2: Transcription and Translation

Question 1

What is the sequence of bases in a polypeptide determined by? How is this sequence stored?

Answer 1

• the base sequence of a gene

- in a coded form

Question 2

What is the first stage in the synthesis of a polypeptide?

Answer 2

to make a copy of the base sequence of the gene

Question 3

When synthesising a protein, what is the copy of the base sequence of the gene made from? Where does the copy go? What is it called

Answer 3

• RNA
• RNA is carried to the ribosomes in the cytoplasm to give them the information needed to synthesize a polypeptide, so is called mRNA (messenger RNA)

Question 4

What is ‘transcription’ of DNA?

Answer 4

the copying of the base sequence of a gene by making an RNA molecule

Question 5

When does transcription begin?

Answer 5

when the enzyme RNA polymerase binds to a site on the DNA at the start of a gene

Question 6

Describe the process of transcription.

Answer 6

(Stages 1, 2, and 3 carried out by RNA polymerase)

1. RNA polymerase moves along gene separating the DNA into two single strands
2. RNA nucleotides are assembled along one of two strands of DNA. Same rules of complementary base pairing are followed as in replication, except that uracil pairs with adenine, as RNA does not contain thymine
3. RNA nucleotides are linked together by covalent bonds between the pentose sugar of one nucleotide and the phosphate of the next
4. RNA strand separates from the DNA strand as it is produced and is released completely when the end of the gene is reached
5. DNA strands pair up again and twist back into double helix

Question 7

What can be deduced from the base sequence of a strand of mRNA?

Answer 7

the base sequence of the DNA strand from which it was transcribed

Question 8

What is the ‘sense strand’? Explain the term.

Answer 8

• sense strand = strand of DNA that is not transcribed

- sense strand is the same as mRNA (apart from having T instead of U) that is why it is called ‘sense’

Question 9

Which strand of DNA is the mRNA transcribed from?

Answer 9

the antisense strand

Question 10

What is ‘translation’ with regard to DNA?

Answer 10

the synthesis of polypeptides on ribosomes, using mRNA and tRNA

Question 11

What does the mRNA determine in translation of DNA?

Answer 11

the amino acid sequence of the polypeptide (it is being used to make)

Question 12

What does the ribosome do in translation of DNA?

Answer 12

translates a genetic code called the triplet code - three bases code for one amino acid

Question 13

What is a codon?

Answer 13

a group of three bases

Question 14

What does translation of DNA depend on?

Answer 14

complementary base pairing between codons on mRNA and anticodons on tRNA

Question 15

Draw and label a diagram of translation. Also indicate 4 points of interest. (p30)

Answer 15

-

basically this was awkward to turn into a flashcard:

1. learn how to draw diagram on p30
2. the “points of interest” (PofI) are the 4 bits of writing
3. learn to label the PofI, but just write 1, 2, 3, or 4.
4. in the upcoming flashcards, you will learn what occurs at each point of interest
5. in the exam you will have to adapt this knowledge to fit the question

Question 16

Point of Interest 1 on translation diagram. (p30)

Answer 16

• mRNA binds to a site on the small sub-unit of the ribosome

- mRNA contains a series of codons consisting of three bases, each of which codes for one amino acid

Question 17

Point of Interest 2 on translation diagram. (p30)

Answer 17

• tRNA molecules are present around ribosome in large numbers
• each tRNA has special triplet of bases called an anticodon and carries the amino acid corresponding to this anticodon

Question 18

Point of Interest 3 on translation diagram. (p30)

Answer 18

• there are 3 binding sites for tRNA molecules on large sub-unit of ribosome by only 2 ever bind at once
• tRNA can only bind if its anticodon is complementary to the codon on the mRNA
• bases on codon and anticodon link together by forming (temporary) hydrogen bonds

Question 19

Point of Interest 4 on translation diagram. (p30)

Answer 19

• amino acids carried by tRNA molecules are bonded together by a peptide linkage
• a dipeptide is formed, attached to the tRNA on the right
• tRNA on the left detaches, ribosome moves along the mRNA to next codon, another tRNA carrying an amino acid binds = a chain of three amino acids is formed
• these stages are repeated until a polypeptide is formed

Question 20

Outline translation.

Answer 20

• mRNA binds to small subunit of ribosome
• free tRNA nucleotides present in large numbers
• tRNA has anticodon and carries corresponding amino acid
• mRNA has codon
• through complementary base pairing, codon on mRNA and anticodon on tRNA bind
• amino acids on tRNA form dipeptides with peptide linkage
• last 2 steps repeat until polypeptide is formed