Biology AS Chapter 8 - DNA, Genes and Protein Synthesis

Question 1

Describe the process of transcription (6 marks)

Answer 1

• DNA helicase binds to a complementary start codon on the DNA and breaks hydrogen bonds
• DNA unwinds forming a template strand
• Each exposed base binds to a complementary, free floating nucleotide
• C with G, A with U
• RNA polymerase moves along the DNA strand, causing the phosphodiester bonds to form between nucleotides
• This forms pre-mRNA which is then spliced to remove the introns so it can fit through nuclear pores

Question 2

Describe the process of translation (4-6 marks)

Answer 2

-mRNA moves out of the nucleus via nuclear pores and attaches to a ribosome
- tRNA carrying amino acids bind their anticodons to the complementary codon on the mRNA
- A second tRNA does the same and the two amino acids form a peptide bond
- The first tRNA molecule moves away
- The process continues and only ends when a stop codon is reached, forming a polypeptide

Question 3

Define the term gene

Answer 3

A section of DNA that contains the coded information for making polypeptides and functional RNA

Question 4

What are scientists reasoning as to why there must be a minimum of three bases that coded for each amino acid?

Answer 4

• only 20 different amino acids occur in proteins
• each amino acid must have its own code for each of the bases on DNA
• only 4 bases in DNA
• if each base coded for an amino acid, there would only be 4 amino acids
• using a pair of bases, 16 (4 squared) different amino acids are possible which is still inadequate
• three bases produce 64 (4 cubed) different codes, more than enough to satisfy the requirements of 20 amino acids

Question 5

Why is the genetic code known as degenerate?

Answer 5

Because most amino acids are coded for by more than one triplet

Question 6

What amino acid always starts the DNA sequence?

Answer 6

Methionine - if it is not later involved in the polypeptide then it is removed

Question 7

What is a stop codon?

Answer 7

A triplet that does not code for any amino acids

Question 8

Why is the genetic code described as non-overlapping?

Answer 8

Each base in the sequence is only read once - 123456 is read 123, 456 and not 123, 234 etc etc

Question 9

Why is the genetic code described as universal?

Answer 9

With few minor exceptions, each triplet codes for the same amino acid in all organisms which provides indirect evidence for evolution

Question 10

What are the difference between introns and exons?

Answer 10

Exons are coding sequences, and introns are not - they are removed after transcription has occurred

Question 11

What are histones?

Answer 11

Proteins which together with DNA, make up the chromosomes of eukaryotic cells

Question 12

How is lots of DNA packed into a single chromosome?

Answer 12

The double helix is wound around histones to fix it in position. This DNA-histone complex is the coiled. The coil is looped and further coiled before being packed into the chromosome

Question 13

What are homologous chromosomes?

Answer 13

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, homologous chromosomes are capable of pairing during meiosis

Question 14

Define the term diploid

Answer 14

Cells in which the nucleus contains 2 sets of chromosomes

Question 15

Define the term allele

Answer 15

One of a number of alternative forms of a gene

Question 16

What is a mutation and how does this lead to the production of a different protein?

Answer 16

Any changes in the base sequence of a gene produces a new allele of that gene (= mutation). This results in a different sequence of amino acids bring coded for, which leads to the production of a different polypeptide and hence a different protein

Question 17

Why can a mutation cause problems for an organism?

Answer 17

The different and new protein produced may not function properly or at all. If it is an enzyme, it may have a different shape which might not fit the enzymes substrate and so the enzyme cannot function, which may lead to serious consequences to the organism

Question 18

Define the term codon

Answer 18

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

Question 19

Define the term genome

Answer 19

The complete set of genes in a cell, including those in the mitochondria and/or chloroplasts

Question 20

Define the term complete proteome

Answer 20

The full range of proteins produced by the genome

Question 21

Define the term proteome

Answer 21

The proteins produced by a given type of cell under a certain set of conditions

Question 22

What is RNA?

Answer 22

A polymer made up of repeating mononucleotide sub-units that forms a single strand

Question 23

What is RNA made up of?

Answer 23

• The pentose sugar ribose
• One of the bases adenine (A), guanine (G), cytosine (C) or uracil (U)
• A phosphate group

Question 24

What is the structure of mRNA, and how is this suited to its function?

Answer 24

It is a long strand that is arranged in a single helix. Its function is to act as a template for protein synthesis and it is suited to this because it processes information in the form of codons and the sequence of codons determines the amino acid sequence of a specific polypeptide

Question 25

Describe the structure of tRNA

Answer 25

It is a relatively small molecule that is made up of around 80 nucleotides. It is single stranded and folded into a clover leaf shape with one end extending beyond the other. This is the part of the tRNA to which an amino acid can easily attach. At the opposite end is the anticodon which is complementary to a codon

Question 26

Why must there be as many tRNA molecules as there are coding triplets?

Answer 26

As the genetic code is degenerate

Question 27

Give 3 differences between the structure of DNA and mRNA

Answer 27

• DNA is a double polynucleotide chain, mRNA is single
• DNA sugar is deoxyribose, mRNA is ribose
• DNA bases are A,T,G and C, mRNA bases are A,G,C and U
• DNA is more chemically stable than mRNA

Question 28

Give 3 differences between the structure of mRNA and tRNA

Answer 28

• mRNA is bigger than tRNA
• mRNA is a single helix molecule, tRNA is clover shaped
• tRNA is more chemically stable than mRNA
• tRNA has an anticodon, mRNA has codons
• tRNA has amino acid attachment site, mRNA does not