3.1.5.1 Structure of DNA and RNA
3.1.5.1 Structure of DNA and RNA
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Both DNA and RNA are polymer molecules.
The monomers that make up their structures are called nucleotides.
Each nucleotide is formed from:
a pentose
a phosphate group
a nitrogen-containing organic base (nitrogenous base)
DNA nucleotide components:
deoxyribose (a pentose)
phosphate group
a nitrogen-containing organic base
Adenine
Cytosine
Guanine
Thymine
RNA nucleotide components:
ribose (a pentose)
phosphate group
a nitrogen-containing organic base
Adenine
Cytosine
Guanine
Uracil
A condensation reaction between two nucleotides forms a phosphodiester bond.
The reaction is catalysed by DNA polymerase.
A DNA molecule is a double helix
with two polynucleotide chains
held together by hydrogen bonds
between specific complementary base pairs.
A RNA molecule is a relatively short polynucleotide chain (single stranded).
RNA can exist as:
messenger RNA (mRNA)
transfer RNA (tRNA)
ribosomal RNA (rRNA).
Q1. What are the monomers of nucleic acids? (1)
Q1. What are the monomers of nucleic acids? (1)
nucleotides
Q2. Where is DNA found in a eukaryotic cell? (2)
In the nucleus, condensed into chromosomes.
There is also circular DNA in mitochondria and chloroplasts.
Q3. Describe the structure of a DNA nucleotide. (3)
Q3. Describe the structure of a DNA nucleotide. (3)
a phosphate group
deoxyribose sugar (pentose)
a nitrogenous base (adenine, thymine, cytosine, or guanine)
Q4. Figure below shows one base pair of a DNA molecule.
Name part F of each nucleotide. (1)
Deoxyribose.
Q5. The diagram below shows part of a DNA molecule.
Q5. The diagram below shows part of a DNA molecule.
Name the type of bond between: (2)
complementary base pairs:
adjacent nucleotides in a DNA strand:
complementary base pairs: hydrogen bonds
adjacent nucleotides in a DNA strand: phosphodiester bonds
Q6. Describe how polynucleotides form. (2)
1. Condensation reaction between nucleotides which involves the loss of water.
2. Strong phosphodiester bonds form between sugar and phosphate group.
3. Catalyses by DNA polymerase
Q7. Figure below shows a short section of a DNA molecule.
a. Name parts R and Q. (2)
R = Deoxyribose
Q = Phosphate group
b. Name the bonds that join A and B. (1)
hydrogen bonds
Q8. Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule. (2)
1. Condensation reaction involving loss of water
2. Between phosphate and deoxyribose
3. Catalysed by DNA polymerase
Q9. Name the complimentary base pairs in DNA. (2)
Adenine joined by double hydrogen bond with Thymine
Cytosine joined by triple hydrogen bond with Guanine
Q10. Describe the bonding between complementary base pairs in DNA. (2)
Adenine (A) pairs with Thymine (T) via double hydrogen bonds.
Cytosine (C) pairs with Guanine (G) via triple hydrogen bonds.
Q11. Some scientists determined that a sample of DNA contained 18% adenine.
What were the percentages of thymine and guanine in this sample of DNA? (2)
1. Thymine 18 %
2. Guanine 32 %
Working out:
100 - (18 x 2) = 64 %
then 64/2 = 32 %
Q12. Why is DNA described as a polymer? (1)
DNA is made from many repeated monomers, called nucleotides.
Q13. Describe the structure of DNA. (5)
1. Polymer of nucleotides
Accept ‘Polynucleotide’
2. Each nucleotide formed from deoxyribose, a phosphate group and an organic (nitrogenous) base
3. Phosphodiester bonds between nucleotides
4. Double helix (2 strands) held by hydrogen bonds
5. Hydrogen bonds between adenine and thymine, and cytosine and guanine
Q14. DNA is a polymer of nucleotides. Each nucleotide contains an organic base.
Explain how the organic bases help to stabilise the structure of DNA. (2)
1. Hydrogen bonds between the base pairs holds two strands together
2. Many hydrogen bonds provides strength
Reject strong hydrogen bonds
Q15. Relate the structure of DNA to its function. (5)
Sugar – phosphate backbone - This protects coding bases on the inside of the helix.
Double stranded - This allows strands to act as templates in DNA replication.
Large molecule - It stores lots of information.
Double helix - This makes the molecule compact.
Complementary base pairing - This allows accurate DNA replication.
Weak hydrogen bonds - This allows strands to separate in DNA replication
Q16. The diagram shows part of a DNA molecule.
a. How many nucleotides are shown in the diagram above? (1)
8
b. Name the type of bond labelled X in the diagram. (1)
Phosphodiester bond
Q17. Name the pentose sugars in DNA and RNA. (1)
DNA: deoxyribose
RNA: ribose
Q18. Describe the structure of a RNA nucleotide. (3)
Q18. Describe the structure of a RNA nucleotide. (3)
1. A phosphate group
2. A ribose sugar (pentose)
3. A nitrogenous base (adenine, uracil, cytosine, or guanine).
Q19. What are the three main types of RNA? (3)
mRNA (messenger RNA)
tRNA (transfer RNA)
rRNA (ribosomal RNA)
Q20. Outline the differences between the structures of DNA and RNA molecules. (3)
1. RNA is single-stranded whereas DNA is double stranded.
2. RNA contains ribose sugar instead of deoxyribose sugar.
3. RNA has uracil (U) instead of thymine (T).
4. RNA is shorter than DNA (it has fewer nucleotides).
Q21. Outline the similarities in the structures of DNA and RNA molecules. (3)
1. Polymers of nucleotides
2. Nucleotide has pentose (5 Carbon sugar), nitrogen-containing organic base and a phosphate group
3. Cytosine, guanine and adenine as nitrogenous bases
4. Have phosphodiester bonds
Spaced practice
Q22. The diagram shows part of a DNA molecule.
a. DNA is a polymer. What is the evidence from the diagram that DNA is a polymer? (1)
Repeating units / nucleotides / monomer / molecules
b. Name the parts of the diagram labelled C, D and E. (3)
1. C = hydrogen bonds
2. D = deoxyribose
Ignore sugar
3. E = phosphate
Q23. Give three differences between DNA molecules and tRNA molecules. (3)
1. DNA contains deoxyribose, whereas tRNA contains ribose.
2. DNA is double-stranded, whereas tRNA is single-stranded.
3. DNA molecules are very long with many nucleotides, whereas tRNA molecules are short with relatively few nucleotides.
4. DNA contains the base thymine, whereas tRNA contains uracil instead of thymine.
5. DNA forms a double helix structure, whereas tRNA folds into a cloverleaf structure.
Q24. Give three differences between mRNA molecules and tRNA molecules. (3)
1. mRNA contains codons, whereas tRNA contains an anticodon.
2. mRNA has no hydrogen bonds or base pairing within the molecule, whereas tRNA has hydrogen bonds forming base pairs.
3. mRNA has no amino acid binding site, whereas tRNA has a specific amino acid binding site.
4. mRNA is linear and not folded, whereas tRNA is folded into a cloverleaf shape.
5. mRNA is long with many nucleotides, whereas tRNA is short with relatively few nucleotides.
6. mRNA does not bind to an amino acid, whereas tRNA does bind to a specific amino acid.
Q25. The nucleus and a chloroplast of a plant cell both contain DNA.
Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus. (3)
Must be comparative statements.
1. Chloroplast DNA is shorter, whereas nuclear DNA is much longer.
2. Chloroplast DNA contains fewer genes, whereas nuclear DNA contains many more genes.
3. Chloroplast DNA is circular (in a loop), whereas nuclear DNA is linear and arranged into chromosomes.
4. Chloroplast DNA is not associated with histone proteins, whereas nuclear DNA is associated with histones.
5. Chloroplast DNA does not contain introns, whereas nuclear DNA contains introns.
Q26. Adenosine triphosphate (ATP) is a nucleotide derivative.
Contrast the structures of ATP and a nucleotide found in DNA to give two differences. (2)
1. ATP has ribose and DNA nucleotide has deoxyribose
2. ATP has 3 phosphate groups and DNA nucleotide has 1 phosphate group
3. ATP – base always adenine and in DNA nucleotide base can be different / varies
Both parts of each MP needed
Accept C, T or G for different bases
Reject Uracil / U
Q27. The diagram shows a short section of a DNA molecule.
a. Use the letters in the diagram to indicate a part of the molecule which is not a base and is different in an RNA molecule. (1)
B
b. Use the letters in the diagram to indicate a part of the molecule which contains nitrogen.(1)
A
c. How are the two strands of the DNA molecule held together? (1)
by hydrogen bonds
d. Give one advantage of DNA molecules having two strands. (1)
stability / protects bases / replication
Q28. The diagram shows one nucleotide pair of a DNA molecule.
a. Name the parts of the nucleotide labelled X, Y and Z. (3)
a. Name the parts of the nucleotide labelled X, Y and Z. (3)
X, phosphate
Y, deoxyribose (pentose / 5-carbon sugar)
Z, nitrogenous base
b. What type of bond holds Z and Q together? (1)
hydrogen bonds
Q29. A sample of DNA was analysed. 28% of the nucleotides contained thymine. Calculate the percentage of nucleotides which contained cytosine. Show your working. (2)
thymine 28% so adenine 28%
100 – 56 = 44%
therefore 44% cytosine and guanine
therefore 22% cytosine
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