Ocr As Biology - Protein Synthesis (Transcription)

Front 1

What is the first stage of protein synthesis?

Back 1

Transcription.

Front 2

What happens during transcription?

Back 2

An mRNA copy of a gene (a section of DNA) is made in the nucleus.

Front 3

[Part 1 of transcription] How is a strand of DNA extracted from a double helix? Mention the enzymes involved.

Back 3

• RNA polymerase attaches to the DNA double-helix at the beginning of a gene.
• The hydrogen bonds between the two DNA strands break, seperating the strands. The DNA molecule uncoils at this point.
• One of the strands is used as a template to make an mRNA copy (this template is also called the antisense strand).

Front 4

[Part 2 of transcription] What occurs to the single strand of DNA? What enzyme is involved? What is this process called?

Back 4

• RNA polymerase lines up free RNA nucleotides alongside the template strand.
• Complementary base pairing occurs, and the mRNA strand becomes a complementary copy of the original DNA strand (except base thymine (T) is replaced with uracil (U) as it is RNA).
• Once the RNA nucleotides have paired up with their specific bases, they are joined together forming the mRNA molecule.

Front 5

[Part 3 of transcription] What occurs once the RNA nucleotides have been paired up with the DNA template strand? What happens to the bonds within the molecule? What enzymes are involved?

Back 5

• RNA polymerase moves along the DNA, separating the mRNA strand from the DNA template and reassembling the DNA strand. (In eukaryotes, its actually a complex of proteins including DNA helicase that seperates the strand. RNA polymerase only assembles the mRNA strand).
• The hydrogen bonds between the uncoiled DNA strands re-form once the RNA polymerase has passed by and the strands coil back into a double-helix.

Front 6

[Part 4 of transcription] When does the RNA polymerase know when to stop making mRNA? What happens after the mRNA strand is completed?

Back 6

• When RNA polymerase reaches a stop codon, it detaches from the DNA and stops making mRNA.
• The mRNA moves out of the nucleus through the nuclear pore and attaches to a ribosome in the cytoplasm, where the next stage of protein synthesis occurs (translation).