Transcription and Translation

Question 1

What are the stages of transcription in eukaryotic cells?

Answer 1

Initiation
Elongation
Termination
Post-Transcriptional Modifications

Question 2

What happens during the initiation phase in transcription?

Answer 2

1. The section of DNA containing the gene unzips and unwinds under the control of DNA helicase.
2. This exposes the bases on the template strand (anti-sense strand).
3. Transcription starts at a start codon on the sense strand.

Question 3

What happens during the elongation phase in transcription?

Answer 3

Free RNA nucleotides align and form complementary base pairs with exposed bases on the anti-sense strand:
* Adenine pairs with uracil (replaces thymine in RNA).
Cytosine pairs with guanine.
RNA polymerase forms phosphodiester bonds between RNA nucleotides, creating a single strand of mRNA.

Question 4

What happens during the termination phase in transcription?

Answer 4

Transcription stops when RNA polymerase reaches a stop codon.
The mRNA strand detaches from the DNA template, and the DNA rewinds into its double-helix form.

Question 5

What happens during the Post-transcriptional Modifications phase in transcription?

Answer 5

A 5’ cap and a poly-A tail are added to the mRNA for stability and protection against degradation.
Introns (non-coding sequences) are removed through splicing, leaving only exons (coding sequences).

Question 6

How does mRNA leave the nucleus, and what happens next?

Answer 6

mRNA exits the nucleus via a nuclear pore in the nuclear envelope.
It travels to a ribosome (free in the cytoplasm or bound to the rough endoplasmic reticulum) for translation.

Question 7

What happens during translation in protein synthesis?

Answer 7

1. mRNA Binding
2. tRNA Binding
3. Codon Recognition
4. Peptide Bond Formation
5. Elongation
6. Termination

Question 8

What happens during the mRNA Binding phase of translation in protein synthesis?

Answer 8

mRNA binds to the small subunit of a ribosome at the start codon (AUG).
The start codon codes for the amino acid methionine.

Question 9

What happens during the tRNA Binding phase of translation in protein synthesis?

Answer 9

A tRNA molecule with the complementary anticodon UAC binds to the start codon.
This tRNA carries methionine.

Question 10

What happens during the Codon Recognition phase of translation in protein synthesis?

Answer 10

Another tRNA molecule with an anticodon complementary to the next mRNA codon binds to the ribosome.
This tRNA brings the corresponding amino acid.

Question 11

What happens during the Peptide Formation phase of translation in protein synthesis?

Answer 11

The amino acid on the first tRNA is transferred to the second tRNA.
A peptide bond forms between the amino acids, catalyzed by peptidyl transferase (an rRNA enzyme).

Question 12

What happens during the Elongation phase of translation in protein synthesis?

Answer 12

The ribosome moves along the mRNA strand, releasing the first tRNA and allowing another tRNA to bind to the next codon.

Question 13

What happens during the Termination phase of translation in protein synthesis?

Answer 13

The process continues until the ribosome reaches a stop codon (e.g., UAA, UAG, or UGA).
The completed polypeptide chain is released and folds into its secondary, tertiary, and sometimes quaternary structures.

Question 14

What is the role of mRNA in protein synthesis?

Answer 14

mRNA: Carries the genetic code from DNA to ribosomes, specifying the sequence of amino acids in a protein.

Question 15

What is the role of tRNA in protein synthesis?

Answer 15

tRNA: Brings specific amino acids to the ribosome. Each tRNA has an anticodon complementary to a codon on the mRNA.

Question 16

What is the role of rRNA in protein synthesis?

Answer 16

rRNA: Forms part of the ribosome structure and catalyzes peptide bond formation during translation.

Question 17

What is the importance of introns in genes? (3)

Answer 17

1. Introns allow for alternative splicing, enabling a single gene to produce multiple proteins.
2. They may contain regulatory sequences that control gene expression.
3. They play a role in evolution, allowing genetic recombination and variation.

Question 18

What are the differences between free-floating and ER-bound ribosomes?

Answer 18

Free-floating ribosomes: Synthesize proteins that function within the cytoplasm.

ER-bound ribosomes: Synthesize proteins destined for secretion, membranes, or lysosomes.

Question 19

Why does RNA need to be present in ribosomes?

Answer 19

RNA stabilizes the ribosomal structure and catalyzes peptide bond formation, acting as a ribosome enzyme (ribozyme).

Question 20

What are the advantages of editing mRNA?

Answer 20

Increases protein diversity through alternative splicing.
Allows the cell to adapt protein expression to environmental changes.
Removes errors or sequences that could cause malfunctions in protein synthesis.

Question 21

Why must unnecessary base sequences (introns) be removed from mRNA before translation?

Answer 21

Introns are non-coding and would disrupt the sequence of amino acids in the protein.
Removing introns ensures accurate protein synthesis.

Question 22

What is the purpose of transcription in protein synthesis?

Answer 22

The purpose of transcription is to create a complementary RNA copy (mRNA) of a DNA gene, enabling the genetic information to be transported out of the nucleus to the ribosomes for protein synthesis.

Question 22

What is the role of the enzyme in ribosome structure?

Answer 22

It catalyzes peptide bond formation between amino acids during translation.

Question 22

Where does transcription occur in eukaryotic cells?

Answer 22

Transcription occurs in the nucleus of eukaryotic cells, where the DNA is enclosed within the nuclear envelope.

Question 23

What is the sense strand, and what is its role in transcription?

Answer 23

The sense strand is the DNA strand that runs from 5’ to 3’ and contains the genetic code for a protein. It is not used as a template but has the same sequence as the mRNA (except thymine is replaced with uracil).

Question 24

What is the antisense strand, and why is it important?

Answer 24

The antisense strand (3’ to 5’) is the template strand used during transcription. RNA polymerase uses this strand to synthesize a complementary mRNA strand.

Question 25

Which enzyme is responsible for catalyzing transcription?

Answer 25

RNA polymerase catalyzes the transcription process by forming phosphodiester bonds between RNA nucleotides.

Question 26

Describe the sequence of events during transcription.

Answer 26

1. DNA helicase unwinds and unzips the DNA, breaking hydrogen bonds between complementary bases at the start codon.
2. Free RNA nucleotides base-pair with complementary bases on the antisense strand (A pairs with U, T pairs with A, G pairs with C, and C pairs with G).
3. RNA polymerase moves along the strand, joining RNA nucleotides via phosphodiester bonds.
4. Transcription stops when RNA polymerase reaches a stop codon.
5. The mRNA detaches and exits the nucleus through nuclear pores, while the DNA rewinds into a double helix.

Question 27

How does RNA differ from DNA in transcription?

Answer 27

RNA is single-stranded, while DNA is double-stranded.
RNA contains uracil (U) instead of thymine (T).
The sugar in RNA is ribose, whereas DNA contains deoxyribose.

Question 28

What happens to the mRNA after transcription?

Answer 28

The mRNA exits the nucleus through nuclear pores and travels to a ribosome in the cytoplasm for translation.

Question 29

What is translation, and where does it occur in eukaryotic cells?

Answer 29

Translation is the process of decoding the mRNA into a sequence of amino acids to form a protein. It occurs at ribosomes, which can be free-floating in the cytoplasm or attached to the rough endoplasmic reticulum.

Question 30

What are the roles of ribosomal RNA (rRNA) and ribosomes in translation?

Answer 30

rRNA helps maintain the structural stability of the ribosome and catalyzes peptide bond formation.
Ribosomes position the mRNA and facilitate the binding of tRNA molecules to ensure accurate amino acid assembly.

Question 31

What is transfer RNA (tRNA), and what is its role in translation?

Answer 31

tRNA is a folded RNA molecule with an anticodon on one end that binds to a complementary mRNA codon. It carries the specific amino acid corresponding to the codon.

Question 32

What happens to the polypeptide chain after translation?

Answer 32

The chain folds into its secondary and tertiary structures, forming hydrogen bonds and other interactions.
It may undergo further modifications in the Golgi apparatus, such as addition of carbohydrates or lipids, to become functional.

Question 33

What is the role of the start codon AUG?

Answer 33

AUG signals the start of translation and codes for methionine, the first amino acid in the polypeptide chain.

Question 34

Why are multiple ribosomes (polysomes) often seen on a single mRNA?

Answer 34

Multiple ribosomes can translate a single mRNA simultaneously, allowing for efficient and rapid synthesis of many protein molecules.

Question 35

How do transcription and translation differ in prokaryotes and eukaryotes?

Answer 35

In prokaryotes, transcription and translation occur simultaneously in the cytoplasm.

In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm, separated by the nuclear membrane.

Question 36

What are introns, and how do they relate to transcription?

Answer 36

Introns are non-coding sequences within a gene. They are transcribed into pre-mRNA but are spliced out during post-transcriptional modification, leaving only exons in the mature mRNA.

Question 37

Why is the 5’ cap and 3’ poly-A tail added to mRNA?

Answer 37

The 5’ cap protects mRNA from degradation and assists ribosome binding during translation.

The 3’ poly-A tail stabilizes mRNA and facilitates export from the nucleus.

Question 38

Why must unnecessary base sequences (introns) be removed before translation?

Answer 38

Introns do not code for amino acids and, if not removed, would result in a non-functional protein.