4.2 protein synthesis, genetic variation & mutations

Question 1

what is a gamete?

Answer 1

a mature haploid male or female reproductive cell that is able to fuse with another of the opposite sex during fertilisation to form a zygote.

Question 2

what is a zygote?

Answer 2

a diploid cell resulting from the fusion of two haploid gametes.

Question 3

explain what chromosomal mutations are caused by.

Answer 3

errors in cell division, resulting in haploid cells that contain variations in the number of chromosomes present.

Question 4

explain how polyploidy occurs.

Answer 4

• polyploidy occurs when organisms have three or more sets of chromosomes rather than the usual two.
• polyploidy occurs mostly in plants.

Question 5

explain ‘non-disjunction’.

Answer 5

• non-disjunction is the failure of a pair of homologous chromosomes to separate properly during meiosis.
• it causes changes in the number of individual chromosomes.

Question 6

explain how non-disjunction can lead to Down’s syndrome in humans.

Answer 6

• Down’s syndrome is caused by an individual having an extra copy of chromosome 21.
• non-disjunction means that chromosome 21 fails to separate properly during meiosis, causing one cell to get an extra copy and another to get no copies.
• when the gamete with the extra copy fuses with another gamete during fertilisation, the resulting zygote will have three copies of chromosome 21.

Question 7

explain what a genetic mutation is, and how genetic mutations occur.

Answer 7

• a genetic mutation is any change to the quantity or base sequence of the DNA of an organism.
• genetic mutations occur spontaneously when DNA is misread during replication.

Question 8

describe the three different types of genetic mutation.

Answer 8

substitution - this occurs when one base in the sequence is substituted with another.

deletion - this occurs when one base in the sequence is deleted, causing a frame-shift to the left.

addition - this is caused by the addition of a nucleotide during DNA replication, resulting in a frame-shift to the right.

Question 9

what is a frameshift mutation?

Answer 9

a mutation that shifts the “reading” frame of the genetic code, by inserting or deleting a nucleotide.

Question 10

explain why some mutations do not affect the order to the amino acid sequence.

Answer 10

• the degenerate nature of the genetic code means that most amino acids are coded for by more than one DNA triplet.
• as a result, not all substitution mutations will result in a change to the amino acid sequence of the protein, as some substitutions will still code for the same amino acid.

Question 11

explain how mutagenic agents increase the rate of mutations. give two examples of mutagenic agents.

Answer 11

• mutagenic agents act by directly altering the sequence of bases in an cell’s DNA, increasing the rate at which mutations can occur.
• ultraviolent radiation and ionising radiation are examples of mutagenic agents.

Question 12

what is a genome?

Answer 12

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

Question 13

what is a proteome?

Answer 13

the full range of proteins produced by the genome.

Question 14

give the main function of messenger RNA (mRNA)

Answer 14

to carry the genetic code from the DNA to the ribosomes, where it is used to form proteins during translation.

Question 15

what is a codon?

Answer 15

a sequence of three adjacent nucleotide bases that code for an amino acid.

Question 16

explain how the structure of transfer RNA (tRNA) relates to its function.

Answer 16

• tRNA is a single polynucleotide, folded into a clover shape and held together by hydrogen bonds.
• it is involved in translation, and carries the amino acids that are used to make specific proteins.
• one end the molecule has a specific sequence of three bases called an anticodon, which can recognise and decode an mRNA codon to assemble proteins.

Question 17

describe where transcription takes place in eukaryotic cells, and explain how is this different to prokaryotic cells.

Answer 17

• transcription in eukaryotic cells takes place in the nucleus of the cell.
• because prokaryotic cells do not have a nucleus, transcription takes place in the cytoplasm.

Question 18

give the enzyme associated with transcription, and explain its role.

Answer 18

• RNA polymerase is associated with transcription.
• its main function is to synthesise RNA molecules from a template DNA strand.
• it does this by lining up free RNA nucleotides alongside the exposed bases on the template strand.
• the free bases are attracted to the exposed bases, and join through complementary base pairing.

Question 19

what is a ‘stop codon’?

Answer 19

a sequence of three nucleotides that halts the production of mRNA from RNA polymerase during transcription.