Genetic Code And Protein Synthesis

Question 1

The structure of a chromosome in prokaryotes

Answer 1

Non-chromosomal shorter, circular loop of DNA.

Question 2

The structure of a chromosome in eukaryotes

Answer 2

Associated with proteins called histones.
Chromosome only visible during cell division.
Interphase- one DNA double helix dispersed.
Cell division- 2 DNA molecules called chromatids which are genetically identical and supercoiled.
Each chromosome is linear and has many genes along its length.

Question 3

What is a gene?

Answer 3

Section of DNA that contains coded information for making

• functional RNA (mRNA, tRNA, rRNA)
• Sequencing amino acids during protein synthesis (translation)
• determines the tertiary structure of a polypeptide.

Question 4

Gene locus

Answer 4

Position of each gene along a chromosome

Question 5

Features of the genetic code

Answer 5

20 different amino acids exist
Each amino acid has its own code of DNA bases
4 different bases (A,T,C,G)
As there are 4 bases, there are 64 different triplets (codons) that code for an amino acid (different combinations of the 4 bases).

Each triplet codon encodes for one specific amino acid.
Degenerate, non-overlapping and universal.

Question 6

Degenerate

Answer 6

Amino acids can have more than one codon.

Question 7

Non-overlapping

Answer 7

Each base is only read once

Question 8

Universal

Answer 8

Each codon codes for the same amino acid in all organisms.

Question 9

Genome

Answer 9

The complete set of genes in a cell including mitochondria/ chloroplast gene.

Question 10

Proteome

Answer 10

Full range of proteins produced by the genome.

Cell proteome- proteins produced by a specific cell as a result of which genes are switched on/ off.

Question 11

Structure and function of DNA.

Answer 11

• double polynucleotide chain.
• larger than mRNA and tRNA
• double helix
• deoxyribose pentose sugar
• bases ATCG
• mostly found in nucleus
• quantity is constant for all cells of a species (except gametes)
• chemically very stable

Question 12

Messenger RNA

Answer 12

Single polynucleotide chain
Smaller than DNA larger than tRNA
Single helix
Ribose pentose sugar
Bases ACG and uracil
Manufactured in nucleus and found throughout cell
Quantity varies from cell to cell and with level of metabolic activity
Not stable- molecules broken down in cells within a few days

Question 13

Transfer RNA

Answer 13

Single polynucleotide chain
Smallest
Clover-shaped
Ribose pentose sugar
Bases AGC and uracil
Manufactured in nucleus but found throughout the cell
Quantity varies throughout the cell and with level of metabolic activity.
More stable than mRNA but less stable than DNA.

Question 14

Transcription

Answer 14

• DNA helicase enzyme breaks hydrogen bonds between complimentary bases. Polynucleotide chain unwinds and separated.
• Free, activated RNA nucleotides from the pool which is present in the nucleus undergo complimentary base pairing (hydrogen bond) with the bases on the template strand.
• A-T, U-A, C-G, G-C.
• RNA polymerase moves along the strand and joins the nucleotides together to form a pre-mRNA molecule while the DNA strands rejoin behind it.
• When the RNA polymerase reaches the sequence of DNA it recognises as a stop triplet codon, it detaches.

Question 15

Splicing

Answer 15

In prokaryotic cells, no splicing is required as transcription results in the production of mRNA.

In eukaryotic cells, the DNA of a gene is made up of introns and exons.
Introns are a sequence of DNA bases that prevent the synthesis of a polypeptide as they don’t code for proteins and exons code for proteins. The introns base sequence is removed and exons are joined together.

-After, the mRNA molecules leave through a nuclear pore and are attracted to ribosomes to which is becomes attached, ready for translation.

Question 16

Translation

Answer 16

• The ribosome attached to the mRNA containing the AUG start codon at one end.
• The tRNA with the complimentary anticodon (UAC anticodon) binds to the mRNA in the ribosome (tRNA carried amino acid methionine).
• A tRNA with a complimentary anticodon pairs with the next codon on the mRNA.
• The tRNA carries a different amino acid and the two tRNA’s sit next to eachother in the ribosome.
• The amino acids from both tRNA’s join using energy from ATP for peptide bond formation.
• The ribosome moves along to the next codon and the uncharged (empty) tRNA leaves the ribosome. It will be charged with an amino acid in the cytoplasm.
• The elongation process continues adding 15 amino acids per second until the ribosome reaches the ‘stop’ codon.
• The ribosome, the mRNA and the last tRNA all separate.