Unit 5: Section 1 - Protein Synthesis and Cellular Control

Question 1

What are the characteristics of messenger RNA?

Answer 1

1) Made in the nucleus
2) Three adjacent bases are called a codon
3) Carries the genetic code from the DNA in the nucleus to the cytoplasm

Question 2

What are the characteristics of transfer RNA?

Answer 2

1) Found in the cytoplasm
2) Has an amino acid binding site at one end and a sequence of three bases at the other end called an anticodon
3) It carries the amino acids that are used to make proteins to the ribosomes during translation

Question 3

What are the stages of transcription?

Answer 3

1) Transcription starts when RNA polymerase attaches attaches to the DNA double helix at the start of a gene
2) The hydrogen bonds between the two DNA strands in the gene break, separating the strands, and the DNA molecule uncoils at that point
3) One of the strands is then used as a template to make an mRNA copy
4) The RNA polymerase lines up free RNA nucleotides alongside the template strand, complementary base pairing means that the mRNA strand formed is a complementary copy of the DNA template strand
5) The RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strands
6) Once the RNA polymerase has passed by the hydrogen bonds re form and the DNA becomes a double helix again
7) When RNA polymerase reaches a stop codon, it stops making mRNA and detaches from the DNA
8) The mRNA leaves the nucleus and attaches to a ribosome

Question 4

What are the stages of translation?

Answer 4

1) The mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to the ribosome
2) A tRNA molecule with an anticodon that is complementary to the first codon on the mRNA attaches itself to the mRNA by complementary base pairings
3) A second tRNA molecule attaches itself to the next codon on the mRNA in the same way
4) The two amino acids are joined together by a peptide bond, the first tRNA molecule moves away, leaving it’s amino acid behind
5) A third tRNA molecule binds to the next codon, it’s amino acid binds to the first two and the second tRNA molecule moves away
6) This process repeats itself until there’s a stop codon on the mRNA molecule

Question 5

What is an operon?

Answer 5

A section of DNA that contains structural genes, control elements and sometimes a regulatory gene

Question 6

What does the structural gene do?

Answer 6

Codes for useful proteins, such as enzymes

Question 7

What do the control elements include?

Answer 7

A promoter (a DNA sequence located before the structural genes that RNA polymerase binds to) and an operator (a DNA sequence that proteins called transcription factors bind to)

Question 8

What does a regulatory gene do?

Answer 8

Codes for a transcription factor

Question 9

Explain the lac operon in E.coli example

Answer 9

1) E.coli is a bacterium that respires glucose, but can use lactose if glucose isn’t available
2) The genes that produce the enzymes needed to respire lactose are found on an operon called the lac operon
3) The lac operon has 3 structural genes - lacZ, lacY and lacA which produce proteins that help the bacteria digest lactose (beta-galactosidase and lactose permease)
4) When lactose isn’t present the regulator gene produces lac repressor, which binds to the operator site and blocks transcription
5) When lactose is present it binds to the repressor and changes it’s shape so it can’t bind to the operator site any more, RNA polymerase can now begin transcription of the structural genes