Nucleotides and nucleic acids PPQs

Lipids, polysaccharides, nucleic acids and proteins are all macromolecules. Which statement about macromolecules is correct?

A All macromolecules are formed in hydrolysis reactions.

B Lipids are not polymers, but polysaccharides, nucleic acids and proteins are polymers.

C Lipids are polymers of fatty acids and glycerol.

D Macromolecules all consist of repeating units of monomers.

The answer is B

Outline why cellular respiration is necessary in cells that are carrying out DNA replication.

Provides ATP to be used in DNA replication

Describe how a polynucleotide is formed from its monomers

Multiple nucleotides are joined together by phosphodiester bonds by condensation reactions

The enzyme helicase is active during DNA replication. Describe the action of helicase

DNA replication conserves genetic information with accuracy. Explain how errors may occur during DNA replication.

A mutation may occur, which is a change in the DNA base sequence of amino acids. It could be a substitution, addition or deletion mutation. Mutations are caused by exposure to mutagens.

Students use this method to extract DNA from the fruit of a strawberry plant

Take a fruit from a strawberry plant and crush it using a mortar and pestle.

Add salt to the crushed strawberry fruit mixture.

Add an enzyme to the mixture.

Add ethanol to the mixture.

Evaluate whether the method used by the students would successfully extract DNA

Yes because…

crushing breaks down cell walls

salt breaks hydrogen bonds between the DNA and water so makes DNA less soluble in water

ethanol added to cause precipitation of DNA

ethanol would break down the plasma membrane

No because…

detergent not added to break down plasma membrane

enzyme must be a protease to digest histone proteins

ethanol is not ice cold so enzyme activity not reduced

Which statement is a correct description of the structure of ADP?

A A pyrimidine is part of its structure.

B It contains deoxyribose.

C It is a nucleotide.

D One phosphate group is present.

The answer is C

State the roles of DNA polymerase and helicase in cells

DNA polymerase catalyses the formation of phosphodiester bonds between nucleotides

Helicase unzips double helix strands

Give three functions of the cytoskeleton

movement of cells

strengthening cells

movement of organelles

holds organelles in place

forms spindle

movement of chromatids / chromosomes

cleavage in cytokinesis

Suggest how a mutation in cytoskeleton genes could cause a disease of the nervous system.

change in structure / function of cytoskeleton protein

less movement of vesicles which may affect exocytosis and the release of neurotransmitters

less synaptic transmission

could change diameter of axon

affects speed of nerve impulses

Which description of biological molecules is correct?

A DNA and RNA are both polymers of nucleotides.

B Hydrolysis of sucrose produces fructose and β-glucose.

C Proteins are polymers of amino acids and are broken down in condensation reactions.

D Starch is a polymer of the monosaccharide maltose

The answer is A

What is the pentose sugar in DNA

Deoxyribose

What is the pentose sugar in RNA

Ribose

Why is DNA replication described as semi-conservative?

Strands from the original DNA molecule act as templates. The new DNA molecule contains 1 old strand and 1 new strand

Explain the role of DNA helicase in semiconservative replication

Breaks hydrogen bonds between complementary base pairs to form 2 single strands, each of which can act as a template

How is a new strand formed during semiconservative replication?

Free nucleotides from nuclear sap attach to exposed bases by complementary base pairing

DNA polymerase joins adjacent nucleotides on new strand in a 5' -> 3' direction via condensation reactions to form phosphodiester bonds

Hydrogen bonds reform

Identify feature of genetic code

Non-overlapping, each triplet is only read once

Degenerate, more than one triplet codes for the same amino acid

Universal, same bases and sequences used by all species

How does a gene determine the sequence of amino acids in a protein?

Consists of base triplets that code for specific amino acids

Describe how DNA can be purified by precipitation

Add ethanol and salt to aqueous solution

Nucleic acids precipitate out of solution

Centrifuge to obtain pellet of nucleic acid

Wash pellet with ethanol and centrifuge again

What does transcription produce and where does it occur?

Produces mRNA, occurs in the nucleus

Outline the process of transcription

RNA polymerase binds to promoter region of a gene

Section of DNA uncoils into 2 strands with exposed bases, antisense strand acts as template

Free nucleotides are attracted to their complementary bases

RNA polymerase joins adjacent nucleotides to form phosphodiester bonds

What happens after a strand of mRNA is transcribed?

RNA polymerase detaches at the terminator region

Hydrogen bonds reform and DNA rewinds

Splicing removes introns from pre-mRNA in eukaryotic cells

mRNA moves out of the nucleus via nuclear pore and attaches to ribosome

What does translation produce and where does it occur?

Produces proteins, occurs in cytoplasm on ribosomes

Outline the process of translation

Ribosome moves along mRNA until start codon

tRNA anticodon attaches to complementary bases on mRNA

Condensation reactions between amino acids on tRNA form peptide bonds

Process continues to form polypeptide chain until stop codon is reached

Describe the structure of ATP and ADP

ATP has 3 inorganic phosphate groups, ADP has 2

What is a mutation?

An alteration in the DNA base sequence

Name the base represented by the letter A

Adenine

Name the base represented by the letter T

Thymine

Name the base represented by the letter G

Guanine

Name the base represented by the letter C

Cytosine

Name the base represented by the letter U

Uracil

True or false, during DNA replication free RNA nucleotides join to bases on the exposed DNA strands

False as free DNA nucleotides join to bases on exposed RNA strands

True or false, during DNA replication both polypeptide strands act as a template

False

True or false, during DNA replication 3 hydrogen bonds form between bases A and T

False, 2 hydrogen bonds form between A and T

True or false, during DNA replication covalent bonds form between the phosphate of one nucleotide and the pentose sugar of the next nucleotide

True

Fill the gap, The type of variation that is caused by differences in DNA is known as ___ variation

Genetic

Fill the gap, Variation can be described as the differences in characteristics between ___

Individuals/organisms/species/phenotypes

Fill the gap, Variation can also be caused by the ___

Environment

Fill the gap, Variation between members of the same species is known as ___ variation

Intraspecific

Fill the gap, Evolution depends on variation and ___ of the best adapted individuals

Selection/ survival

State the number of DNA nucleotide bases that code for a single amino acid.

3

There is a maximum of 64 different base combinations in DNA that could each code for an amino acid. How is this number of combinations calculated?

4 x 4 x 4 (There are 4 different bases and 3 bases in a triplet code)

Twenty different amino acids are commonly used for protein synthesis. In theory, this would need only 20 different base combinations. Explain the uses of the remaining 44 combinations

Several triplets code for the same amino acid

Same are used as start or stop codons

May result in a silent mutation if mutation does occur

Which nucleotide bases are common to DNA and RNA?

Adenine, guanine and cytosine

Describe how a nucleotide base sequence in a gene is used to synthesise a polypeptide

Transcription – The gene is transcribed into mRNA

Free RNA nucleotides undergo complementary base pairing

The template strand is synthesised by RNA polymerase

Translation – mRNA moves into ribosomes

tRNA molecules bind to mRNA, anticodons pair with codons

A specific amino acid attaches to tRNA and peptide bonds

Fill the blank, A gene is a section of DNA that codes for the production of a ___

Polypeptide

Fill the blank, The molecule that copies a gene and carries the information to a ___ is called RNA

Ribosome

State two ways in which a diagram of part of an RNA molecule would appear different from the DNA molecule shown in Fig. 5.1

Uracil instead of thymine

Single stranded

Explain why complementary base-pairing is important in DNA replication

Allows reformation of hydrogen bonds

Reduces occurrence of mutation

DNA can be replicated without error

Glucose is a hexose sugar and is a monomer in many carbohydrates. Name the precise group of carbohydrate molecules of which glucose is an example

Monosaccharide

The cells of living organisms require glucose. State and explain two ways in which the glucose molecule is well suited to its function in living organisms

soluble so can be easily transported

small so can diffuse across cell membranes

quickly respired to release energy and produce ATP

molecules can join to produce disaccharides / polysaccharides

Deoxyribose is a pentose sugar that is a component of the double-stranded DNA molecule. Describe the structural relationship between deoxyribose and the other components of the DNA molecule.

Part of nucleotide

joined to base and phosphate

phosphate joined to C5 and C3 / base joined to C1

deoxyribose is part of backbone of DNA

linking with second phosphate on adjacent nucleotide

nucleotide is monomer of DNA / polynucleotide

A length of DNA has the base sequence of AATCGCGGTCGCTCA

Select the row that shows the correct complementary DNA strand and the sequence of mRNA made during transcription of the DNA sequence above

Complementary DNA sequence

mRNA sequence

A

AATCGCGGTCGCTCA

UUAGCGCCAGCGAGU

B

TTAGCGCCAGCGAGT

UUAGCGCCAGCGAGU

C

TTAGCGCCAGCGAGT

TTAGCGCCAGCGAGT

D

TTAGCGCCAGCGAGT

AAUCGCGGUCGCUCA

B

What bases are purine and what bases are pyrimidine?

Purine – adenine and guanine

Pyrimidine – thymine, uracil and cytosine

The following passage has four key terms missing, which are names of molecules involved in protein synthesis.

The enzyme (1) joins nucleotides together to make a copy of the gene. This makes the molecule (2), which leaves through the nuclear pore to bind to an organelle that is made of protein and (3). The amino acids are assembled here when (4) brings the specific amino acid to be joined to the polypeptide.

Which row gives the correct names of these missing molecules?

1

2

3

4

A

DNA polymerase

(t)RNA

(m)RNA

(r)RNA

B

RNA polymerase

(m)RNA

(r)RNA

(t)RNA

C

RNA polymerase

(m)RNA

(t)RNA

(r)RNA

D

RNA polymerase

(r)RNA

(t)RNA

(m)RNA

B

Name the base represented by the letter T

Thymine

Which of these statements about DNA replication are incorrect?

1

The DNA molecule unwinds

2

Hydrogen bonds between the base pairs break

3

Free RNA nucleotides join to bases on the exposed DNA strands

4

Both polypeptide strands act as a template

5

Hydrogen bonds form between complementary bases

6

Three hydrogen bonds form between bases A and T

7

DNA polymerase links the new nucleotides

8

Covalent bonds form between the phosphate of one nucleotide and the pentose sugar of the next nucleotide

Statements 3, 4 and 6 are incorrect

When a virus infects a human host, it causes the host's cells to produce many new copies of the virus

The influenza vaccination must be given each year because there are frequent mutations in the RNA of the virus

The antigens on the surface of the virus are made of protein

The virus uses the organelles and enzymes in the host's cells to produce new copies of itself

Suggest the role of the viral RNA in the production of viral proteins

The viral RNA acts as the host cell's mRNA and the viral RNA carries the code for the viral protein to the ribosomes

Explain why a mutation in viral RNA leads to a change in the 3D shape of the protein antigens on its surface

A mutation in the RNA alters the sequence of amino acids, the primary structure

This consequently affects the tertiary structure, which changes the 3D shape of the antigens