RNA synthesis

Question 1

What is transcription? Does the language remain the same?

Answer 1

• Transcription is the synthesis of mRNA transcript from DNA.
• The language stays the same - nucleic acids.

Question 2

What is translation? Does the language stay the same?

Answer 2

• Translation is the protein production from mRNA transcript.
• Th language changes - from nucleic acids to protein.

Question 3

What are the differences between RNA and DNA?

Answer 3

1. ​RNA has ribose not deoxyribose.
2. RNA has uracil instead of thymine.
3. RNA is single stranded.
4. RNA is shorter than DNA.

Question 4

How many nucleotides are there in the human genome?

Answer 4

The human genome has 3.2 x 10⁹ nucleotides.

Question 5

What is an autosome? How many autosomes do we have in a genome?

Answer 5

• An autosome is any chromosome that isnt a sex chromosome.
• We have 22 autosomes in a genome numbered 1 - 22 (where 1 is the largest chromosome and 22 is the smallest chromosome).

Question 6

How many homologs do chromosomes have and what are they called?

Answer 6

Chromsomes have 2 homologs and they are known as maternal and paternal homologs.

Question 7

What is the function of chromosomes?

Answer 7

They carry the genes and assist in the process of cell division by distributing DNA.

Question 8

What is the main function of centromeres?

Answer 8

The key function of centromere is to keep the chromosomes attached to the mitotic spindle during mitosis.

Question 9

Where are telomeres located and what is their structure and main function?

Answer 9

• Telomeres are found at the end of the chromosome.
• They consist of a 6 base pair repeat.
• They protect the genes at the end of the chromosome from deteriorating.
• It also protects the fusion of strands of neighbouring chromosomes.

Question 10

What are homologous chromosomes?

Answer 10

• Homologous chromosomes are chromosome pairs (one from each parent) that are similar in length, gene position, and centromere location.
• The position of the genes on each homologous chromosome is the same, however, the genes may contain different alleles.

Question 11

What is the intergenic region in the genes? What does it consist of?

Answer 11

• The intergenic region is a stretch of DNA sequences located between genes.
• They consist of non-coding DNA ( pseudogene, and repetitive DNA).

Question 12

What is a gene?

Answer 12

• A gene is a sequence of DNA or RNA the codes for a molecule that has a function.
• It is the unit of heredity - it contains instructions for an organisms phenotype.
• Genes can differ in size
• They can differ in the number of introns and exons.
• Genes can cluster into different families - because of evolution different genes are developed for different functions. They will have similar regulations.

Question 13

Where is the promoter found and what is its function?

Answer 13

• The promoter is found ~20 base pairs upstream of transcription start site.
• Its function is to make sure that RNA polymerase II can bind in order to start transcribing the mRNA molecule.

Question 14

What are the introns?

Answer 14

• They are the non-coding parts of the gene.
• They will be removed.

Question 15

What are the exons?

Answer 15

• They are the parts of the DNA that will be expressed as proteins.

Question 16

Explain the process of transcription.

Answer 16

1. RNA polymerase unwinds a part of the DNA in order to transcribe it into RNA. RNA polymerase is an enzyme that catalyses the addition of the bases and the formation of phosphodiester bonds.
2. Watson-crick base pairing:- A->T (on RNA A->U), C-G
3. RNA molecule is being made from the 5’ to 3’ direction (look at the RNA strand being made). It is the template DNA strand that is being copied in the 3’ to 5’ direction (look at the lower DNA strand).

Question 17

What are the three different types of RNA polymerase in humans? What are the genes they transcribe?

Answer 17

Question 18

During RNA synthesis how many bases are transcribed per minute?

Answer 18

Around 1.25 - 1.75kb per minute so it’s relatively quick.

Question 19

What are transcription factors?

Answer 19

Transcription factors are proteins that are required to initiate or regulate transcription in eukaryotes.

Question 20

What is the TATA box?

Answer 20

• It is part of the promoter region.
• It helps initiate the process of transcription.
• It is found in eukaryotes.
• It is ~30 base pairs long.
• RNA polymerase binds to it.

Question 21

Describe transcription (initiation).

Answer 21

1. TFIID (transcription factor II D) recognises the TATA-binding protein (TBP) in the promoter region. (TFIID is a subunit of TBP)
2. TFIID binds to the TATA box and modifies the DNA by creating a slight bend in the DNA.
3. TFIIA and TFIIB helps recruite RNA polymerase II in the promoter region.
4. RNA polymerase needs TFIIF in order for to bind to the promoter.
5. Once the RNA polymerase binds to the promoter it forms the transcription initiation complex.
6. TFIIE and TFIIH associate with the RNA polymerase.
7. TFIIH can act as a kinase or helicase. In order to separate the two DNA strands TFIIH acts as DNA helicase and separates the two strands (by breaking hydrogen bonds).
8. TFIIH acts as a kinase and phosphorylates (adds a phosphate group) the C terminal of RNA polymerase II. It uses ATP breaking it down to ADP and phosphate.
9. The phosphorytated RNA polymerase II is released from the complex and begins transcription.

Question 22

Describe transcription (elongation).

Answer 22

1. The RNA strand gets longer by the addition of nucleotides at the growing 3’ end of the new strand.

Question 23

Explain transcription (termination) And the two steps that happen at the same time.

Answer 23

1. When a sequence of DNA, known as a terminator and located after the stop codon is reached transcription ends.
2. Two things happen:-
   
   1. Polyadenylate A tail at the 3’end
   2. 5’ cap with the help of guanine.
3. Without polyadenylation and capping the presence of mRNA and lifespan of mRNA is not that long inside the cell.

Polyadenylation

• mRNA is trimmed by a specific endonuclease at the 3’ end.
• The transcript is finished off by poly(A) polymerase enzyme that adds repeated adenine bases at the cut end.
• A poly A tail is formed making the transcript more stable and protects pre-mRNA from RNAses which digest RNA in the cytoplasm as it moves out the nucleus.
• The longer the poly A tail the more stable the RNA is.

Capping

• The 5’ end becomes modified after 25 nucleotides of RNA have been synthesised.
• A guanine is added to the 5’ end and an enzyme called methyl transferase, methylates guanine to give rise to 7 methylguanine cap.
• This cap protects the degradation of mRNA from exonucleases.

Question 24

What are the untranslated regions called?

Answer 24

Introns.

Question 25

What is splicing? Explain the steps.

Answer 25

Splicing is the removal of introns and joining together the exons.

1. Splicosome recognises the sequence after the exon 1 (5’ site) and cleaves it there.
2. The inton loops around and the GU sequence will then join at A (branch point). This forms a structure called the lariat structure.
3. Another cleavage forms before exon 2 (3’ site).
4. Then ligation happens and exon 1 and 2 are joined together.

Question 26

What is alternative splicing?

Answer 26

• During splicing there can be variation in ligation of exons.
• So the sequence of mRNA also changes.
• The proteins made my have similar structures as there are common exons, but different functions as the sequence of exons differs.

Question 27

What are the different types of alternative splicing?

Answer 27