Topic 4

Question 1

What is a gene

Answer 1

A gene is a base sequence of DNA that codes for:
• the amino acid sequence of a polypeptide 

• a functional RNA (including ribosomal RNA and tRNAs).
A gene occupies a fixed position, called a locus, on a particular DNA molecule.
A sequence of three DNA bases, called a triplet, codes for a specific amino acid. The genetic code is universal, non-overlapping and degenerate.

Question 2

What do these words mean when referring to the genetic code?

Answer 2

Universal: The same three bases code for the same amino acid in all organisms

b. Non-overlapping: Each base is only part of one ‘triplet’ (it is only ‘read’ once)
c. Degenerate: Some amino acids are coded for by more than one set of three bases

Question 3

What is an Exon?

Answer 3

A base sequence within a gene which codes for a sequence of amino acids

Question 4

What is an intron?

Answer 4

A base sequence within a gene which is non-coding

Question 5

What is meant by a ‘non-coding’ base sequence

Answer 5

Does not code for amino acid/tRNA/rRNA;

Question 6

What is a “non-coding multiple repeat base sequence”

Answer 6

A short sequence of bases which repeats a number of times, e.g. CAGCAGCAGCAG, or ATATATAT
These can be found between genes and do not function as sequences that code for amino acids

Question 7

What is a genome

Answer 7

The complete set of genes in a cell

Question 8

What is a preteome

Answer 8

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

Question 9

What is tRNA

Answer 9

A folded strand of RNA which is held in a clover shape due to H-bonds. Each tRNA carries a specific amino acid.

Question 10

What is mRNA

Answer 10

A linear strand of RNA which is transcribed from a DNA template and has no H-bonds. Its length depends on the length of the gene. (NOTE: in eukaryotes, pre-mRNA is transcribed from DNA)

Question 11

Structure of mRNA

Answer 11

Compare the structure of mRNA to the structure of tRNA

Linear
No hydrogen bonds
Doesn’t have a binding site for an amino acid
Always the same length

Question 12

Structure of tRNA

Answer 12

Folded and clover shaped
Has hydrogen bonds
Has a binding site for a specific amino acid
Always the same length

Question 13

DNA triplet

Answer 13

three bases on DNA which code for an amino acid

Question 14

Codon

Answer 14

three bases on mRNA which code for an amino acid

Question 15

Anticodon

Answer 15

three bases on tRNA which are complementary to a codon. Each tRNA carries a specific amino acid

Question 16

What is transcription

Answer 16

In eukaryotes: The formation of pre-mRNA using DNA as a template
In prokaryotes: The formation of mRNA using DNA as a template

Question 17

Describe how mRNA is formed:

Answer 17

1. DNA Helicase Breaks hydrogen bonds between DNA strands so that they separate
2. Only one DNA strand acts as template;
3. RNA nucleotides attracted to exposed bases;
4. (Attraction) according to base pairing rule;
5. (Complementary) base pairing so A→U, T→A, C→G, G→C;
6. RNA polymerase joins (RNA) nucleotides together;
7. Pre-mRNA spliced to remove introns;

Question 18

Splicing

Answer 18

The removal of introns from the pre-mRNA to form mRNA

Question 19

How and why is transcription in prokaryotes different to eukaryotes?

Answer 19

In eukaryotes pre-mRNA is formed and then splicing occurs to remove introns and form mRNA
In prokaryotes mRNA is formed and it doesn’t contain any introns so there is no splicing

Question 20

What is translation

Answer 20

The production of polypeptides at ribosomes, using information in the sequence of codons carried by mRNA

Question 21

Describe how a protein is formed from the mRNA template

Answer 21

mRNA leaves (nucleus) through nuclear pore

2. mRNA attaches to ribosome;

3. codon on mRNA binds to an anti-codon on tRNA;
4. The tRNA brings a specific amino acid;
5. Another tRNA binds to the 2nd binding site in the ribosome (bringing another specific amino acid)
6. A peptide bond forms between the amino acids in a condensation reaction (using energy from ATP)
7. the tRNA detaches and collects another amino acid
8. The ribosome moves along to the next codon on the mRNA (and the process repeats)
9. so the sequence of codons determines the order of amino acids in the polypeptide

Question 22

What is meant by genetic diversity?

Answer 22

The number of different alleles of each gene

Question 23

What is a mutagenic agent? Give three examples of mutagenic agents

Answer 23

Something which increases the chance of mutations occurring, e.g.

1. High energy radiation, e.g. gamma rays, x-rays, UV light
2. Ionising radiation, e.g. alpha or beta radiation
3. Chemicals such as benzene or mustard gas

Question 24

What is a deletion mutation?

Answer 24

When there is one or more bases missing from the DNA

Question 25

What is the consequence of deleting 1 or 2 bases?

Answer 25

There is frame shift, so all the following triplets will be different. This results in a completely different sequence of amino acids

Question 26

Why are deletions of 3 bases (or multiples of 3) less serious than deletions of 1 or 2 bases (or multiples of 2 bases)?

Answer 26

Deletions of multiples of 3 bases do not cause frame shift – so only one amino acid is affected

Question 27

What is a substitution mutation?

Answer 27

When an incorrect nucleotide is inserted in place of another nucleotide

Question 28

What is a silent mutation? What is the consequence for the polypeptide?

Answer 28

When the new codon still codes for the same amino acid because the genetic code is degenerate. The polypeptide formed is identical.

Question 29

What is a non-sense mutation? And what is the consequence for the polypeptide?

Answer 29

When the new codon codes for a stop codon, so the protein is ‘truncated’ (i.e. it is shorter than it should be

Question 30

What is a missense mutation? And what is the consequence for the polypeptide?

Answer 30

When the new codon codes for a different amino acid. So the protein has one different amino acid.

Question 31

What is chromosome non-disjunction? And when does it occur? What is the consequence?

Answer 31

When homologous pairs of chromosomes do not separate during meiosis. This results in daughter cells which either have extra chromosomes or have chromosomes missing.

Question 32

What is the definition of a haploid cell? How many chromosomes does a human haploid cell? Give an example of a haploid cell? Draw an example

Answer 32

Haploid cells contain just one copy of each chromosome. • In humans haploid cells have 23 chromosomes. • The gametes are examples (egg and sperm cell).

Question 33

Why is it so important that gametes are haploid?

Answer 33

TO MAINTAIN CHROMOSOME NUMBER AT FERTILISATION

Question 34

What is a the definition of a diploid cell? A human diploid cell has how many chromosomes? Draw an example

Answer 34

Compared with diploid cells which contain 2 copies of each chromosome. • In humans there are 46 chromosomes (23 pairs) in a diploid cell

Question 35

What is an allele

Answer 35

Different forms of the same gene

Question 36

What are homologous chromosomes and explain why they are not genetically identical?

Answer 36

Homologous chromosomes have the same genes at the same loci | • They are not genetically identical as they different forms of the gene (alleles).

Question 37

At what division does crossing over occur? What is crossing over? Why does crossing over occur?

Answer 37

* Crossing over happens during the first division of meiosis. * Crossing over is when the ‘arms’ of homologous pairs twisted up. * The point at which the cross over is called a chiasma * This leads to the exchange of alleles between homologous chromosomes * THIS IS A RARE PROCESS

Question 38

At what division does independent assortment occur? What is it? And why does it occur?

Answer 38

Independent assortment happens during the first division of meiosis. • It is when the homologous pairs separate and go to opposite poles of the cell. Independent assortment means that the combination of chromosomes that go into each gamete is random.

Question 39

Give 2 reasons that it is vital for meiosis to take place

Answer 39

To make gametes which are haploid – so that when fertilisation takes place the chromosome number is maintained from one generation to the next • To create genetic variation in gametes and offspring

Question 40

Describe what happens to chromosomes in meiosis.

Answer 40

* DNA replication takes place so that each chromosomes replicates * The chromosomes become condensed
 * Homologous chromosomes pair up (forming bivalents) * Crossing-over can occur where there is an exchange of alleles between homologous pairs * spindle fibres attach to the chromosomes at their centromere 
 * so the bivalents line up at the equator of the cell * The spindle fibres contract causing the homologous chromosomes to separate and be pulled to opposite poles of the cell * In the 2nd division of meiosis the sister chromatids are separated

Question 41

HOW does meiosis cause variation

Answer 41

? 1. Crossing-over 2. Independent segregation of homologous chromosomes in meiosis I (1st division) 3. Independent segregation of chromatids in meiosis II (2nd division)

Question 42

Explains the advantage of variation?

Answer 42

Any three from:
 1. Different adaptations / some better adapted;
 2. Some survive 3. To reproduce;
 4. Pass on gene / allele;
 5. Allows for survival in changing environment / different environment

Question 43

Meiosis

Answer 43

Reduces the chromosome number Homologous chromosomes associate in pairs Crossing-over / chiasmata formation Two divisions / → 4 offspring cells Genetically different cells produced

Question 44

Mitosis

Answer 44

Mitosis Maintains the same chromosome number as in the parent nucleus Homologous chromosomes do not pair up No crossing-over One division / → 2 offspring cells Genetically identical cells produced