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Nucleic Acids > Transcription > Flashcards

Transcription Flashcards

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
Q

What is the 5’ cap?

A

a modified guanine nucleotide

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26
Q

What happens when a polyadenylation sequence appears in an RNA molecule during transcription?

A
  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

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27
Q

What is the purpose of the poly-A tail?

A

It makes the RNA more stable

It promotes the RNA to be exported from the nucleus to the cytosol

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28
Q

What happens when pre-mRNAs are spliced?

Why is it important?

A

The introns are removed and the exons are rejoined

In order for mRNA to encode a protein, the introns must be removed

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29
Q

What is the spliceosome and what does it recognise?

A

It is a protein-RNA complex that recognises specific sites at the start and end of each intron

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30
Q

What is a shared feature of all the intron splice sites?

A

They contain GU dinucleotide at the 3’ end of the upstream intron

They contain AG dinucleotide at the 5’ end of the downstream intron

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31
Q

What is meant by ‘alternative splicing’?

A

The same pre-mRNA can be spliced in multiple different ways depending on which exons are kept

32
Q

what is significant about ‘alternative splicing’?

A

It means that more than one mRNA molecule can be made from a single gene

One gene codes for multiple proteins

33
Q

What is the initiation site?

A

The site on the DNA from which the first RNA nucleotide is transcribed

34
Q

How do ‘upstream’ and ‘downstream’ relate to the initiation site?

A

Any nucleotides before the initiation site are upstream

Any nucleotides before the initiation site are downstream

35
Q

What is the enzyme involved in transcription?

A

RNA polymerase 2

36
Q

What are the roles of RNA polymerases 1 and 3?

A

Polymerase 1 transcribes ribosomal RNA

Polymerase 3 transcribes tRNA

37
Q

What is a TATA box?

A

It is a sequence within a promoter region of a gene

38
Q

What will the TATA box be recognised by?

What does this allow for?

A

It is recognised by a general transcription factor

This allows other transcription factors, and eventually RNA polymerase II to bind

39
Q

What is significant about the TATA box containing lots of As and Ts?

A

This makes it easier to pull the 2 strands of DNA apart

A and T only form 2 hydrogen bonds

40
Q

How is the energy needed for transcription generated?

A

When ATP is reduced to ADP and Pi

After this, the transcription factors dissociate and transcription begins

41
Q

What are the enhancer elements that are further away from the TATA box usually involved in?

A

Tissue-specific regulated transcription

42
Q

What is a CpG island?

A

CpG islands are stretches of DNA where there are multiple points where C is followed by G

(p is phosphate)

43
Q

Where are CpG islands usually found?

A

Upstream of many genes

They appear to have promoter activity

44
Q

How does the state of the C in the CpG island vary depending on its location?

A

Inside the CpG island, the C is protected from methylation

Outside the island, the C is often methylated

45
Q

what does methylation of a CpG island lead to?

A

It is a way of switching off expression of a gene

This happens in inactivation of X chromosomes and some cancers

46
Q

What are the enzymes that add methyl groups?

A

Methyltransferases

47
Q

What does DNA methylation often involve?

A

Addition of a methyl group to the 5th carbon atom of a cytosine ring

48
Q

What happens to the chromatin in the B-globin gene?

A

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
Q

what is the locus control region (LCR)?

A

A sequence that can isolate a gene or group of genes from external influences

50
Q

How many amino acids are there in the body?

A

20

Each is coded for by a 3-base codon

51
Q

What is the initiation codon?

A

Methionine (AUG)

52
Q

What are the 3 stop codons?

A

UAA

UAG

UGA

53
Q

What type of “codon” is present on a transfer RNA molecule?

A

An anticodon

This is a set of 3 nucleotides that are complementary to a codon on the mRNA

54
Q

What is found on the opposite end of the tRNA molecule to the anticodon?

A

the amino acid that is specified by the codon

55
Q

What happens when tRNA molecules reach the ribosome?

A
  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
Q

What are the 3 key components involved in the initiation stage of translation?

A
  1. ribosome
  2. mRNA
  3. initiator tRNA that carries the first amino acid in the protein

THIS IS METHIONINE

57
Q

Where will the initiator tRNA bind to?

What will this form?

A

It attaches to the small ribosomal subunit

This forms the tRNA-ribosome complex

58
Q

What will the tRNA-ribosome complex bind to and why?

A

The 5’ end of the mRNA

It does this by recognising the 5’ GTP cap

59
Q

What happens after the tRNA-ribosome complex has bound to the mRNA?

A

the initiation complex walks along the mRNA in the 3’ direction and stops when it reaches the start codon

60
Q

What happens once the initiation complex reaches the start codon?

A

The large subunit of the ribosome binds and translation commences at the start codon

61
Q

During the elongation stage of translation, where does the initiator tRNA start?

A

The tRNA carrying methionine starts in the P site of the ribosome

62
Q

What happens when the initiator tRNA enters the P site of the ribosome?

A

A new codon is exposed in the A site

The A site is next to the P site

63
Q

What is significant about the A site in the ribosome?

A

The A site is where the next tRNA molecule will land

The anticodon must be complementary to the exposed codon

64
Q

What happens once the next tRNA molecule has entered the A site?

A

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
Q

What is formed by the methionine and the second amino acid added to the polypeptide?

A

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
Q

What happens in the ribosome after the first peptide bond has formed?

A

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
Q

When does the termination stage of translation occur?

A

When a stop codon in the mRNA enters the A site of the ribosome

68
Q

what are stop codons in the mRNA recognised by?

A

Release factors, which fit neatly into the P site

69
Q

What is the role of release factors?

A

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
Q

what happens to the newly synthesised peptide after the release factors have entered the P site?

A

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
Q

What happens to a newly synthesised polypeptide after translation?

A

It folds into a distinct 3D structure

Or it may join with other polypeptides to form a multi-part protein

72
Q

What is the role of chaperones?

A

They help some proteins fold into their 3D shape

This prevents them from sticking together incorrectly during folding

73
Q

Where are special amino acid sequences that direct proteins to certain parts of the cell found?

A

Often found close to the N- or C-terminus

74
Q

Why is the coding region of mRNA usually covered with ribosomes?

A

To help protect the mRNA from degradation by RNAses

75
Q

What happens if there are no ribosomes on the RNA?

A

Both protein and mRNA may be trashed as the RNA is exposed to nucleases

76
Q

What is nonsense mediated decay?

How does it affect the mRNA?

A

When a mutation causes a premature stope codon

This causes a bit of the mRNA to become exposed and liable to degradation

77
Q

why must antibiotics which target prokaryotic ribosomes be used with care?

A

Ribosomes in the mitochondria are susceptible to the same antibiotics

Nucleic Acids (4 decks)

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