S7) Gene Expression

Question 1

What is a codon?

Answer 1

Codons are triplets of nucleotides in mRNA that encode the information for a specific amino acid in a protein

Question 2

What is a stop codon?

Answer 2

Stop codons are codons in mRNA that signal the end of translation (end of protein synthesis)

Question 3

Identify 3 stop codons

Answer 3

• UAA
• UAG
• UGA

Question 4

What is a start codon (AUG)?

Answer 4

A start codon is always the first codon in a gene, present in mRNA to signal the location for translation to begin.

AUG = methionine

Question 5

How is genetic code written?

Answer 5

Genetic code is written as it appears in an RNA copy of the DNA template

Question 6

What is transcription?

Answer 6

Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA (especially mRNA) by the enzyme RNA polymerase.

Question 7

Identify the 3 stages in transcription

Answer 7

• Initiation
• Elongation
• Termination

Question 8

Describe initiation in transcription

Answer 8

Initiation: RNA polymerase binds to a specific nucleotide sequence (called a promoter) that marks the beginning of a gene.

Question 9

Describe elongation in transcription

Answer 9

- Elongation: DNA strands unwind, and RNA polymerase reads the nucleotide sequence of the template strand.

• As it moves along, it inserts and pre-mRNA molecule.

Question 10

Describe termination in transcription

Answer 10

• Termination: RNA polymerase reaches the end of the gene, marked by nucleotides called a 3’ termination sequence
• RNA-polymerase stops adding nucleotides to pre-mRNA which falls off the DNA template strand and is released
• DNA strands re-form a double helix

Question 11

What is an intron?

Answer 11

An intron is a sequence in the pre-mRNA molecule that is not transcribed

Question 12

What is an exon?

Answer 12

An exon is a sequence in a pre-mRNA molecule that is transcribed

Question 13

Outline the process of RNA splicing

Answer 13

• RNA splicing is when pre-mRNAs are processed in the nucleus to remove introns which occur between exons
• As introns are removed, the exons are spliced together to form mature mRNA molecules

Question 14

How are mRNA molecules processed?

Answer 14

Ends of the mRNA are modified:

• A nucleotide cap is attached to the 5’ end of eukaryotic mRNA
• A poly-A tail is added at the 3´ end (aids in export of the mRNA from nucleus)

Question 15

How is the mature mRNA molecule released?

Answer 15

The final product, mature mRNA is exported from the nucleus into the cytoplasm, where translation takes place

Question 16

What is translation?

Answer 16

Translation is the process in which ribosomes in the cytoplasm/rER produce polypeptides from information in mRNA

Question 17

What is the structure and function of ribosomes?

Answer 17

• Structure: cytoplasmic organelles with a large and a small subunit
• Function: site of polypeptide synthesis

Question 18

What kind of ribosomes are there?

Answer 18

• Free ribosomes: float in the cytoplasm
• Bound ribosomes: attached to the outer membrane of the rough endoplasmic reticulum

Question 19

Explain the role of ribosomes in translation

Answer 19

The large subunit contains ribosomal RNA (rRNA) which acts as an enzyme, linking amino acids together to form a polypeptide during translation

Question 20

What is tRNA?

Answer 20

Transfer RNA (tRNA) molecules bring amino acids to the mRNA ribosome complex during translation.

Question 21

Each tRNA molecule has two attachment sites. Identify and describe them

Answer 21

• An anticodon that pairs with a complementary codon sequence in mRNA
• A site for attachment of the amino acid specified by the mRNA codon

Question 22

Identify the three steps in translation

Answer 22

• Initiation
• Elongation
• Termination

Question 23

Describe initiation in translation

Answer 23

• Initiation: mRNA binds to a small ribosomal subunit and the anticodon of the initiator
• tRNA carrying the amino acid methionine pairs with the AUG codon of mRNA.

Question 24

Describe elongation in translation

Answer 24

• Elongation: amino acids are added to the growing polypeptide chain
• The rRNA of the large subunit forms peptide bond between amino acids

Question 25

Describe termination in translation

Answer 25

- **Termination** occurs when the ribosome reaches a stop codon (UAA, UAG, and UGA) - Proteins called release factors bind to stop codons - Polypeptide, mRNA, and tRNA are released from the ribosome

Question 26

What is a gene?

Answer 26

**Gene**: a unit of inheritance which is transferred from a parent to offspring and is held to determine some characteristic of the offspring

Question 27

Distinguish between DNA replication, transcripion and translation in terms of its overall function

Answer 27

**- DNA Replication:** making DNA **- DNA Transcription:** making RNA **- DNA Translation:** making a polypeptide

Question 28

Distinguish between DNA replication, transcription and translation in terms of the enzymes involved

Answer 28

**- DNA Replication:** DNA polymerase **- DNA Transcription:** RNA polymerase **- DNA Translation:** rRNA

Question 29

Distinguish between DNA replication, transcription and translation in terms of the activated substrates used

Answer 29

**- DNA Replication:** dNTPs **- DNA Transcription:** NTPs **- DNA Translation:** amino acids

Question 30

Distinguish between DNA replication, transcription and translation in terms of the template needed

Answer 30

**- DNA Replication:** DNA **- DNA Transcription:** DNA **- DNA Translation:** mRNA

Question 31

Compare and contrast the ribosomes of prokaryotes and eukaryotes in terms of: - Size - Subunits - rRNAs - Proteins

Answer 31

Question 32

Identify 5 factors which characterise protein synthesis in bacteria.

Answer 32

- Simple promoters - Single RNA polymerase - No post-transcriptional processing - Short-lived mRNAs - Simpler ribosomes