1.3 Gene Expression

Question 1

what is a gene, a genotype, a phenotype & gene expression?

Answer 1

• genes are DNA sequences that code for particular proteins
• different forms of genes are called alleles
• the genes a person has makes up their genotype
• a person’s phenotype is determined by the proteins produced as a result of gene expression
• a person’s phenotype can also be influenced by environmental factors
• gene expression involves transcription and translation of DNA sequences

Question 2

what is the structure of RNA?

Answer 2

RNA = ribonucleic acid

The basic sub-unit for any nucleic acid is the nucleotide.

The three components of an RNA nucleotide are:
- the phosphate
- the ribose sugar
- the base

There are 4 bases: adenine, uracil, cytosine and guanine

The base pairing rule is that adenine pairs with uracil and cytosine pairs with guanine.

Question 3

what are the differences between DNA & RNA?

Answer 3

DNA is found in the nucleus.
RNA is found in the cytoplasm and the nucleus.

DNA has 2 strands.
RNA has 1 strand.

DNA contains a deoxyribose sugar within the nucleotide.
RNA contains a ribose sugar within the nucleotide.

DNA — adenine pairs with thymine
RNA — adenine pairs with uracil

Question 4

what are the 3 types of RNA?

Answer 4

mRNA (messenger RNA)
- found in the nucleus and the cytoplasm
- it carries a copy of the DNA code from the nucleus to the ribosome
- has a triplet of bases known as a codon

tRNA (transfer RNA)
- found in the cytoplasm
- it carries a specific amino acid to the ribosome
- have an exposed triplet of bases, known as an anticodon

rRNA (ribosomal RNA)
- found in the ribosome
- rRNA and proteins from the ribosome

Question 5

describe ‘transcription’

Answer 5

Transcription takes place in the nucleus.

During transcription, a complementary copy of the DNA code is made. The molecule is called mRNA.

1. RNA polymerase moves along the DNA, unwinding the double helix and breaking the hydrogen bonds between bases.
2. RNA polymerase synthesises a primary transcript of mRNA from RNA nucleotides using complementary base pairing.

Question 6

describe ‘RNA splicing’

Answer 6

After transcription, RNA splicing forms a mature transcript of mRNA.

During RNA splicing, introns (non-coding regions) are removed and exons (coding regions) are joined together (the order of the exons does not change).

Question 7

describe ‘alternative RNA splicing’

Answer 7

In some instances, some exons may be removed from the primary transcript (as well as introns). This means they will not make up the mature transcript mRNA and the amino acids they code for will not be in the final protein. This means a different protein will be made.

Because of alternative RNA splicing, different proteins can be expressed from one gene.

Once the mature transcript mRNA has been produced, it leaves the nucleus and enters the cytoplasm to find a ribosome for the next stage in protein synthesis, translation.

Question 8

describe ‘translation’

Answer 8

Translation takes place at a ribosome within the cytoplasm.

During translation, the mRNA code is ‘read’ producing a polypeptide chain which will later be packaged to produce a protein. The order of bases on the mRNA determines the order of amino acids in the polypeptide chain.

1. A ribosome attaches to the mRNA and recognises a ‘start’ codon.
2. tRNA molecules bring the correct amino acid to the ribosome.
3. Anticodons (on the tRNA) bond to codons (on the mRNA) using complementary base pairing.
4. Peptide bonds form between the amino acids forming a polypeptide chain.
5. The ribosome continues along the mRNA until it recognises a ‘stop’ codon.

Question 9

what is the function of a protein?

Answer 9

proteins come in all sorts of different shapes and sizes which determines their function

Question 10

What are the 3 types of proteins and their functions with named examples?

Answer 10

Fibrous Protein
- e.g. keratin and collagen (inelastic) actin and myosin (elastic)
- rope-like structure
- polypeptides wind round each other to form strong fibres
- can be elastic or inelastic
- elastic is used in muscle contraction
- inelastic is used for strength/support

Globular Protein
- e.g. enzymes, antibodies and hormones
- folded up like a ball of string forming specific shapes (which can be important for their functions)

Conjugated Proteim
- e.g. iron in the haem of haemoglobin and magnesium in chlorophyll
- a globular protein which contains a non-protein part