B Coding for Proteins

Question 1

What are the features of the genetic code?

Answer 1

1. Universal (the same triplet codes for the same amino acid in all organisms).
2. Degenerate (more than one triplet codes for the same amino acid).
3. Non-overlapping (each nucleotide is only part of one triplet).

Question 2

What is a gene?

Answer 2

Base sequence of DNA that codes for the amino acid sequence of a polypeptide.

Question 3

What are introns?

Answer 3

Non-coding base sequences.

Question 4

What are exons?

Answer 4

Base sequences that code for an amino acid.

Question 5

What is the locus?

Answer 5

The fixed position of each gene on a DNA molecule.

Question 6

What is the genome?

Answer 6

The complete set of genes in a cell.

Question 7

What is the proteome?

Answer 7

The full range of proteins a cell is able to produce.

Question 8

What is the structure of an RNA nucleotide?

Answer 8

• A pentose (ribose) sugar, a phosphate group and a base.
• RNA contains the bases adenine, cytosine, guanine and uracil (which is a pyrimidine that forms 2 hydrogen bonds with adenine).
• RNA nucleotides join together as a result of a condensation reaction forming a phosphodiester bond.

Question 9

What is the function of RNA?

Answer 9

Transfers genetic information from the nucleus, to the ribosomes to produce proteins (protein synthesis).

Question 10

What is mRNA (messenger RNA)?

Answer 10

• A single strand of RNA nucleotides arranged in a nucleus.
• Carries genetic information out of the nucleus, to the ribosomes where it is used for protein synthesis.

Question 11

Describe the process of translation

Answer 11

1. RNA polymerase binds to DNA and runs along it, breaking the hydrogen bonds between bases, forming two exposed strands.
2. RNA nucleotides from the nucleus are added to the exposed, complementary base pairs by RNA polymerase on one strand (the template strand), and RNA polymerase then joins the nucleotides together in a condensation reaction.
3. As RNA polymerase moves along the DNA it reattaches the DNA helix behind it. Once the RNA strand is complete the RNA strand and RNA polymerase detach from the DNA, forming the original DNA molecule and a stand of mRNA.
4. Since RNA is single stranded, it is complementary to the template stand of DNA, and identical to the coding strand (with U not T).

Question 12

What is the difference between a triplet and a codon?

Answer 12

• A triplet is a set of 3 bases in DNA that code for 1 amino acid.
• A codon is a set of 3 bases in RNA that code for 1 amino acid.
• Therefore transcription produces codons that are complementary to the triplets from the template strand.

Question 13

What are anticodons?

Answer 13

A sequence of bases complementary to a codon, attached to a small molecule (tRNA) that also carries an amino acid.

Question 14

Describe the process of translation:

Answer 14

1. Once the mRNA strand reaches the ribosome, the ribosome attaches to the strand. Then a tRNA molecule with the complementary anticodon (and carrying an amino acid) binds to the first codon.
2. Another tRNA molecule carrying the next anticodon then binds, and the ribosome forms a peptide bond between these amino acids in a condensation reaction.
3. The ribosome then moves to the next codon on the mRNA strand, causing the anticodon (therefore the tRNA molecule) to detach from the amino acid and the ribosome.
4. The ribosome continues to move along the mRNA strand, adding amino acids to the polypeptide chain until the chain is complete. The polypeptide chain, ribosome, amino acid and tRNA molecule then all detach.

Question 15

What is tRNA?

Answer 15

• A single strand of RNA that contains fewer nucleotides than a strand of mRNA.
• The RNA strand is folded to form a clover shape where base pairs are held together by hydrogen bonds.
• tRNA contains an amino acid binding site (where part of one strand extends beyond the other) and an anticodon.
• tRNA transfers amino acids to the polypeptide chain.

Question 16

What is rRNA (ribosomal RNA)?

Answer 16

• rRNA combines with other proteins to form ribosomes.
• rRNA is responsible for catalysing the peptide bonds that form during translation.