5. RNA synthesis

Question 1

Describe the human genome (3 pts)

Answer 1

1. Humans have 3.2 x 10’9 nucleotides in human cells.
2. The human genome is made up of 22 pairs of autosomal homologous chromosomes which are arranged in order of size- 1 is the biggest whilst 22 is the smallest.
3. Humans have 1 pair of sex chromosomes which determine the sex of the individual-

xx= females 
xy= males

Question 2

Describe the structure of a chromosome (3pts)

Answer 2

1. Chromosome= Is a linear double stranded molecule of DNA. It carries genes.
2. Telomere= Is the end of a chromosome. It protects the end of the chromosome.
3. Centromere= Primary constriction. It is made up of lots of repeats of DNA known as megabases and it keeps the chromosomes attached to the mitotic spindle during mitosis.

Question 3

Describe genes (4pts)

Answer 3

1. Gene= A gene is a DNA segment containing instructions for making a particular protein including regulatory elements.
2. Gene is a unit of hereditary which contains instructions for an organisms phenotype.
3. The chromosome consists of an intregenic region which consists of 98% of the genome.
4. Genes:
5. Differ in size- e.g the histone genes are the smallest
6. Differ in numbers of exons and introns
7. Cluster into families

Question 4

Describe the structure of a gene ( 5 pts)

Answer 4

1. Promoter= Determines whether a gene is switched on and off. It is important in the regulation of gene expression.
2. 5 UTR= Untranslated region. It is part of the first exon
3. Exon= expressed sequences which code for the proteins
4. Introns= spliced.
5. 3 UTR= untranslated region. Part of the last exon.

Question 5

What is Transcription (1pt)

Answer 5

Synthesis of MRNA from DNA

Question 6

What is translation (2pts)

Answer 6

1. Produced proteins from MRNA transcript

2. Nucleic acid to protein

Question 7

Describe Human RNA polymerase (3pts)

Answer 7

1. RNA polymerase I= ribosomal RNA
2. RNA polymerase II= Protein-coding
3. RNA polymerase III= Transfer RNA

Question 8

Describe transcription factors (3 pts)

Answer 8

1. Transcription factors= Proteins required to initiate or regulate transcription in eukaryotes.
2. They assemble on the promoter to position RNA pol II.
3. Pull apart DNA helix and expose template strand.

Question 9

Describe the steps of Transcription (5 pts)

Answer 9

1. TATA box is recognised by TATA-binding protein subunit of TFID.
2. TFIIA and TFIIB bind. The TFIIA stabilises the complex.
3. Other general transcription factors bind, then RNA polymerase II assembles at the promoter forming the transcription initiation complex.
4. TFIIH pulls apart the DNA helix and phosphorylates RNA poll II.
5. Phosphorylated RNA poll II is released from the complex and begins transcription.

Question 10

Describe UTRS (3 pts)

Answer 10

UTR’s are transcribed but not translated

1. 5 UTR= Regulation of translation
2. 3 UTR= MRNA stability and MiRNA binding.

Question 11

Describe RNA processing (3pts)

Answer 11

The primary transcript is processed in 3 steps: (CPS)

1. Capping
2. Polyadenylation
3. Splicing

Question 12

Describe capping (2pts)

Answer 12

1. Capping occurs at the 5 prime end of the molecule.
2. The cap acts as a marker of RNA polymerase to transcribed RNA. It stimulates splicing and is used for the recognition of MRNA by the protein translation machinery in the cytoplasm of the cell.

Question 13

Describe Polyadenylation (4 pts)

Answer 13

Polyadenylation= Is the addition of a poly A tail to the end of the primary transcript.

1. The DNA strand has been transcribed into the RNA molecule
2. The 5 prime cap has been added at the beginning of transcription
3. The cleavage signal is within the RNA molecule transcript and is recognised and cleaned by specific endonucleases.
4. Then the addition of a poly A tail by the enzyme polymerase occurs.

Question 14

Why is capping and Polyadenylation used (3pts)

Answer 14

1. For stability- nucleus contains many nucleases enzymes to degrade RNA. The poly A tail prevents this degradation.
2. important for transport to cytoplasm- RNA must be fully formed before it leaves the nucleus which is ensured by processing steps.
3. Integrity prior to translation- cell does not want to waste energy and resources in translating a mrna molecule that is incomplete or incorrect. checks if the processing steps have been done correctly before translation can take place.

Question 15

Describe Splicing (3pts)

Answer 15

Splicing= Removal of introns and joining of exons

1. The splicesome first cleaves at the 5 prime splice site (sequence GU)
2. It forms a lariat intermediate with the UG binding to the branch point.
3. Cleavage then occurs again at the 3 prime site and the two exons either side of the intron are then ligated.

Question 16

Describe introns (3pts)

Answer 16

1. introns= non-coding sequences within genes
2. introns vary in size
3. Introns vary in the number of genes

Question 17

Describe alternative splicing (2pts)

Answer 17

1. Alternative splicing= Selecting different exons to make up the final mature mrna molecule
2. In alternative splicing the exons can be joined up in different arrangements and various combinations of exons produce different proteins. This means that one gene can code for multiple proteins.

Question 18

How do proteins indicate they have been properly processed (1pt)

Answer 18

The proteins binding successfully indicates it has been properly processed allowing the mrna to be exported through the nuclear pore to the cytoplasm to be translated.

Question 19

Describe a CBC (1pt)

Answer 19

Cat binding complex= recognises the cap

Question 20

Describe a TREX (1PT)

Answer 20

Transcription-coupled export complex= Binds to make a large complex

Question 21

Describe a EJC (1pt)

Answer 21

Exon junction complex= Recognises successful splicing events