RNA Synthesis

Question 1

What is transcription?

Answer 1

Synthesis of mRNA transcript from DNA

Question 2

What is Translation?

Answer 2

Production of proteins from the mRNA transcript

Question 3

How many nucleotides does the human genome contain?

Answer 3

3.2 x 10^9

Question 4

What are the 2 types of chromosome?

Answer 4

Autosomes and Sex Chromosomes

Question 5

How many pairs of autosomes are present within the human genome?

Answer 5

22 pairs

Question 6

How many pairs of sex chromosomes are present within the human genome?

Answer 6

1 pair

Question 7

What is a centromere and what is its function?

Answer 7

Structure made up of lots of repeats of DNA that holds chromatids of a chromosome together
During mitosis it keeps chromosomes attached to mitotic spindle during mitosis

Question 8

What is a telomere and what is its function?

Answer 8

Long repetitive piece of DNA (6 base pair repeat) at each end of the chromosome.
Its function is to Protect genes at end of chromosome from being lost and prevents cell from “sticking together” the ends of chromosomes with each other.

Question 9

What is the intergenic region of a chromosome?

Answer 9

region between genes on a chromosome

Question 10

How much of the human genome do intergenic regions make up?

Answer 10

98%

Question 11

What other functions can the regions that don’t code for proteins have?

Answer 11

Pseudo genes (duplicates of genes that are non-functional) and repetitive DNA

Question 12

What are the 2 different definitions of a gene?

Answer 12

1. Unit of heredity; contains instructions for an organism’s phenotype
2. DNA segment containing instructions for making a particular product

Question 13

What is the transcription start site?

Answer 13

Location on a chromosome where the transcription of a particular gene occurs. Always at the 5’ end of a gene.

Question 14

What is an exon?

Answer 14

Part of gene that get expressed as proteins

Question 15

What is an intron?

Answer 15

Part of the gene that gets removed before mRNA is released from the cell

Question 16

What is the promoter region?

Answer 16

Region 20 or 30 base pairs Upstream of transcription start site. Makes sure that RNA polymerase II binds to the gene to start transcription

Question 17

Key properties of RNA polymerase

Answer 17

Synthesises RNA molecule in the 5’ to 3’ direction
Uridine-triphosphate (U) Replaces (T) in RNA compared with DNA
Unwinds small portion of DNA
Catalyses production of phosphodiester bonds

Question 18

What are the 3 types of RNA polymerase present within human cells and what do they each transcribe?

Answer 18

RNA Polymerase I - Transcribes most ribosomal RNA
RNA polymerase II - Transcribes mRNA, miRNA and non-coding RNA
RNA polymerase III - Transcribes tRNA and other small RNA’s

Question 19

Why does more than one RNA polymerase work on the same gene?

Answer 19

Allows multiple mRNA transcripts to be produced at the same time meaning more than one protein will eventually be produced. Speeds up the process of translation

Question 20

What are the 5 things required for transcription?

Answer 20

1. RNA polymerase
2. rNTPs - the building blocks of RNA
3. Promoter region - acts as a ‘bookmark’ so RNA polymerase knows where to transcribe
4. Transcription factors
5. Enhancers and silencers - elements which enhance and suppress the transcription of a gene.

Question 21

What are transcription factors and what function do they serve?

Answer 21

They are proteins required to initiate or regulate transcription in eukaryotes
They assemble on the promoter to position RNA polymerase II and pull apart DNA helix and expose template strand

Question 22

Describe the events that occur during the initiation stage of transcription (production of transcription initiation complex).

Answer 22

A) General transcription factor TFIID recognises and then binds to promoter region

TFIID is made of multiple sub-units and one of the subunits is called TBP (TATA binding protein) TBP is responsible for allowing TFIID to bind to the TATA box. This then forms a complex which allows other proteins to bind

B) TFIIA and TFIIB then bind (TFIIA stabilises the complex while TFIIB is responsible for linking RNA polymerase to the TFIID and TBD)

C) Other general transcription factors, e.g. TFIIE and TFIIH bind. This causes conformational change which allows RNA polymerase II to recognise the complex and bind to it. This causes the formation of the Transcription initiation complex

D) TFIIH pulls apart the DNA helix around the initiation site to form a cistron. TFIIH also phosphorylates RNA polymerase II which requires ATP.

E) Phosphorylation of RNA polymerase II allows it to be released from the transcription initiation complex and gives it the energy needed to translocate along DNA and begin transcription

Question 23

What are untranslated regions?

Answer 23

regions of a gene that are transcribed into mRNA but not translated into proteins

Question 24

What are the 2 types of untranslated region and what do they do?

Answer 24

5’ UTR - involved in regulation of transcription

3’ UTR - involved in mRNA stability and miRNA binding

Question 25

Describe the events that occur during the elongation stage of Transcription

Answer 25

RNA polymerase translocates along the anti-sense strand of the DNA in the 5' to 3' direction. As it does this it catalyses the reaction that joins the next nucleotide of the newly synthesised mRNA strand to the 3'OH end of the previous nucleotide. (Formation of phosphodiester bond between them). Complementary mRNA strand produced is exact copy of sense strand (coding strand) except that uracil replaces thymine. During elongation capping also occurs

Question 26

Describe the process of capping?

Answer 26

Capping occurs during the elongation phase of Transcription. After 25 nucleotides of RNA are synthesized the 5’ end becomes modified. A guanine nucleotide is added to the 5’ end of the mRNA strand and an enzyme called methyl transferase methylates the guanine at position 7 to give rise to 7 methylguanine cap. The polymerase II continues along the DNA making pre-mRNA until it reaches the termination sequence.

Question 27

What purpose does the 5' cap serve?

Answer 27

Protects the 5’ end from the activity of 5’ exonucleases which digest the RNA. Also acts as a checkpoint for the ribosome as if it doesn’t recognise a 5’ cap then the mRNA will not be exported out of the nucleus

Question 28

Describe the process of polyadenylation

Answer 28

After the stop codon there is a sequence called AAUAAA which is the Polyadenylation signal or termination signal. A specific endonuclease recognizes the termination signal. The CPSF (cleavage and polyadenylation specificity factor) is the enzyme that cleaves the mRNA strand after the polyadenylation signal. Polyadenylate polymerase, Poly(A) polymerase, then adds 250 adenosine monophosphates to the 3’ end of pre-mRNA to form the poly (A) tail. This process is called Polyadenylation.

Question 29

What is the purpose of polyadenylation?

Answer 29

The poly a tail protects the pre-mRNA from degradation. It protects it from RNAses which digest the RNA in the cytoplasm as it moves out of the nucleus. (The longer the poly A tail the more stable the RNA is). It also promotes the nuclear transport of the mRNA

Question 30

What is splicing?

Answer 30

Splicing is the removal of introns and joining of exons together within the pre-mRNA to produce mature mRNA.

Question 31

Describe the process of splicing

Answer 31

snRNPs bind to the pre-mRNA at the GU site and the AG site which marks where the intron is within the pre-mRNA Spliceosome then cleaves the GU site (5' splice site) of intron GU sequence then loops around and joins with the branch point (A) to form lariat – like intermediate The bonding of the guanine and adenine takes place via transesterification in which a hydroxyl group (OH) on the carbon atom of the adenine ‘attacks’ the bond of the guanine nucleotide at the splice site Spliceosome then cleaves at AG site (3' splice site) of the intron Ligase then joins the 2 exons together

Question 32

What is the idea of alternative splicing?

Answer 32

Idea that not all of the exons within a pre-mRNA molecule are needed to produce mature mRNA so many different mRNA molecules can be produced from one gene.

Question 33

What are the 3 proteins that need to bind to mRNA in order to guide it to the nuclear membrane for export out of the nucleus?

Answer 33

Poly-A tail binding protein Exon junction complex Cap binding protein

Question 34

What are exon junctions?

Answer 34

Exon junctions are the gaps between exons which are filled with introns if they are still present within the mRNA