Nucleic Acids

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DNA as the Genetic Material

DNA (deoxyribonucleic acid) carries the genetic code in all living organisms, which is why the genetic code is described as universal.
DNA is mainly found in the nucleus where it forms chromosomes; it is also found in chloroplasts and mitochondria of eukaryotic cells.
RNA (ribonucleic acid) is another nucleic acid and is the main component of ribosomes, playing an important role in protein synthesis.
Some RNA is found in the nucleus and cytoplasm.
Certain viruses (e.g. SARS-CoV-2) contain RNA as their genetic material instead of DNA, causing diseases such as COVID-19, Ebola, mumps and influenza.
Viruses are not considered living organisms because they cannot replicate by themselves, depend on other cells, and lack a cellular structure.

From cell to gene

Nucleotide Structure

Both DNA and RNA are polymers made of repeating units called nucleotides.
Each nucleotide is formed from a pentose sugar (5 carbon atoms), a nitrogen-containing organic base (1 or 2 rings), and a phosphate group (acidic and negatively charged).
The base and phosphate group are both covalently bonded to the sugar.
The nitrogenous bases in DNA are adenine (A), guanine (G), cytosine (C) and thymine (T).
RNA shares the same bases as DNA except thymine is replaced by uracil (U).
Adenine and guanine are purine bases; cytosine, thymine and uracil are pyrimidine bases.

The sugar-phosphate backbone

Drawing Nucleotides

Simple shapes can be used to draw nucleotides: pentagons for pentose sugars, circles for phosphates (often with P inside), and rectangles for bases.
Covalent bonds can be shown with solid lines; hydrogen bonds with dashed lines or complementary shapes.
Advanced drawing skills are not required; clear, large diagrams are easier for examiners to award marks for.

Linking Nucleotides

Nucleotides join in chains to form DNA or RNA strands; the phosphate group of one nucleotide forms a covalent bond to the pentose sugar of the next.
This polymerisation occurs by condensation reactions, releasing a molecule of water for each bond formed.
This creates a sugar-phosphate backbone with a base linked to each sugar.
The polymer of nucleotides is known as a strand; DNA is double-stranded, RNA is usually single-stranded.
There are just 4 separate bases that can be joined in any combination/sequence because the sugar and phosphate are the same in every nucleotide.

RNA Structure

RNA molecules are relatively short (a hundred to a few thousand nucleotides) and usually form a single-stranded polynucleotide.
RNA nucleotides contain ribose as the pentose sugar and the bases A, G, C and U (uracil instead of thymine).
Carbon atoms in nucleotides are numbered from the right in a clockwise direction, which helps identify bonds and orientation.
Adjacent RNA nucleotides are linked by condensation reactions, forming phosphodiester bonds between the pentose sugar of one nucleotide and the phosphate group of the next.
Types of RNA include mRNA (formed in the nucleus, transported to ribosomes), tRNA (transports amino acids to ribosomes), and rRNA (forms part of ribosomes).

DNA Structure

DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.
Each DNA strand has alternating deoxyribose sugars and phosphate groups forming the sugar-phosphate backbone.
Each strand has a 3' end and a 5' end, relating to which carbon atom on the pentose sugar could bond with another nucleotide.
The strands run in opposite directions: one is 5' to 3' and the other is 3' to 5'.
The nitrogenous bases project from the backbone towards the interior of the double-stranded molecule.
Complementary base pairing: A pairs with T (two hydrogen bonds), G pairs with C (three hydrogen bonds).

The DNA double helix

The Genetic Code

DNA carries the genetic code as a sequence of nitrogenous bases (A, G, C, T).
One strand carries the base sequence read by enzymes: the coding strand.
The sequence of bases in genes determines the order of amino acids in proteins.
The code is read as a triplet of bases called a codon, each coding for one amino acid.
There are 20 different amino acids that can be coded for; the sequence determines the shape and function of the protein.
The genetic code is universal: almost every organism uses the same code, so genetic information is transferable between species.
The universal code provides evidence for a universal common ancestor and is why genetic engineering is possible.
Conserved sequences are coding and non-coding sequences that have remained unchanged in all organisms; highly conserved sequences are found in genes for transcription, translation and histone proteins.

DNA vs RNA

Pentose sugar: DNA contains deoxyribose; RNA contains ribose.
Bases: DNA contains A, C, G, T; RNA contains A, C, G, U (uracil replaces thymine).
Strands: DNA is double-stranded (double helix); RNA is single-stranded.
Length: RNA polynucleotide chains are relatively short compared to DNA.

DNA Function and Information Storage

Complementary base pairing means the base sequence on one strand determines the sequence of the other; one strand acts as a template for the other.
This allows DNA to be copied precisely during DNA replication, ensuring the genetic code is accurately copied and expressed.
Despite only four bases, they can combine to form a very diverse range of sequences in DNA molecules of different lengths.
DNA has an almost limitless capacity for storing genetic information.
Storage capacity can be measured by the number of genes or the number of base pairs in the genome.
The DNA in a human cell nucleus contains about 3.2 gigabases (about 10⁹ base pairs) with a length of about 2 meters, fitting inside a microscopic nucleus.
This indicates how incredibly well packaged DNA is, giving it enormous capacity for storing genetic data with great economy.

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练习题

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1.Which of the following is the main role of DNA?
Easy
- ACarrying the genetic code in all living organisms
- BTransporting amino acids to ribosomes
- CForming the structural component of ribosomes
- DCatalysing metabolic reactions
2.In the term 'polynucleotide', what does the prefix 'poly' mean?
Easy
- AMany
- BFew
- COne
- DTwo
3.A section of DNA contains 17% of its nucleotides with the base cytosine. What percentage of the nucleotides in this section contain thymine?
Medium
- A17%
- B33%
- C34%
- D66%
4.Why could the calculation of thymine percentage from a cytosine percentage not be performed for a piece of RNA?
Medium
- ARNA is single-stranded, so there is no complementary base pairing to determine the proportion of other bases
- BRNA contains uracil instead of cytosine
- CRNA contains deoxyribose instead of ribose
- DRNA does not contain any phosphate groups
5.RNA is usually single-stranded, whereas DNA is double-stranded.
Easy
True or false?
6.Which of the following statements apply only to RNA? (select all that apply)
Medium
- AThe molecule is in the shape of a double helix, with antiparallel strands
- BThe four bases are adenine, cytosine, guanine, thymine
- CRibose is present as the pentose sugar in its nucleotides
- DIt is a polymer formed by the linkage of many nucleotides
- EUracil is present instead of thymine
7.Which of the following correctly identifies the components of a nucleotide?
Medium
- AA pentose sugar, a nitrogen-containing organic base and a phosphate group
- BA hexose sugar, an amino acid and a phosphate group
- CA pentose sugar, a nitrogen-containing organic base and a carboxyl group
- DA glycerol molecule, a fatty acid and a phosphate group
8.The diagram represents the structure of a nucleotide. Which row correctly identifies the components labelled 1, 2 and 3?
Easy
- A1 = phosphate group, 2 = pentose sugar, 3 = adenine
- B1 = adenine, 2 = pentose sugar, 3 = phosphate group
- C1 = pentose sugar, 2 = phosphate group, 3 = adenine
- D1 = phosphate group, 2 = adenine, 3 = pentose sugar

更多题目

9.The diagram shows a section of a DNA molecule with three base pairs. Which of these base pairs are not correct?
Medium
- ANone
- B1
- C2
- D3
10.Which of the following correctly describes complementary base pairing in DNA?
Medium
- AAdenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds)
- BAdenine pairs with guanine (three hydrogen bonds) and cytosine pairs with thymine (two hydrogen bonds)
- CAdenine pairs with uracil (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds)
- DAdenine pairs with thymine (three hydrogen bonds) and guanine pairs with cytosine (two hydrogen bonds)
11.Which of the following statements about the genetic code is correct?
Medium
- AThe genetic code is universal, meaning almost every organism uses the same code
- BThe genetic code is read as a pair of bases, each coding for one amino acid
- CThe genetic code is unique to each species, with no shared sequences
- DThe genetic code is read as a triplet of bases, each coding for one protein
12.Which of the following are found in a DNA nucleotide? (select all that apply)
Medium
- ADeoxyribose
- BRibose
- CPhosphate group
- DUracil
- EThymine
13.Which of the following statements correctly describe the sugar-phosphate backbone of a DNA strand? (select all that apply)
Medium
- AIt is formed by covalent bonds between the phosphate of one nucleotide and the pentose sugar of the next.
- BIt is formed by hydrogen bonds between the phosphate of one nucleotide and the pentose sugar of the next.
- CIt consists of alternating deoxyribose sugars and phosphate groups.
- DIt is formed by condensation reactions that release water.
- EThe nitrogenous bases are attached to the phosphate groups.
14.Which of the following statements about the genetic code are correct? (select all that apply)
Medium
- AThe genetic code is read as triplets of bases called codons.
- BEach codon codes for one amino acid.
- CThere are 20 different amino acids that can be coded for.
- DThe genetic code is universal, meaning almost all organisms use the same code.
- EThe genetic code is carried by the sequence of phosphate groups.
15.Match each structural feature of a nucleic acid with its correct description.
Medium
- Purine base
- Pyrimidine base
- Phosphodiester bond
- Hydrogen bond
- A base with two rings of atoms, such as adenine or guanine
- A base with one ring of atoms, such as cytosine or thymine
- A covalent bond linking the sugar of one nucleotide to the phosphate of the next
- A bond that holds complementary bases together between two DNA strands
16.Match each type of RNA with its correct role.
Medium
- mRNA
- tRNA
- rRNA
- Carries the genetic message from the nucleus to the ribosomes
- Transports amino acids to the ribosomes during protein synthesis
- Forms part of the structure of ribosomes
17.Place the following steps in the correct order to show how a DNA polynucleotide strand is formed.
Medium
- Nucleotides are activated
- A condensation reaction occurs between the phosphate group of one nucleotide and the pentose sugar of the next
- A molecule of water is released
- A phosphodiester bond forms
- A sugar-phosphate backbone is produced
18.Arrange the following components of a nucleotide from the phosphate group to the base, as they are attached to the pentose sugar.
Medium
- Phosphate group
- Pentose sugar
- Nitrogenous base
19.In DNA, adenine always pairs with thymine by two hydrogen bonds.
Easy
True or false?
20.The two strands of a DNA molecule are parallel, running in the same direction.
Medium
True or false?
21.Which of the following correctly describes the 3' end of a DNA polynucleotide strand?
Medium
- AIt is the end where the third carbon atom of the pentose sugar is available to bond with another nucleotide.
- BIt is the end where the fifth carbon atom of the pentose sugar is available to bond with another nucleotide.
- CIt is the end where the third phosphate group is attached.
- DIt is the end where the third nitrogenous base is located.
22.A DNA molecule has 1,500 base pairs. If 30% of the bases are cytosine, how many nucleotides of adenine are present?
Medium
- A300
- B450
- C600
- D900
23.A sample of DNA was analysed and found to contain 24.7% adenine, 24.1% guanine, 18.5% cytosine and 32.7% thymine. Which of the following conclusions can be drawn from these data? (select all that apply)
Hard
- AThe DNA is likely to be single-stranded because the percentages of A and T are not equal.
- BThe DNA is double-stranded because A pairs with T.
- CThe DNA is double-stranded because G pairs with C.
- DThe data show that base pairing rules do not apply to this DNA.
- EThe DNA is from a virus.
24.The diagram represents the structure of a nucleotide. Which row correctly identifies components 1, 2 and 3?
Medium
- A1 = Adenine, 2 = Pentose sugar, 3 = Phosphate group
- B1 = Phosphate group, 2 = Adenine, 3 = Pentose sugar
- C1 = Pentose sugar, 2 = Phosphate group, 3 = Adenine
- D1 = Phosphate group, 2 = Pentose sugar, 3 = Adenine
25.Which of the following statements only apply to RNA? I. The molecule is in the shape of a double helix, with antiparallel strands. II. The four bases are adenine, cytosine, guanine, thymine. III. Ribose is present as the pentose sugar in its nucleotides. IV. It is a polymer formed by the linkage of many nucleotides.
Medium
- AI and III
- BI, II and IV
- CII, III and IV
- DIII and IV
26.A sample of DNA was analysed and found to contain 24.7% adenine, 24.1% guanine, 18.5% cytosine and 32.7% thymine. Which conclusion is best supported by these data?
Medium
- AThe DNA is double-stranded because A = T and G = C.
- BThe DNA is single-stranded because A does not equal T and G does not equal C.
- CThe DNA is from a virus because it contains uracil.
- DThe DNA is from a eukaryote because the percentages are not equal.
27.A DNA molecule has 1,200 base pairs. If 30% of the bases are cytosine, how many nucleotides of adenine are present?
Medium
- A240
- B360
- C480
- D720
28.The DNA in the nucleus of a fruit fly cell is 5.6 cm long. One kilobase (Kb) of double-stranded DNA has a length of 0.34 μm. Calculate the number of Kb in the DNA of the fruit fly. Give your answer to the nearest whole number.
Hard
- A1.65 × 10⁴ Kb
- B1.65 × 10⁵ Kb
- C1.65 × 10⁶ Kb
- D1.65 × 10⁷ Kb
29.Which of the following statements about the structure of ATP compared with an adenine DNA nucleotide are correct? (select all that apply)
Medium
- AATP contains ribose, while a DNA nucleotide contains deoxyribose.
- BATP contains three phosphate groups, while a DNA nucleotide contains one phosphate group.
- CATP contains the base adenine, while a DNA nucleotide contains the base thymine.
- DATP is a nucleotide derivative, while a DNA nucleotide is a monomer of DNA.
- EATP contains uracil, while a DNA nucleotide contains thymine.
30.A section of DNA contains 1,200 base pairs. The number of guanine molecules on strand one was counted as 156. The number of cytosine molecules on strand one was counted as 209. The number of adenine molecules on strand two was counted as 264. Which of the following statements are correct? (select all that apply)
Hard
- AThe total number of guanine molecules in the section is 365.
- BThe total number of cytosine molecules in the section is 365.
- CThe total number of adenine molecules in the section is 471.
- DThe total number of thymine molecules in the section is 471.
- EThe percentage composition of adenine is approximately 19.6%.
31.The genetic code is universal, meaning that almost every organism uses the same triplet codes for the same amino acids.
Easy
True or false?
32.RNA molecules are usually longer than DNA molecules.
Easy
True or false?
33.A student measures the length of a DNA molecule as 5.6 cm. Given that 1 kilobase (Kb) of double-stranded DNA has a length of 0.34 μm, what is the length of this DNA molecule in kilobases? (1 cm = 10,000 μm)
Medium
- A164,706 Kb
- B1,647 Kb
- C16,471 Kb
- D164.7 Kb
34.In the sugar-phosphate backbone of DNA, the phosphate group of one nucleotide is linked to the pentose sugar of the next nucleotide by a phosphodiester bond formed through a condensation reaction.
Medium
True or false?
35.A section of DNA contains 1,200 base pairs. The number of guanine molecules on strand one was counted as 156. The number of cytosine molecules on strand one was counted as 209. The number of adenine molecules on strand two was counted as 264. What is the total number of thymine molecules in this section of DNA?
Challenge
- A264
- B471
- C936
- D1,200
36.A scientist sequences a gene and finds it consists of 1,500 base pairs. 30% of the total bases are cytosine. How many nucleotides of adenine are present in this gene?
Hard
- A600
- B450
- C300
- D900
37.The two strands of a DNA molecule are antiparallel, meaning they run in opposite directions.
Easy
True or false?
38.A DNA molecule has 30% of its bases as cytosine. What percentage of adenine is present?
Hard
- A20%
- B30%
- C40%
- D70%
39.Which pentose sugar is found in DNA nucleotides?
Easy
- ARibose
- BDeoxyribose
- CGlucose
- DFructose
40.Which nitrogenous base is found in RNA but not in DNA?
Easy
- AThymine
- BUracil
- CAdenine
- DGuanine
41.Name the type of bond that forms between the pentose sugar of one nucleotide and the phosphate group of the next nucleotide in a polynucleotide strand.
Medium
42.A DNA molecule has 30% of its bases as cytosine. Calculate the percentage of adenine in this DNA molecule.
Medium
43.Complete the sentence about the genetic code.
Easy
The genetic code is read as a triplet of bases called a ____.
44.A fruit fly DNA molecule is 5.6 cm long. One kilobase (Kb) of double-stranded DNA has a length of 0.34 μm. Calculate the number of Kb in the DNA of the fruit fly. Give your answer to the nearest whole number. (1 cm = 10,000 μm)
Hard
45.Complete the table comparing DNA and RNA by filling in the missing words.
Medium
DNA contains the pentose sugar ____, while RNA contains the pentose sugar ____. In DNA, the base ____ pairs with adenine, whereas in RNA, the base ____ pairs with adenine.
46.The DNA in the nucleus of a human cell contains about 3.2 gigabases. If one base pair is 0.34 nm long, calculate the length of this DNA in metres. Give your answer to 2 significant figures.
Hard
47.State the part of a DNA molecule that contains nitrogen.
Medium
48.Complete the sentence about the pentose sugars in nucleic acids.
Easy
DNA nucleotides contain the pentose sugar ____, whereas RNA nucleotides contain the pentose sugar ____.
49.Complete the sentence about the nitrogenous bases of RNA.
Easy
In RNA, the base ____ replaces the base ____ found in DNA.
50.Complete the sentence about the bases that differ between DNA and RNA.
Easy
The four bases found in DNA are adenine, guanine, ____ and ____.
51.Complete the sentence about the bond that links nucleotides in a strand.
Easy
The phosphate group of one nucleotide is joined to the pentose sugar of the next nucleotide by a ____ bond, which is formed during a ____ reaction.
52.Complete the sentence about the bonds that hold the two strands of DNA together.
Easy
The two antiparallel strands of a DNA molecule are held together by ____ bonds between the nitrogenous bases, while the nucleotides within each strand are joined by ____ bonds.
53.Explain how the structure of DNA allows it to carry out semi-conservative replication.
Hard
54.Compare and contrast the structures of DNA and RNA.
Medium
55.Explain why both covalent and hydrogen bonds are important for the functioning of DNA.
Hard
56.Explain how the data provide evidence that the virus contains single-stranded DNA.
Hard
57.A DNA molecule has 30% of its bases as cytosine. Calculate the percentage of adenine in this DNA molecule.
Medium
- A20 %
- B18 %
- C22 %
- D24 %
58.A fruit fly DNA molecule is 5.6 cm long. One kilobase (Kb) of double-stranded DNA has a length of 0.34 μm. Calculate the number of Kb in the DNA of the fruit fly. Give your answer to the nearest whole number. (1 cm = 10,000 μm)
Hard
- A197648 Kb
- B148235 Kb
- C181177 Kb
- D164706 Kb
59.The DNA in the nucleus of a human cell contains about 3.2 gigabases. If one base pair is 0.34 nm long, calculate the length of this DNA in metres. Give your answer to 2 significant figures.
Hard
- A3.1 m
- B2.1 m
- C0.1 m
- D1.1 m

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