Protein Synthesis

Question 1

What is transcription?

Answer 1

• when base sequences of genes have to be copied and transported into a molecule of messenger RNA (mRNA) to go to the ribosomes.
• first stage of protein synthesis

Question 2

How does mRNA differ from DNA?

Answer 2

• single stranded
• ribose sugar
• uracil base

Question 3

What is the process of transcription?

Answer 3

1. Helices (enzyme) acts on region of DNA to break hydrogen bonds between the bases.
2. RNA polymerase moves along template (antisense) strand of the DNA, beginning at ‘start’ codon.
3. RNA polymerase joins phosphodiester bonds between RNA nucleotides
4. Complementary base pairing occurs
5. RNA nucleotides join together and the pre-mRNA is formed.
6. DNA behind RNA polymerase rejoins into double helix
7. When RNA polymerase reaches ‘stop’ codon, the chain is terminated and the pre-mRNA attaches.
8. the mRNA leaves the nucleus through a pore and then to the ribosomes.

Question 4

Why is transcription necessary?

Answer 4

• DNA too large to leave the nucleus

- DNA very well protected

Question 5

What is the structure of ribosomes like?

Answer 5

• large and small subunits
• elements of ribosomal RNA
• protein element

Question 6

What is translation?

Answer 6

When the ribosome holds the mRNA on its small subunit while it is decoded, or translated, into a sequence of amino acids.

Question 7

What are the stages of translation?

Answer 7

1. mRNA attaches to ribosome at ‘start’ codon. tRNA with complementary anticodon attaches to mRNA.
2. ribosome moves along mRNA ‘holding’ two tRNA molecules at any one time.
3. enzyme and ATP are used to join amino acids with a peptide bond.
4. first tRNA is released and can collect another specific amino acid.
5. process repeated until ‘stop’ codon reached.

Question 8

Which enzyme catalyses the formation of peptide bonds in translation?

Answer 8

• peptidyl transferase

Question 9

What is polysome?

Answer 9

When many ribosomes travel along the mRNA at the same time.

Question 10

Where will the protein/polypeptide go after being made at the ribosome?

Answer 10

• everywhere

- golgi apparatus

Question 11

What does the sequence of bases determine?

Answer 11

The sequence of bases on a gene determines the sequence of amino acids in a polypeptide.

Question 12

What is the structure of tRNA like?

Answer 12

• composed of a strand of RNA folded, so the three bases (anticodon) are at one end of the molecule
• the anticodon will bind to a complementary codon on mRNA
• hydrogen bonds
• 3 leaf clover shape

Question 13

What are the roles of mRNA, tRNA and rRNA in protein synthesis?

Answer 13

• mRNA: copies genes from DNA and takes copy to ribosome
• tRNA: brings amino acid to ribosome
• rRNA: formation of ribosomes

Question 14

Enzymes are part of the structure of ribosomes. What is their role?

Answer 14

• catalyse the formation of peptide bond between two amino acids

Question 15

Why does rRNA need to be present in a ribosome?

Answer 15

• bind to tRNA

- complementary base pairing

Question 16

Why can ribosomes both be found free-floating and attached to the ER?

Answer 16

• free floating ribosomes produce proteins for use in cells/cytoplasm
• bound ribosomes produce proteins for export from the cell

Question 17

Why do unnecessary base sequences need to be removed before protein synthesis begins?

Answer 17

• role of protein dependent on structure
• shape/3D structure dependent on primary structure/ sequence of amino acids
• base triplets/codons on mRNA code for amino acids
• unnecessary amino acids/stop signal codons could cause frameshift

Question 18

Why are introns present within genes?

Answer 18

• originally functional genes could mutate
• changes base sequences
• no longer code for (useful) amino acids