Molecular Genetics
SEAB syllabus topic 12: DNA, genes and chromosomes, base pairing, the genetic code, transgenic organisms, making human insulin with bacteria, and the benefits and ethics of genetic engineering, with the why, the traps and the exam checks.
DNA is a double helix of two strands of nucleotides, and the strands are joined by complementary base pairing.
A gene is a section of a DNA molecule that codes for one polypeptide.
Genetic engineering transfers a gene from one organism into another, which then becomes a transgenic organism.
DNA, genes and chromosomes
outline the relationships among DNA, genes and chromosomes
DNA is the molecule that carries genetic information, a gene is a section of a DNA molecule, and a chromosome, before its DNA is copied, is one very long DNA molecule, packed with proteins, that carries many genes.
- DNA
- DNA: deoxyribonucleic acid, the molecule that carries the genetic information of an organism, found mainly in the chromosomes in the nucleus
- Gene
- Gene: a section of a DNA molecule, made of a sequence of nucleotides, that codes for one polypeptide
- Allele
- Allele: one of the different forms of the same gene, found at the same position on a pair of homologous chromosomes
- Chromosome
- Chromosome: before the DNA is copied, one very long DNA molecule wound with proteins into a compact rod, that carries many genes; it is easiest to see when the cell divides, when it has two identical chromatids
- Locus
- Locus: the position of a gene on a chromosome supports K325-12(a)
- Homologous chromosomes
- Homologous chromosomes: a pair of chromosomes that carry the same genes in the same order, with one chromosome of the pair inherited from each parent supports K325-12(a)
From nucleus to gene
- 1The nucleus of a cell contains the chromosomes.
- 2Each chromosome is made of DNA and protein, and before the DNA is copied it contains one very long DNA molecule.
- 3Along the DNA molecule are sections called genes, and each gene has its own locus.
- 4Each gene is a sequence of nucleotides, which are the building blocks of DNA.
- 5A human body cell has 46 chromosomes, arranged in 23 pairs, and the two chromosomes of a pair are homologous, apart from the X and Y chromosomes of a male, which are not a full matching pair.
DNA, gene, allele and chromosome compared
| Term | What it is | How it relates to the others |
|---|---|---|
| DNA | DNA is the molecule itself, a double helix built from nucleotides. | Genes are sections of DNA, and each chromosome contains DNA. |
| Gene | A gene is a short section of a DNA molecule. | A chromosome carries many genes, and each gene has its own locus on the chromosome. |
| Allele | An allele is a version of a gene. | For a gene on the autosomes, a body cell has two alleles, one on each chromosome of a homologous pair, but a male has one allele of a gene that is found just on the X chromosome. |
| Chromosome | Before the DNA is copied, a chromosome is one long DNA molecule, packed with proteins into a compact rod. | The chromosomes are easiest to see when the cell divides. |
Chromosomes condense before division
When a cell prepares to divide, its chromosomes coil up and become tightly packed, so they can be seen under a microscope and are easier to share out between the two daughter cells than long loose threads of DNA. supports K325-12(a)
Copied chromosomes
After the DNA has been copied and before the cell divides, each chromosome has two identical chromatids, and each chromatid contains one DNA molecule. supports K325-12(a)
Alleles at a locus
Two homologous chromosomes carry the same genes at the same loci, but they may carry different alleles of a gene. supports K325-12(a)
Depth: why and how
- Why do we say that a chromosome is not the same as a gene? A chromosome carries many genes along its DNA, so a gene is a small part of a chromosome.
- Why is DNA called the genetic material? The sequence of bases in the DNA of the genes carries the instructions for making polypeptides, and DNA is copied and passed on when cells divide.
- What is the difference between a gene and an allele? A gene is a section of DNA that codes for a polypeptide, and alleles are the different forms of that gene.
- Cross-link to Inheritance: the alleles of a gene are the letters used in genetic crosses, and a change in the base sequence of a gene is a mutation, which can produce a new allele.
- Cross-link to Cell Structure and Organisation: the nucleus is the organelle that contains the chromosomes, and it controls the activities of the cell.
Common mistakes
A gene is the same thing as a chromosome.
A chromosome carries many genes along its DNA, and a gene is a section of it.
Alleles are different genes.
Alleles are different forms of the same gene, found at the same locus on homologous chromosomes.
The chromosomes of a body cell are found in the cytoplasm.
The chromosomes of a body cell are found in the nucleus.
DNA is a protein.
DNA is a nucleic acid, and it carries the code for making proteins.
N C D G: the Nucleus contains chromosomes, Chromosomes are made of DNA and protein, DNA carries genes along its length, Genes code for polypeptides.
Read the four letters as a ladder from big to small: nucleus, chromosome, DNA, gene.A gene is a section of a DNA molecule, and a chromosome, before its DNA is copied, is one very long DNA molecule packed with proteins, so a chromosome carries many genes.
Exam check
Q1 Which statement about a chromosome is correct?
- A It is a single gene
- B It is a very long DNA molecule that carries many genes
- C It is a protein found in the cytoplasm
- D It is a section of an allele
Show answer
MCQ answer key B (It is a very long DNA molecule that carries many genes): before the DNA is copied, a chromosome is one very long DNA molecule, packed with proteins, that carries many genes.
Q2 What is an allele?
- A A section of a chromosome that has no gene
- B A pair of homologous chromosomes
- C A sequence of three bases
- D One of the different forms of a gene
Show answer
MCQ answer key D (One of the different forms of a gene): an allele is one of the different forms of the same gene, found at the same locus on homologous chromosomes.
Q3 Where are the chromosomes of a human body cell found?
- A In the nucleus
- B In the cell membrane
- C In the vacuole
- D In the ribosomes
Show answer
MCQ answer key A (In the nucleus): the chromosomes of a body cell are found in the nucleus.
Structured A student studies a human body cell under a microscope.
- State what a gene is, in terms of DNA. [1]
- State where the chromosomes of a human body cell are found, and state how many chromosomes the cell has. [2]
- Explain why the two chromosomes of a homologous pair can carry different alleles of the same gene. [2]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: a section of a DNA molecule
- Mark scheme: accept a sequence of nucleotides that is part of a DNA molecule
(b) 2 marks
- Mark scheme: in the nucleus
- Mark scheme: 46 (in 23 pairs)
(c) 2 marks
- Mark scheme: homologous chromosomes carry the same genes at the same loci
- Mark scheme: but each gene can have different forms, called alleles, and one chromosome of the pair is inherited from each parent
DNA structure
state that DNA is a double helix comprising two strands of nucleotides, each nucleotide formed of a sugar, a phosphate group and one of four different bases
DNA is a double helix made of two strands of nucleotides, and each nucleotide is formed of a sugar, a phosphate group and one of four different bases.
- Double helix
- Double helix: the shape of a DNA molecule, in which two strands are twisted around each other like a spiral ladder
- Nucleotide
- Nucleotide: the unit from which DNA is built, made of one deoxyribose sugar, one phosphate group and one nitrogenous base
- Deoxyribose
- Deoxyribose: the five-carbon sugar in DNA, which is why DNA is called deoxyribonucleic acid supports K325-12(b)
- Phosphate group
- Phosphate group: the part of a nucleotide that joins to the sugar of the next nucleotide, so it is part of the backbone
- Nitrogenous base
- Nitrogenous base: the part of a nucleotide that carries the code; the four bases in DNA are adenine (A), thymine (T), guanine (G) and cytosine (C)
- Sugar-phosphate backbone
- Sugar-phosphate backbone: the side of the ladder, made of alternating sugar and phosphate groups, with one backbone in each strand
- Hydrogen bond
- Hydrogen bond: a weak bond between the bases of the two strands, which holds the two strands together like the rungs of a ladder
The shape and the parts
The three parts of a nucleotide
| Part of a nucleotide | Position | Job |
|---|---|---|
| Phosphate group | The phosphate group joins the sugar of one nucleotide to the sugar of the next nucleotide in the strand. | Together with the sugar, it forms the sugar-phosphate backbone of the strand. |
| Deoxyribose sugar | The deoxyribose sugar is in the middle of the nucleotide, joined to both the phosphate group and the base. | Together with the phosphate group, it forms the sugar-phosphate backbone of the strand. |
| Nitrogenous base | The base is one of adenine, thymine, guanine or cytosine, and it sticks out sideways from the sugar. | The sequence of the bases along a strand is the genetic code, and the bases pair with the bases on the other strand. |
Two strands
A DNA molecule has two strands, and each strand is a long chain of many nucleotides.
Backbone
Each strand has a sugar-phosphate backbone on the outside, and the bases point inwards towards the other strand.
Hydrogen bonds
The bases of one strand are joined to the bases of the other strand by hydrogen bonds, which hold the double helix together.
Depth: why and how
- Why are the sugar and phosphate groups on the outside? They form the strong backbone of each strand, while the bases point inwards, where they pair with the bases of the other strand.
- Why does a DNA molecule need four different bases? The order of the four bases along a strand is the code, and the code is used to make specific polypeptides.
- Why are hydrogen bonds a good way to join the two strands? Hydrogen bonds are weak, so the strands can be separated when DNA is copied, but there are many of them, so the molecule is stable.
- Cross-link to Biological Molecules: a nucleotide is one of the small units that make up a nucleic acid, in the same way that an amino acid is a unit of a protein.
- Cross-link to Cell Structure and Organisation: the DNA is found in the nucleus, which is separated from the cytoplasm by the nuclear envelope.
Common mistakes
DNA is a single strand.
DNA is a double helix, made of two strands of nucleotides.
The rungs of the DNA ladder are made of sugar.
The rungs are pairs of bases held together by hydrogen bonds, and the sides are made of sugar and phosphate groups.
A nucleotide is made of a sugar and a base.
A nucleotide is formed of a sugar, a phosphate group and one base.
DNA has five different bases.
DNA has four different bases, adenine, thymine, guanine and cytosine.
Say D N A: Double helix of two strands held together by hydrogen bonds, Nucleotides make up each strand, each with a sugar, a phosphate group and one base, Adenine, thymine, guanine and cytosine are the four bases.
Each letter is a fact: the shape and the bonds, the parts of a nucleotide, and the four bases that carry the code.A DNA molecule is a double helix of two strands, and each strand is a chain of nucleotides formed of a sugar, a phosphate group and one of four bases.
Exam check
Q1 Which list shows the three parts of a DNA nucleotide?
- A Sugar, amino acid, base
- B Sugar, phosphate group, base
- C Phosphate group, fatty acid, base
- D Sugar, phosphate group, protein
Show answer
MCQ answer key B (Sugar, phosphate group, base): a nucleotide is formed of a sugar, a phosphate group and one of four different bases.
Q2 How many different bases are found in DNA?
- A Two
- B Three
- C Four
- D Five
Show answer
MCQ answer key C (Four): DNA has four different bases, which are adenine, thymine, guanine and cytosine.
Q3 What joins the two strands of a DNA molecule together?
- A Hydrogen bonds between the bases
- B Phosphate groups between the sugars
- C Peptide bonds between the bases
- D Sugar molecules between the phosphates
Show answer
MCQ answer key A (Hydrogen bonds between the bases): the two strands are joined by hydrogen bonds between the bases, and the sugar-phosphate backbone is within each strand.
Structured The diagram in a textbook shows part of a DNA molecule.
- State the shape of a DNA molecule. [1]
- Name the three parts of one nucleotide. [3]
- State what joins the two strands of the molecule. [1]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: double helix
(b) 3 marks
- Mark scheme: a sugar, which is deoxyribose
- Mark scheme: a phosphate group
- Mark scheme: a nitrogenous base
(c) 1 mark
- Mark scheme: hydrogen bonds between the bases
You've read the free 28%. Unlock the rest of Topic 12.
Still to read in this topic:
- Complementary base pairing
- Genes
- The genetic code and polypeptides
- Transgenic organisms
- Making human insulin with bacteria
- Benefits and ethical considerations of genetic engineering
$9 one-time · all 14 topics · no subscription
Less than a fast-food meal.
Yours for the whole O-Level.
Premium members get all 14 topics included.
Notes version 1.0 · Last updated 30 Sept 2026 · Topic 12 of 14