Mock Revision Topic 4

Question 1

Describe how DNA is arranged in prokaryotes (2)

Answer 1

1. DNA molecules are short, circular
2. And not associated with proteins

Question 2

Describe how DNA is arranged in eukaryotes (2)

Answer 2

1. In the nucleus, DNA molecules are very long, linear
2. And associated with proteins called histones

Question 3

Name the organelles (other than the nucleus) which contain DNA and describe how it is arranged there (3)

Answer 3

1. The mitochondria and chloroplasts of eukaryotic
   cells also contain DNA
2. Which, like the DNA of prokaryotes, is short, circular
3. And not associated with protein

Question 4

What is a chromosome?

Answer 4

DNA molecule and its associated proteins

Question 5

What is a gene and what does it code for? (3)

Answer 5

1. A base sequence of DNA that codes for
2. The amino acid sequence of a polypeptide
3. A functional RNA (including ribosomal RNA and
   tRNAs)

Question 6

What is a locus?

Answer 6

The position a gene occupies on a particular DNA molecule

Question 7

What is a triplet and what does it code for? (2)

Answer 7

1. A sequence of three DNA bases
2. Which codes for a specific amino acid

Question 8

What is meant by saying that the genetic code is universal?

Answer 8

The same base triplets code for the same amino acids in all organisms (with some exceptions)

Question 9

What is meant by saying that the genetic code is non-overlapping?

Answer 9

Each base is only read once as part of a specific triplet

Question 10

What is meant by saying that the genetic code is degenerate?

Answer 10

Different base triplets or codons code for the same amino acid

Question 11

What is an exon?

Answer 11

Sequences of DNA bases in a gene code for amino acid sequences

Question 12

What is an intron?

Answer 12

Non-coding sequences of DNA bases in a gene (i.e. do not code for amino acid sequences)

Question 13

What is the genome?

Answer 13

The complete set of genes in a cell

Question 14

What is the proteome?

Answer 14

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

Question 15

Describe the similarities between mRNA and tRNA (3)

Answer 15

1. Both polymers of nucleotides
2. Nucleotides contain phosphate, ribose, adenine,
   uracil, cytosine, guanine
3. Both contain phosphodiester bonds

Question 16

Describe the differences between mRNA and tRNA (4)

Answer 16

1. mRNA has a linear shape, tRNA has a clover leaf shape
2. mRNA has no hydrogen bonds, tRNA does have hydrogen bonds
3. mRNA is variable in length, tRNA is not variable in length
4. mRNA has no amino acid binding site, tRNA has an amino acid binding site

Question 17

How does transcription differ in pro/eukaryotes? (3)

Answer 17

1. In prokaryotes, transcription results directly in the
   production of mRNA from DNA
2. In eukaryotes, transcription results in the production of pre-mRNA
3. This is then spliced to form mRNA

Question 18

What is transcription and where does it take place? (8)

Answer 18

1. In eukaryotes, transcription takes place in the nucleus
2. Hydrogen bonds between DNA bases break
3. One DNA strand acts as a template
4. Free RNA nucleotides align by complementary base pairing
5. In RNA, uracil base pairs with adenine on DNA ,or in RNA, uracil is used in place of thymine
6. RNA polymerase joins adjacent RNA nucleotides
7. By phosphodiester bonds between adjacent nucleotides
8. Pre-mRNA is spliced to form mRNA (introns are removed)

Question 19

What is the role of RNA polymerase? (3)

Answer 19

1. Joins adjacent RNA nucleotides
2. With phosphodiester bonds
3. To form pre-mRNA

Question 20

What is translation and where does it occur? Include the roles of tRNA and ATP (8)

Answer 20

1. Translation occurs at the ribosome in the cytoplasm
2. mRNA attaches to ribosomes
3. tRNA anticodons bind to complementary mRNA codons
4. tRNA brings a specific amino acid
5. Amino acids join by peptide bonds
6. With the use of ATP
7. tRNA released after the amino acid is joined to polypeptide
8. The ribosome moves along the mRNA to form the polypeptide

Question 21

What is a gene mutation in general?

Answer 21

A change in the DNA base sequence of chromosomes

Question 22

What is deletion?

Answer 22

The removal of one or more bases

Question 23

What is substitution?

Answer 23

The replacement of one or more bases by one or more different bases

Question 24

What is a mutagenic agent and what do they do? Include some named examples (6)

Answer 24

1. They can increase the rate of gene mutation
2. X-rays
3. Gamma rays
4. UV light
5. Alpha and beta particles
6. Benzene

Question 25

Why might a substitution not cause a change in the sequence of encoded amino acids? (2)

Answer 25

1. The genetic code is degenerate, more than one base triplet could code for the same amino acid 2. So the polypeptide remains unchanged

Question 26

What is genetic diversity?

Answer 26

The number of different alleles of genes in a population

Question 27

What are the principles of natural selection? (7)

Answer 27

1. Variation will already be present in a population due to random mutations which can result in new alleles of a gene 2. Many mutations are harmful but, in certain environments, the new allele of a gene might benefit an organism 3. Leading to increased reproductive success 4. A change in the environment results in organisms with the advantageous allele for a particular phenotype being selected for as they are better adapted 5. Organisms with the allele for this selected phenotype survive and reproduce, passing on this beneficial allele 6. While those without this allele do not (differential reproductive success) 7. Over many generations, the frequency of the advantageous allele increases in the population

Question 28

What is directional selection? Include an example (3)

Answer 28

1. This occurs when the environment is changing 2. The change in the environment selects for those organisms with alleles for a phenotype towards one extreme of a range 3. Such as antibiotic resistance in bacteria

Question 29

What is stabilising selection? Include an example (6)

Answer 29

1. This occurs when the environment is stable 2. Organisms with alleles for characteristics towards the middle of the range organisms survive 3. To reproduce and pass on their alleles to the next generation 4. Organisms with alleles for the extremes of this range are selected against 5. So are less likely to survive and reproduce so are less likely to pass on their alleles to the next generation 6. Such as human birth weights

Question 30

What is a species? (2)

Answer 30

1. Organisms with similar characteristics 2. Which can interbreed to produce fertile offspring

Question 31

What is courtship behaviour and why is it necessary? (6)

Answer 31

1. Acts as a necessary precursor to successful mating. It Allows organisms to: 2. Recognise members of their own species 3. Recognise members of the opposite sex 4. Identify a mate who is capable of breeding (i.e. sexually mature, fertile and biologically fit) 5. Synchronise mating so that it takes place when there is maximum probability of the sperm and egg meeting 6. Form a pair bond that will lead to successful mating and raising of offspring

Question 32

What is phylogenetic classification? Include an explanation of the term hierarchy and taxon (2)

Answer 32

1. Hierarchy is where smaller groups are placed within larger groups, with no overlap between groups 2. Each group a species is put into is called a taxon

Question 33

List the taxa in order starting with the largest (8)

Answer 33

1. Domain 2. Kingdom 3. Phylum 4. Class 5. Order 6. Family 7. Genus 8. Species