strand will act as the template

Question 10

What are the 3 different types of RNA and what is their function

Answer 10

1. Ribosomal RNA (rRNA) -forms part of the structure of ribosomes
2. Messenger RNA (mRNA)-carries genetic info from nucleus to the ribosomes and codes for a particular protein
3. Transfer RNA (tRNA)- involved in the process of translation. It brings the amino acids to the ribosomes to allow proteins synthesis