RNA Synthesis And The Genetic Code

Question 1

What is the purpose of transcription?

Answer 1

Retrieval of information for synthesis of the cells proteins in the form of messenger RNA (mRNA)

Question 2

What are the classes of RNA?

Answer 2

Transfer RNA (tRNA)-carrier for Amina acids
Ribosomal RNA (rRNA)-major component of the protein synthesizing machinery
Messenger RNA (mRNA)- carries information encoded in the DNA to the cytoplasm for translation into proteins

Question 3

What is the sequence in transcription?

Answer 3

Transcription has a start site on the DNA
There are a few consensus sequences:
-10 region-Tata box
-35 region
Consensus sequences at -10 and -35 form the promoter
RNA polymerase binds to region near the promoter
Helicase unwinds the DNA

Question 4

What are the characteristics of termination?

Answer 4

Rho independent terminators-hairpin loops

Rho dependent-Rho binds to 5 prime end of messages, ribosomes stall and termination occurs

Question 5

What is the location and product of RNA polymerase?

Answer 5

RNA polymerase I in nucleolus -product ribosomes; RNA
RNA polymerase II in nucleoplasm- hnRNA (precursor of mRNA)
RNA polymerase III in nucleoplasm- small RNA

Question 6

What is the structure of the promoter?

Answer 6

TATA box-directs RNA polymerase to correct start site
CAAT box-controls frequency of transcription initiation
Enhancer/silencer elements-can be located either upstream or downstream

Question 7

What is the difference in MRNA between prokaryotes and eukaryotes?

Answer 7

In prokaryotes:
MRNA is polycistronic message
No introns
Message short lived and immediately translated
In eukaryotes:
mRNA is monocistronic
Possess introns and exons
mRNA must be processed 
Long lived

Question 8

What are the modifications of mRNAs?

Answer 8

Caps-consisting of methylated guanine and methylated ribose at the 5 prime end of the molecule.
Function in protecting 5 prime from attack by Rnase, permit binding of mRNA to the 40S ribosomal subunit
PolyA tails-Prevents early degradation of message.
messenger RNBAs which code for histones don’t contain polyA

Question 9

What are introns and exons?

Answer 9

Exons: coding sequences
Introns: non coding sequences removed when mRNA processed

Question 10

What is splicing?

Answer 10

Removal of introns
GU rich found in 5 prime end where intron meet 5 prime exon
AG site where 3 prime end of intron meets 3 prime of exon
Adenosine is part of the branch site located 40 nucleotides upstream from 3 prime splice site
Intron removed by 2 transesterification reactions: between 5 prime splice site and branch site, between 5 prime exon and 3 prime splice site.

Question 11

What is a spliceosome?

Answer 11

Large protein plus RNA complex which catalyzed slicing out of the intron

Question 12

What is a snurp?

Answer 12

Complex associated with a particular small RNA molecule
Protein portion has at least 7 proteins
Hydrogen bonding between small RNas and sequences near intron splice sites are required for proper splicing

Question 13

What is the genetic code?

Answer 13

Sequence of 3 nucleotide coding for amino acids
64 possible codes: 61 for amino acids, 3 termination scodons (UAA, UGA, UAG)
Initiation codon and methionine similar=AUG

Question 14

What are the characteristics of the genetic code/

Answer 14

Degenerate-more than one code for most AA
Degeneracy is not uniform
Wobble-3rd position AA less specific than first 2
Code almost universal except mitochondria and chloroplasts

Question 15

What are the mutations of the genetic code?

Answer 15

Transitions-purine is replaced by another purine (A->G, T–>C)
Transversions-Purine is replaced by pyrimidine or vice versa (T–>A or G, or G–>T or C)
Transitions and transversions also known as point mutations

Question 16

What are the consequences of point mutations?

Answer 16

Silent mutations-Change in codon but no change in AA no effects is seen
Missense mutations-inactive or partially active protein (sickle cell anemia)
Nonsense mutation- Results in synthesis of nonfunctional protein, introduction of an early stop codon

Question 17

What are the types of frame shift mutations?

Answer 17

Insertions-new base pair inserted into the DNA

Deletions-Base pair removed from the DNA