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1
Q

Where are premRNAs produced?

What are they processed to?

A

premRNAs are produced in the nucleoplasm

They are processed to mature mRNAs

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2
Q

What happens to mRNA after it travels through the nuclear pores?

A

It enters the cytosol

Each mRNA molecule becomes covered in ribosomes to form a polysome

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3
Q

Where do proteins synthesised on a cytosolic polysome get deposited?

A

In the cytosol

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4
Q

What are the ER and Golgi apparatus involved in?

A

Packaging and secreting proteins that will be released from cells

Production of these proteins is slightly different from cytosolic proteins

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5
Q

What is the first stage in endocytosis?

A

The plasma membrane folds inwards on itself to form a cavity that fills with extracellular fluid and other substances

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6
Q

What is the second stage in endocytosis?

A

The plasma membrane then folds back on itself until the ends of the folded membrane meet

This traps the fluid in the vesicle

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7
Q

What is the third stage in endocytosis?

A

The vesicle is pinched off from the membrane as the ends of the membrane fuse together

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8
Q

What is contained within lysosomes?

A

Hydrolytic enzymes

These degrade molecules imported into the cell or old/damaged cellular components

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9
Q

How are the degradative enzymes delivered to the site of degradation?

A

via the ER and Golgi apparatus, which have signals for lysosomal targeting

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10
Q

Where are nuclear proteins synthesised?

What happens after synthesis?

A

In the cytosol

They are imported into the nucleus through nuclear pores after they are made

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11
Q

What is significant about nuclear proteins?

A

They have nuclear targeting sequences

This is an amino acid sequence that tags a protein for import into the nucleus by nuclear transport

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12
Q

Where are most mitochondrial proteins synthesised?

A

In the cytosol by polysomes

A few mitochondrial proteins are synthesised within the mitochondria

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13
Q

What is the definition of transcription?

A

The process in which information in a gene is used to construct a physical product; a protein

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14
Q

What is the overview of transcription?

A

It involves making an RNA copy of a sequence of DNA found in a gene

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15
Q

What is the first step in the initiation stage transcription?

A

RNA polymerase binds to the promoter region of a gene

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16
Q

What must bind to a promoter region before RNA polymerase can bind?

A

Transcription factors

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17
Q

what happens after RNA polymerase has bound to the promoter region?

A

It separates the DNA strands, leaving a single-stranded template needed for transcription

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18
Q

What happens after a single-stranded template has been provided?

A

RNA polymerase builds an RNA strand in the 5’ to 3’ direction

It adds each new nucleotide to the 3’ end of the strand

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19
Q

When does the elongation stage of transcription begin?

A

Once one strand of DNA becomes the template strand

It acts as a template for RNA polymerase

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20
Q

What happens during the elongation stage of transcription?

A

RNA polymerase reads the template strand one base at a time

It builds a new RNA molecule (premRNA) out of complementary nucleotides

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21
Q

What is significant about the premRNA produced during the elongation stage of transcription?

A

It carries the same information as the coding strand

It contains uracil instead of thymine

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22
Q

What happens during the termination stage of transcription?

A
  1. sequences called terminators signal that the mRNA transcript is complete
  2. once the terminators are transcribed, RNA is released from RNA polymerase
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23
Q

How do the pre-mRNAS have their ends modified?

A

Through the addition of a 5’ cap at the beginning and a 3’ poly-A tail at the end

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24
Q

Where is the 5’ cap added?

What are its purposes?

A

To the first nucleotide in the premRNA molecule

It prevents the RNA from being broken down

It helps the ribosome attach to the mRNA and begin reading it during translation

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25
What is the 5' cap?
a modified guanine nucleotide
26
What happens when a polyadenylation sequence appears in an RNA molecule during transcription?
1. an enzyme chops the RNA into two | 2. poly-A polymerase adds 100-200 adenine nucleotides to the cut end to form a poly-A tail
27
What is the purpose of the poly-A tail?
It makes the RNA more stable It promotes the RNA to be exported from the nucleus to the cytosol
28
What happens when pre-mRNAs are spliced? Why is it important?
The introns are removed and the exons are rejoined In order for mRNA to encode a protein, the introns must be removed
29
What is the spliceosome and what does it recognise?
It is a protein-RNA complex that recognises specific sites at the start and end of each intron
30
What is a shared feature of all the intron splice sites?
They contain GU dinucleotide at the 3' end of the upstream intron They contain AG dinucleotide at the 5' end of the downstream intron
31
What is meant by 'alternative splicing'?
The same pre-mRNA can be spliced in multiple different ways depending on which exons are kept
32
what is significant about 'alternative splicing'?
It means that more than one mRNA molecule can be made from a single gene One gene codes for multiple proteins
33
What is the initiation site?
The site on the DNA from which the first RNA nucleotide is transcribed
34
How do 'upstream' and 'downstream' relate to the initiation site?
Any nucleotides before the initiation site are upstream Any nucleotides before the initiation site are downstream
35
What is the enzyme involved in transcription?
RNA polymerase 2
36
What are the roles of RNA polymerases 1 and 3?
Polymerase 1 transcribes ribosomal RNA Polymerase 3 transcribes tRNA
37
What is a TATA box?
It is a sequence within a promoter region of a gene
38
What will the TATA box be recognised by? What does this allow for?
It is recognised by a general transcription factor This allows other transcription factors, and eventually RNA polymerase II to bind
39
What is significant about the TATA box containing lots of As and Ts?
This makes it easier to pull the 2 strands of DNA apart A and T only form 2 hydrogen bonds
40
How is the energy needed for transcription generated?
When ATP is reduced to ADP and Pi After this, the transcription factors dissociate and transcription begins
41
What are the enhancer elements that are further away from the TATA box usually involved in?
Tissue-specific regulated transcription
42
What is a CpG island?
CpG islands are stretches of DNA where there are multiple points where C is followed by G (p is phosphate)
43
Where are CpG islands usually found?
Upstream of many genes They appear to have promoter activity
44
How does the state of the C in the CpG island vary depending on its location?
Inside the CpG island, the C is protected from methylation Outside the island, the C is often methylated
45
what does methylation of a CpG island lead to?
It is a way of switching off expression of a gene This happens in inactivation of X chromosomes and some cancers
46
What are the enzymes that add methyl groups?
Methyltransferases
47
What does DNA methylation often involve?
Addition of a methyl group to the 5th carbon atom of a cytosine ring
48
What happens to the chromatin in the B-globin gene?
It folds up to enclose several genes The long DNA segment acts as a self-contained unit that isn't influenced by genes on either side of the chromosome
49
what is the locus control region (LCR)?
A sequence that can isolate a gene or group of genes from external influences
50
How many amino acids are there in the body?
20 Each is coded for by a 3-base codon
51
What is the initiation codon?
Methionine (AUG)
52
What are the 3 stop codons?
UAA UAG UGA
53
What type of "codon" is present on a transfer RNA molecule?
An anticodon This is a set of 3 nucleotides that are complementary to a codon on the mRNA
54
What is found on the opposite end of the tRNA molecule to the anticodon?
the amino acid that is specified by the codon
55
What happens when tRNA molecules reach the ribosome?
1. tRNAs enter slots in the ribosome and bind to complementary codons 2. as this happens, amino acids are linked to the growing peptide chain 3. this results in a polypeptide whose amino acid sequence mirrors the sequence of codons in the mRNA
56
What are the 3 key components involved in the initiation stage of translation?
1. ribosome 2. mRNA 3. initiator tRNA that carries the first amino acid in the protein THIS IS METHIONINE
57
Where will the initiator tRNA bind to? What will this form?
It attaches to the small ribosomal subunit This forms the tRNA-ribosome complex
58
What will the tRNA-ribosome complex bind to and why?
The 5' end of the mRNA It does this by recognising the 5' GTP cap
59
What happens after the tRNA-ribosome complex has bound to the mRNA?
the initiation complex walks along the mRNA in the 3' direction and stops when it reaches the start codon
60
What happens once the initiation complex reaches the start codon?
The large subunit of the ribosome binds and translation commences at the start codon
61
During the elongation stage of translation, where does the initiator tRNA start?
The tRNA carrying methionine starts in the P site of the ribosome
62
What happens when the initiator tRNA enters the P site of the ribosome?
A new codon is exposed in the A site The A site is next to the P site
63
What is significant about the A site in the ribosome?
The A site is where the next tRNA molecule will land The anticodon must be complementary to the exposed codon
64
What happens once the next tRNA molecule has entered the A site?
A peptide bond forms between amino acids This transfers the methionine from the first tRNA onto the amino acid of the second tRNA in the A site
65
What is formed by the methionine and the second amino acid added to the polypeptide?
The methionine forms the N-terminal as it has an exposed amino group The second amino acid forms the C-terminal as it has an exposed carboxyl group
66
What happens in the ribosome after the first peptide bond has formed?
The mRNA is pulled through the ribosome by exactly one codon The empty tRNA can drift out via the E (exit) site and a new codon is exposed in the A site
67
When does the termination stage of translation occur?
When a stop codon in the mRNA enters the A site of the ribosome
68
what are stop codons in the mRNA recognised by?
Release factors, which fit neatly into the P site
69
What is the role of release factors?
They affect the enzyme involved in forming peptide bonds They cause it to add a water molecule to the last amino acid in the chain
70
what happens to the newly synthesised peptide after the release factors have entered the P site?
The peptide chain is separated from the tRNA and the protein is released The small and large ribosomal subunit separate from each other and the mRNA
71
What happens to a newly synthesised polypeptide after translation?
It folds into a distinct 3D structure Or it may join with other polypeptides to form a multi-part protein
72
What is the role of chaperones?
They help some proteins fold into their 3D shape This prevents them from sticking together incorrectly during folding
73
Where are special amino acid sequences that direct proteins to certain parts of the cell found?
Often found close to the N- or C-terminus
74
Why is the coding region of mRNA usually covered with ribosomes?
To help protect the mRNA from degradation by RNAses
75
What happens if there are no ribosomes on the RNA?
Both protein and mRNA may be trashed as the RNA is exposed to nucleases
76
What is nonsense mediated decay? How does it affect the mRNA?
When a mutation causes a premature stope codon This causes a bit of the mRNA to become exposed and liable to degradation
77
why must antibiotics which target prokaryotic ribosomes be used with care?
Ribosomes in the mitochondria are susceptible to the same antibiotics

Nucleic Acids (4 decks)

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