2.1.2l Transcription & Translation

Question 1

see slide 2 for recap on base combinations for amino acids using chart

Question 2

How does DNA code for polypeptides

Answer 2

1. Genes are inside the cell nucleus but proteins are made in the cytoplasm, at ribosomes
2. As the instructions inside the genes, on chromosomes, cannot pass out of the nucleus, a copy of each gene has to be TRANSCRIBED (copied) into a length of mRNA.
3. In the form of mRNA, the sequence of bases (codons), can pass out the nucleus to the ribosome, ensuring that the coded instructions are TRANSLATED & the protein is assembled correctly from amino acids.

Question 3

see slide 4 for diagram of making a protein from DNA

Question 4

What is Transcription

Answer 4

The process of copying the code exactly to form a template of mRNA.

Question 5

RNA polymerase attaches to…

Stage 1 of Transcription

Answer 5

1. RNA polymerase attaches to the DNA
   - Transcription starts when RNA polymerase attaches to the DNA double-helix at the beginning of a gene
   - The hydrogen bonds between the 2 DNA strands in the gene break, separating the strands & the DNA molecules begins to uncoil
   - One of the strands is used as a template strand to make the mRNA copy
   (see slide 6-10 for dia)

Question 6

What is the DNA template in transcription also known as

Answer 6

The antisense strand

Question 7

Complementary…

Stage 2 of Transcription

Answer 7

2. Complementary mRNA is formed
   - The RNA polymerase lines up free RNA nucleotides alongside the template strand
   - Complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand (except T has been replaced w U in RNA)
   - Once RNA nucleotides have paired up w the specific bases on the DNA, the RNA polymerase joins them tg to form an mRNA strand

Question 8

RNA polymerase…

Stage 3 of Transcription

Answer 8

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 & the strands coil back into a double helix

Question 9

Nucleus…

Stage 4 of Transcription

Answer 9

4. mRNA leaves the nucleus
   - It will inhibit protein synthesis. By inhibiting RNA polymerase, {a}-amanitin will prevent the transcription of mRNA from DNA, preventing protein synthesis from taking place

Question 10

What is {a}-amanitin

Answer 10

A deadly toxin produced by some mushrooms. It works by inhibiting RNA polymerase

Question 11

What is Translation

Answer 11

The process by which info in the code from the sequence of bases in mRNA controls the sequence of amino acids that a ribosome assembles.

Question 12

What happens during Transcription (simple overview)

Answer 12

The 1st stage of protein synthesis. During transcription, an mRNA copy of a gene is made in the nucleus

Question 13

What happens during Translation (simple overview)

Answer 13

The 2nd stage of protein synthesis. During translation, amino acids are joined tg by a ribosome in the cytoplasm to make a polypeptide chain (protein), following the sequence of codons carried by mRNA

Question 14

Ribosome…

Stage 1 of Translation

Answer 14

1. mRNA attaches to ribosome
   - mRNA attaches itself to a ribosome at its start codon (AUG) & tRNA (transfer RNA) molecules carry amino acids to the ribosome
   (slide 11-15)

Question 15

tRNA –> mRNA

Stage 2 of Translation

Answer 15

2. tRNA molecule attaches to mRNA
   - A tRNA molecule, w an anticodon that’s complementary to the start codon on the mRNA, attaches itself to the mRNA by complementary base pairing.
   - A 2nd tRNA molecule then attaches itself to the next codon on the mRNA in the same way

Question 16

rRNA

Stage 3 of Translation

Answer 16

3. rRNA joins amino acids
   - Ribosomal RNA (rRNA) in the ribosome catalyses the formation of a peptide bond between the 2 amino acids attached to the tRNA molecules
   - This joined the amino acids tg & the first tRNA molecule moves away, leaving the amino acid behind

Question 17

stop codon

Stage 4 of Translation

Answer 17

4. Chain keep forming until stop codon
   - A 3rd tRNA molecule binds to the next codon on mRNA. Its amino acid binds to the first two, & the second tRNA molecule moves away
   - This process repeats continuously until there’s a ‘stop codon’ on the mRNA molecule

Question 18

complete

Stage 5 of Translation

Answer 18

5. Translation is complete
   - The polypeptide chain (protein) moves away from the ribosomes & translation is complete

Question 19

EXAM Q: An error occurs during transcription that accidentally inserts a stop codon into the middle of an mRNA sequence.
What effect could this have on the protein that is eventually produced?

Answer 19

It could be shorter & so could be a different protein. Translation of the mRNA sequence only continues until a stop codon is reached. Any codons after the stop codon would not be translated into amino acids

Question 20

Quick summary: where does transcription take place

Answer 20

Takes place in nucleus of cell. Involves RNA polymerase & mRNA

Question 21

Quick summary: where does translation take pace

Answer 21

Takes place in the ribosomes in the cytoplasm of the cell. involves mRNA, tRNA, rRNA

Question 22

What is RNA

Answer 22

• A single-stranded polynucleotide, which means it is made up of a no. of RNA nucleotide molecules joined to eachother w covalent bonds that form by condensation reactions
• RNA forms relatively short lengths of up to a few thousand nucleotides
• There are 3 types of RNA: mRNA, tRNA, rRNA

Question 23

see slide 17 for dia of 3 types of RNA

Question 24

3 points

What is mRNA

Answer 24

Messenger RNA
- Made in the nucleus during transcription
- Carries the genetic code from the DNA in the nucleus to the cytoplasm, where it’s used to make a protein during translation. It is small enough to leave the nuclear pores
- In mRNA, groups of 3 adjacent bases are called CODONS
(slide 18 for dia)

Question 25

Key info about mRNA

Answer 25

mRNA contains Uracil (U) instead of Thymine (T)

Question 26

3 points

What is tRNA

Answer 26

Transfer RNA
- tRNA is folded into a clover shape. Hydrogen bonds between specific base pairs hold the molecule tg in this shape
- Every tRNA molecule has a specific sequence of 3 bases at one end called an anticodon
- They also have an amino acid binding site at the other end

Question 27

Function of tRNA

Answer 27

• tRNA is found in the cytoplasm where it’s involved in translation. It carries the amino acids that are used to make the proteins in the ribosomes

Question 28

What is rRNA

Answer 28

Ribosomal RNA
- rRNA forms the 2 subunits in a ribosome, along with proteins
- The ribosome moves along the mRNA strand during protein synthesis
- The rRNA in the ribosome helps to catalyse the formation of peptide bonds between the amino acids

Question 29

Importance of rRNA

Answer 29

rRNA is important in maintaining the structural stability of the protein synthesis sequence

Question 30

see images 07.12 for dia of 3 types of RNA