8.1 - 8.5 DNA, genes and protein synthesis

Question 1

Describe the differences between eukaryotic and prokaryotic DNA

Answer 1

Eukaryotic:
- Linear DNA
- DNA associated with histones
- Longer DNA molecules

Prokaryotic:
- Circular DNA & plasmids
- DNA not associated with histones
- Shorter DNA molecules

Question 2

Describe the structure of a chromosome

Answer 2

• DNA double helix made up of nucleotides with complimentary base pairs between strands
• DNA wraps twice around each histone protein to form nucleosomes
• DNA-histone complex is coiled
• Coils fold to form loops
• Loops pack together to form a chromosome

Question 3

Define locus

Answer 3

The position of a gene on a chromosome

Question 4

Define allele

Answer 4

A version of a gene

Question 5

Define homologous pair of chromosomes

Answer 5

A pair of chromosomes (1 paternal, 1 maternal) that contain the same genes in the same loci but may have different alleles

Question 6

Define autosomes

Answer 6

Non-sex chromosomes

Question 7

What is the human karyote?

Answer 7

All 23 pairs of chromosomes

Question 8

Define gene

Answer 8

• A sequence of bases that code for the amino acid sequence of a polypeptide
• Or a sequence of bases that codes for a functional RNA (rRNA & tRNA)

Question 9

Define base triplet

Answer 9

Three DNA bases that code for an amino acid

Question 10

Define a codon

Answer 10

An mRNA base triplet that codes for one amino acid

Question 11

What are the 3 key characteristics of the genetic code and what do they mean?

Answer 11

1. Degenerate - Each amino acid may be coded for by more than one codon
2. Non-overlapping - A base belongs to only one codon
3. Universal - Almost all organisms have the same codons for the same amino acids

Question 12

Define introns

Answer 12

Introns = interruptions

Non-coding regions of DNA

Question 13

Define exons

Answer 13

Exons = expressed

Coding regions of DNA

Question 14

What are telomeres and why are they important?

Answer 14

• Protective structures made of non-coding DNA which ‘seal’ the ends of chromatids in chromosomes
• This acts as a buffer region of non-essential DNA & ensures that no important coding sections near the ends of the DNA molecules are left out of the replication process

Question 15

Define genome

Answer 15

The complete set of genes in a cell

Question 16

Define complete proteome

Answer 16

The full range of proteins that an organism is able to produce

Question 17

Define cellular proteome

Answer 17

The collection of proteins found in a particular cell type under a particular set of environmental conditions

Question 18

Describe the differences between DNA, mRNA and tRNA

Answer 18

See table

Question 19

What are the 2 stages of protein synthesis?

Answer 19

1. Transcription
2. Translation

Question 20

Describe transcription

Answer 20

1. Section of DNA encoding the desired gene/protein unwinds
2. DNA helicase attaches at the start of the gene and the hydrogen bonds between the complementary base pairs are broken
3. The non-coding DNA strand is used as a template
4. Free RNA nucleotides attach via hydrogen bonds to the exposed complimentary bases (Adenine-Uracil, Cytosine-Guanine)
5. RNA polymerase forms pre-mRNA by joining adjacent RNA nucleotides in condensation reactions forming phosphodiester bonds
6. The pre-mRNA strand detaches from the DNA
7. The pre-mRNA is spliced to remove non-coding introns
8. mRNA leaves the nucleus through a nuclear pore

Question 21

What is the difference in transcription between eukaryotes and prokaryotes?

Answer 21

The splicing stage of transcription only happens in eukaryotes not prokaryotes (as prokaryotic DNA does not contain introns)

Question 22

Describe translation

Answer 22

1. mRNA enters the cytoplasm and attaches to a ribosome
2. The ribosome moves to the start codon (AUG)
3. A tRNA molecule (which carries a specific amino acid depending on its anticodon) binds to the mRNA via hydrogen bonding between the complementary base pairs of the codon and anticodon
4. The ribosome moves along the mRNA to the next codon and a second tRNA molecule attaches (the ribosome has 2 tRNA binding sites)
5. A peptide bond forms between adjacent amino acids using energy from the hydolysis of ATP
6. The first tRNA detaches and collects another specific amino acid
7. The ribosome continues to move along the mRNA, reading the codons until a stop codon is reached
8. The stop codon causes the polypeptide to be released from the ribosome
9. The polypeptide then undergoes post-translational modifications that produce a functional protein (or a protein awaiting activation)

Question 23

List some examples of post-translational modifications

Answer 23

• Some proteins are folded in the RER
• Proteins are often cleaved (made shorter)

Modifications in the Golgi include:
- Addition of carbohydrate groups to make glycoproteins
- Addition of lipid groups to make lipoproteins
- Addition of one or more poylpeptides to make a quaternary protein
- Addition of prosthetic groups