8. DNA, Genes and Protein Synthesis

Question 1

What is a gene?

Answer 1

A section of DNA containing the codedInformation for making polypeptides and functional RNA.

Question 2

what is the minimum number of bases that code for for each amino acid?

Answer 2

Three

Question 3

what is a degenerate code?

Answer 3

Most amino acids are coded for by more than one triplet.

Question 4

The start of a DNA sequence that codes for a polypeptide is always the same triplet, what is this triplet?

Answer 4

Methionine/ AUG

Question 5

what does it mean if the genetic code is non-overlapping?

Answer 5

Each base in the sequence is only read once, e.g. 123 456

Question 6

What does it mean if the genetic code is universal?

Answer 6

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

Question 7

what are exons?

Answer 7

Coding sequences.

Question 8

What are introns?

Answer 8

Non coding sequences.

Question 9

Calculate how many bases are required for a chain of six consecutive amino acids?

Answer 9

18

Question 10

Explain how a change in one base along the DNA molecule may result in an enzyme becoming non-functional

Answer 10

A different base might code for a different amino acid. The sequence of amino acids in the polypeptide produced will be different. This change in the primary structure of a protein might result in a different tertiary structure. The enzyme shape will be different and may not fit the substrate. The enzyme substrate complex cannot be formed and so the enzyme is non-functional.

Question 11

Describe the differences in the DNA of prokaryotic cells and eukaryotic cells?

Answer 11

In prokaryotic cells the DNA molecules are shorter, form a circle and are not associated with the protein molecules; therefore they do not have a chromosomes.
In eukaryotic cells DNA molecules are longer, linear rather than a circle, and occur in association with proteins called histones to form structures called chromosomes.

Question 12

State the function of the protein found in chromosomes.

Answer 12

Proteins called histones fix the DNA into position.

Question 13

Describe the structure of a gene?

Answer 13

Chromosomes only visible when a cell is dividing. Chromosomes appear as two threads joined by a single point. Each thread is called a chromatid and are attached to the centromere. The length of DNA found in each cell is highly coiled. The double helix is wound around histones to fix it in position. The DNA-histone complex is coiled. The coil is further coiled before being packaged into the chromosome. This allows a considerable length of DNA to be condensed into a single chromosome.

Question 14

what is a homologous pair of chromosomes?

Answer 14

A pair of chromosomes one maternal and one paternal, that have the same gene loci and therefore determine the same features. They are not necessarily identical, however, as individual alleles of the same gene may vary, e.g. one chromosome may carry the allele of blue eyes and the other for brown eyes. Homologous chromosomes are capable of pairing during meiosis.

Question 15

What is an allele?

Answer 15

One number of alternative forms of a gene.

Question 16

What is the role of mRNA?

Answer 16

Transfers the DNA code from the nucleus to the cytoplasm and acts as a type of messenger. It is small enough to leave the nucleus through the nuclear pore and enter the cytoplasm where the coded information that it contains is used to determine the sequence of amino acids and the proteins which are synthesised there.

Question 17

Define codon

Answer 17

The sequence of three bases on mRNA that codes for a single amino acid.

Question 18

Define ‘genome’

Answer 18

The complete set of genes in a cell including those in mitochondria and chloroplasts

Question 19

Define ‘proteome’?

Answer 19

The full range of proteins produced by the genome.

Question 20

State three ways in which the molecular structure of RNA differs from the structure of DNA.

Answer 20

RNA is smaller than DNA, RNA is normally a single strand and DNA is a double helix, the sugar in RNA is ribose while the sugar in DNA is deoxyribose, in RNA the base uracil replaces the base thymine found in DNA.

Question 21

Describe the structure of mRNA

Answer 21

mRNA is a long strand that is arranged in a single helix. The base sequence of mRNA is determined by the sequence of bases on a length of DNA in transcription. It possesses information in the form of codons, which determine the amino acid sequence of a specific polypeptide.

Question 22

Describe the structure of tRNA

Answer 22

tRNA is a relatively small molecule that is made up of around 80 nucleotides. It is a single-stranded chain folded into a cloverleaf shape, with one end of the chain extending beyond the other, this is the part of the tRNA molecule to which an amino acid can easily attach. Opposite end of the tRNA molecule is a sequence of three other organic bases, known as the anticodon. Each tRNA is specific to one amino acid and has an anti codon that is specific to that amino acid.

Question 23

Distinguish between a codon and an anticodon

Answer 23

A codon is the triplet of bases on mRNA that codes for an amino acid. An anticodon is the triplet of bases on tRNA that is complimentary to a codon.

Question 24

Describe the process of transcription

Answer 24

An enzyme acts on a specific region of DNA causing the two strands to separate and expose the nucleotide bases.
The nucleotide bases on the two DNA strands, known as the template strand, pair with the complementary nucleotides from the pool which is present in the nucleus.The enzyme RNA polymerase then moves along the strand and joins the nucleotides together to form a pre-mRNA molecule.
When the RNA polymerase reaches a particular sequence of the bases on the DNA that it recognises as a stop triplet code, it detaches and the production of pre-mRNA is complete.

Question 25

Describe the role of RNA polymerase in transcription

Answer 25

The enzyme RNA polymerase moves along the template DNA strand, causing the bases on the strand to join with the individual complementary nucleotides from the pool that is present in the nucleus. The RNA polymerase adds the nucleotides one at a time, to build a strand of pre-mRNA until it reaches a particular sequence of bases on the DNA that recognises as a stop code.

Question 26

State which other enzyme is involved in transcription and describe its role

Answer 26

DNA helicase acts on a specific region of the DNA molecule to break the hydrogen bonds between the bases, causing the two strands to separate and expose the nucleotide bases in that region.

Question 27

why is splicing necessary?

Answer 27

Splicing is necessary because pre-mRNA has nucleotide sequences dervived from introns in DNA. These introns are non-functional and, if left on the mRNA, would lead to the production of non-functional polypeptides or no polypeptides at all. Splicing removes these non-functional introns from the mRNA

Question 28

Describe the process of translation

Answer 28

A ribosome becomes attached to the starting codon at one end of the mRNA molecule.
The tRNA molecule with a complimentary anticodon sequence moves to the and pairs up with the codon on the mRNA. The tRNA carries a specific amino acid.
A tRNA molecule with a complimentary anticodon pairs with the next codon on the mRNA. This tRNA molecule carries another amino acid.
The ribosome moves along the mRNA, bringing together two tRNA molecules at any one time, each pairing up with the corresponding two codons on the mRNA.
The two amino acids on the tRNA are joined by peptide bond using an enzyme and ATP which is hydrolysed to provide energy.
The ribosome moves onto the third codon in the sequence of the mRNA, linking the amino acids on the 2nd and 3rd tRNA molecules.
As this happens, the first tRNA is released from its amino acid and is free to collect another amino acid from the amino acid pool in the cell.
The process continues this way until a polypeptide chain is built.
The polypeptide synthesis continues until the ribosome reaches a stop codon, at this point the ribosome, mRNA and the last tRNA molecule all separate and the polypeptide chain is complete.

Question 29

After translation, how is the protein assembled?

Answer 29

• The polypeptide structure is coiled, producing its secondary structure.
• The secondary structure is folded, producing its tertiary structure.
• Different polypeptide chains, along with non-protein groups associate to form the quaternary structure.

Question 30

A codon found on a section of mRNA has the sequences of bases AUC. List the sequence of bases found on:

• The tRNA anticodon that attaches to your the codon
• The template strand of DNA that formed the mRNA codon

Answer 30

• tRNA anticodon= UAG

- DNA template strand= TAG

Question 31

A strand of mRNA has 64 codons but the protein produced from it has only 63 amino acids. Suggest a reason for this difference.

Answer 31

On one of the codons is a stop codon that indicates the end of polypeptide synthesis. Stop codons do not code for any amino acid so there is one less amino acid and than codons.