Topic 3 - The Genome

Question 1

What is DNA?

Answer 1

A double-stranded polymer of nucleotides, wound to form a double helix through weak hydrogen bonds

Question 2

What are the monomers of DNA?

Answer 2

Nucleotides

Question 3

What are DNA nucleotides made up of?

Answer 3

● Common sugar
● Phosphate group
● One of four bases: A, T, C or G

Question 4

State the full names of the four bases found in nucleotides

Answer 4

● Adenine
● Thymine
● Cytosine
● Guanine

Question 5

Describe how nucleotides interact to form a molecule of DNA

Answer 5

● Sugar and phosphate molecules join to form a
sugar-phosphate backbone in each DNA strand
● Base connected to each sugar
● Complementary base pairs (A pairs with T, C pairs with G) joined by weak hydrogen bonds

Question 6

Define genome

Answer 6

The entire genetic material of an organism

Question 7

What is a chromosome?

Answer 7

A long, coiled molecule of DNA that carries genetic information in the form of genes

Question 8

Define gene

Answer 8

A section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein

Question 9

Describe the method used to extract DNA from fruit

Answer 9

1. Place a piece of fruit in a beaker and crush it
2. Add detergent and salt, mix
3. Filter the mixture and collect the liquid in a test tube
4. Pour chilled ethanol into the test tube
5. DNA precipitates forming a fibrous white solid
6. Use a glass rod to collect the DNA sample

Question 10

Why is detergent added to the crushed fruit?

Answer 10

It disrupts the cell membranes, releasing

DNA into solution

Question 11

Why is salt added to the crushed fruit?

Answer 11

Salt encourages the precipitation of DNA

Question 12

Why is chilled ethanol added rather water?

Answer 12

DNA is insoluble in ethanol, encouraging its precipitation

Question 13

Explain how a gene codes for a protein

Answer 13

● A sequence of three bases in a gene forms a triplet
● Each triplet codes for an amino acid
● The order of amino acids determines the structure
(i.e. how it will fold) and function of protein formed

Question 14

Why is the ‘folding’ of amino acids important in proteins such as enzymes?

Answer 14

The folding of amino acids determines the shape of the active site which must be highly specific to the shape of its substrate.

Question 15

What is protein synthesis?

Answer 15

The formation of a protein from a gene

Question 16

What are the two stages of protein synthesis?

Answer 16

1. Transcription

2. Translation

Question 17

What does transcription involve?

Answer 17

The formation of mRNA from a DNA template

Question 18

Outline transcription

Answer 18

1. DNA double helix unwinds
2. RNA polymerase binds to a specific base sequence of non-coding
   DNA in front of a gene and moves along the DNA strand
3. RNA polymerase joins free RNA nucleotides to complementary bases
   on the coding DNA strand
4. mRNA formation complete. mRNA detaches and leaves the nucleus

Question 19

Describe the differences between mRNA and DNA

Answer 19

● mRNA is single stranded whereas
DNA is double stranded
● mRNA uses U whereas DNA uses T

Question 20

Why is mRNA used in translation rather than DNA?

Answer 20

DNA is too large to leave the nucleus so cannot reach the ribosome.

Question 21

What does translation involve?

Answer 21

A ribosome joins amino acids in a specific order dictated by mRNA to form
a protein.

Question 22

Outline translation

Answer 22

1. mRNA attaches to a ribosome
2. Ribosome reads the mRNA bases in triplets. Each triplet codes
   for one amino acid which is brought to the ribosome by a tRNA molecule
3. A polypeptide chain is formed from the sequence of amino
   acids which join together

Question 23

How is a tRNA molecule adapted to its function?

Answer 23

Each tRNA molecule has an anticodon which is specific to the codon of the amino acid that it carries.

Question 24

What is a mutation?

Answer 24

A random change in the base sequence of DNA which results in genetic variants

Question 25

Describe the effect of a gene mutation in coding DNA

Answer 25

● If a mutation changes the amino acid sequence,
protein structure and function may change
● If a mutation does not change amino acid sequence, there is no effect on protein structure or function

Question 26

What is non-coding DNA?

Answer 26

DNA which does not code for a protein but instead controls gene expression

Question 27

Describe the effect of a gene mutation in non-coding DNA

Answer 27

● A mutation may affect the ability of RNA polymerase to bind to non-coding DNA
● This may affect protein production and the resulting phenotype of the organism

Question 28

Outline how the work of Mendel helped scientists to develop their understanding
of genetics

Answer 28

● Mendel studied the inheritance of different phenotypes of pea plants
● He established a correlation between parent and offspring phenotypes
● He noted that inheritance was determined by ‘units’ passed on to
descendants
● Using gene crosses, he devised the terms ‘dominant’ and ‘recessive’

Question 29

Why was Mendel’s work initially overlooked?

Answer 29

Scientist’s didn’t understand Mendel’s

work as there was no knowledge of genes or DNA at the time.