4. RNA and protein synthesis

Question 1

3 types of RNA

Answer 1

Messenger RNA, mRNA
Transfer RNA , tRNA
Ribosomal RNA, rRNA
which with proteins makes up the structure of ribosomes.

Question 2

mRNA

Answer 2

A single, short, helical polynucleotide strand.
Made in nucleus during transcription.
Carries the genetic code into the cytoplasm for translation as small enough to pass through pores in the nuclear envelope.
3 bases = A CODON
Chemically unstable – broken down within days
Its length is variable depending on the length of the gene from which it is transcribed.

Question 3

tRNA

Answer 3

Smallest, single polynucleotide strand which is folded into a clover leaf shape.
Hydrogen bonds hold it in shape.
Made in the nucleus, found throughout the cell.
Contains an anticodon of 3 bases
Has amino acid binding site – binds specific amino acid and carries it to a ribosome.
3 Bases = Anticodon, able to form complimentary base pairs with specific mRNA codons so lining up the amino acid it carries.
More stable than mRNA, less stable than DNA.

Question 4

AO2: Complete the table

Answer 4

Question 5

So how is the structure of DNA related to it’s function?

Answer 5

Long – so can store a lot of genetic information
Double helix – nitrogenous bases protected on inside
Many weak hydrogen bonds between complementary base pairs (A,T and C,G) – collectively make DNA stable, individually are weak so the 2 strands can be separated and act as templates.
Wrapped around Histone proteins – compact so can store large amounts in a small space

Question 6

So how is the structure of mRNA related to it’s function?

Answer 6

Short – able to leave the nucleus through the nuclear pores in the nuclear envelope and carry the genetic code for a single polypeptide to the ribosomes.
Has groups of 3 bases called codons – complementary to tRNA anticodons, so accurate translation of the DNA code

Question 7

So how is the structure of tRNA related to it’s function?

Answer 7

Has a specific amino acid binding site – so binds only 1 amino acid and carries it to the ribosome
Has an anticodon complementary to a specific mRNA codon so lines the amino acids up in a particular order.

Question 8

Protein Synthesis - overview

Answer 8

Transcription- DNA uncoils, strands separate and RNA polymerase is used to make pre-mRNA.

Pre-mRNA is edited- introns are removed by splicing. Exons are joined together.

Translation-mRNA travels to cytoplasm and attaches to a ribosome where it is translated.

Question 9

Transcription

Answer 9

The process of copying the genetic code to form mRNA is called transcription.
1. RNA polymerase binds at a promoter region at the start of the gene. It breaks the hydrogen bonds between bases – unzipping the DNA helix
2. One strand of DNA acts as a template (the sense strand), and forms complementary base pairs with free RNA nucleotides present in the nucleus. C base pairing with G, but A base pairing with U.

3. RNA polymerase also joins the adjacent nucleotides together by condensation reactions to form phosphodiester bonds.
4. The molecule produced at this stage is called pre-mRNA.
5. The DNA strands rejoin to form the double helix – only about 12 bases are exposed at any 1 time
6. When RNA polymerase reaches the terminator region at the end of the gene it detaches and the production of pre-mRNA is complete.

Question 10

Splicing pre-mRNA in eukaryotes

Answer 10

snRNPs are proteins that come together to form the spliceosome (along with other protein units)

Question 11

1. What will be the order of nucleotides in the pre-mRNA strand produced by this section of DNA? AGC TTA CGT ACG TAG GAT CCA

Answer 11

UCG AAU GCA UGC AUC CUA GGU

Question 12

2. A gene is made up of 756 base pairs. The mRNA transcribed from this gene is only 524 nucleotides long. Explain why there is a difference

Answer 12

There are promoter and terminator codes at the beginning and end of the gene that are not transcribed AND the introns within the gene will have been spliced out

Question 13

3. Why is splicing of pre-mRNA necessary in eukaryotic cells but not prokaryotic cells?

Answer 13

Prokaryotic DNA does not contain introns

Question 14

4. Describe the role of RNA polymerase in transcription.

Answer 14

It binds to the promoter region of DNA
Breaks hydrogen bonds exposing the sense strand of DNA
Joins adjacent nucleotides

Question 15

Complete the questions

Answer 15

Question 16

Explain why the percentages of bases from the middle part of the chromosome and the end part are different.

Answer 16

1. Different genes;
2. Have different (base) sequences/combinations of (bases);
3. (Pre-mRNA) transcribed from different DNA/codes for different proteins;

Question 17

Translation

Answer 17

The process of using the genetic code on mRNA, to assemble the amino acids of a polypeptide chains (that make up proteins), in the correct order.
1. In eukaryotes mRNA passes through the pores of the nuclear envelope and into the cytoplasm.
2. From then on the process in eukaryotic and prokaryotic cells is the same.
3. mRNA attaches to the ribosomes at the start codon (AUG).

4. A tRNA molecule with a complementary anticodon moves to the ribosome pairing up with the mRNA codon by complementary base pairing.

5.This tRNA carries a specific amino acid
6. A second tRNA attaches itself to the next codon on mRNA in the same way, lining up the next amino acid.
7. The 2 amino acids are joined together by a peptide bond; this releases the first tRNA back into the cytoplasm so that it can pick up another amino acid.

8. The energy to form the bond comes from the hydrolysis of ATP.
9. The process continues, producing a chain of linked amino acids (a polypeptide chain) – the chain folds as it is produced.
10. At the stop codon on the mRNA, the polypeptide chain is complete and moves away from the ribosome.