A4. Transcription and Translation

Question 1

Transcription
During transcription an ______copy of a gene is made from ____. In ______________cells, transcription takes place in the __________. _____________don’t have a _______, so transcription takes place in the __________.
Here’s how transcription happens: (4 steps, 4,3,2,2 things)

Answer 1

Transcription
During transcription an mRNA copy of a gene is made from DNA. In eukaryotic cells, transcription takes place in the nucleus. Prokaryotes don’t have a nucleus, so transcription takes place in the cytoplasm. Here’s how transcription happens:

1. RNA polymerase attaches to the DNA
   - Transcription starts when RNA polymerase (an enzyme) attaches to the DNA double-helix at the beginning of a gene.

• In eukaryotes, the hydrogen bonds between the two DNA strands in the gene are broken by RNA polymerase.
• This separates the strands, and the DNA molecule uncoils at that point, exposing some of the bases.
• One of the strands is then used as a template to make an mRNA copy

2. Complementary mRNA is formed
   - The RNA polymerase lines up free RNA nucleotides alongside the exposed bases on the template strand.

• The free bases are attracted to the exposed bases. Specific complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand (except the base T is replaced by U in RNA).
• Once the RNA nucleotides have paired up with their specific bases on the DNA strand, they’re joined together by RNA polymerase, forming an mRNA strand

3. RNA polymerase moves down the DNA strand
   - The RNA polymerase moves along the DNA, assembling the mRNA strand.

• The hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by and the strands coil back into a double-helix

4. RNA polymerase reaches stop signal
   - When RNA polymerase reaches a particular sequence of DNA called a stop signal, it stops making mRNA and detaches from the DNA.

• In eukaryotes, mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, where the next stage of protein synthesis takes place

Question 2

Editing mRNA

What is pre-mRNA?
What is splicing?
Where does this take place?
What happens after?

What about prokaryotes?

Answer 2

Transcription produces different products in eukaryotes and prokaryotes. In eukaryotes, the introns and exons are both copied into mRNA during transcription. mRNA strands containing introns and exons are called pre-mRNA. A process called splicing then occurs introns are removed and the exons joined together-forming mRNA strands. This takes place in the nucleus. The mRNA then leaves the nucleus for the next stage of protein synthesis (translation).

In prokaryotes, mRNA is produced directly from the DNA- without splicing taking place. There’s no need for splicing because there are no introns in prokaryotic DNA.

Question 3

Translation
Translation is the second stage of protein synthesis. In both eukaryotes and prokaryotes, translation occurs at the ___________in the ___________. During translation, ______ ______are joined together to make a ____________chain (protein), following the sequence of codons carried by the mRNA.

Here’s how it works: (4 steps without titles, 2,2,3,3 things)

Answer 3

Translation is the second stage of protein synthesis. In both eukaryotes and prokaryotes, translation occurs at the ribosomes in the cytoplasm. During translation, amino acids are joined together to make a polypeptide chain (protein), following the sequence of codons carried by the mRNA. Here’s how it works:

1) - The mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to it.

• ATP provides the energy needed for the bond between the amino acid and the tRNA molecule to form.

2) - A tRNA molecule (carrying an amino acid), with an anticodon that’s complementary to the first codon on the mRNA, attaches itself to the mRNA by complementary base pairing.

• A second tRNA molecule attaches itself to the next codon on the mRNA in the same way.

3) - The two amino acids attached to the tRNA molecules are joined by a peptide bond.

• The first tRNA molecule moves away, leaving its amino acid behind.
• A third tRNA molecule binds to the next codon on the mRNA.

4) - Its amino acid binds to the first two and the second tRNA molecule moves away.

• This process continues, producing a chain of linked amino acids (a polypeptide chain), until there’s a stop signal on the mRNA molecule.
• The polypeptide chain (protein) then moves away from the ribosome and translation is complete.