Chromosomes, DNA and genes
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16.1.1 Understand that chromosomes contain DNA, which carries genetic information in the form of genes.
The nucleus contains chromosomes. Each chromosome contains one long molecule of DNA. A gene is a short length of that DNA, and it carries the instructions for making one protein.
What is inside the nucleus
In the Cell Structure and Organisation chapter you learned that the nucleus controls the cell. It does this with the genetic information it holds — the instructions for building and running the organism.
The genetic information is stored in thread-like structures called chromosomes. You can only see them clearly when a cell is dividing, because then they coil up tightly and become short and thick.
- Each chromosome contains one very long molecule of DNA, wound up tightly around proteins.
- DNA is the chemical that carries the genetic information.
- A gene is a short length of DNA that codes for a protein. One DNA molecule carries hundreds or thousands of genes, one after another along its length.
So the genetic information is carried in the form of genes. Each gene tells the cell how to make one protein. Proteins produce our characteristics — for example, an enzyme makes the pigment that gives eye colour. This is why genes control characteristics. (How the DNA code is read is taught in the Inheritance chapter.)
From large to small: a cell contains a nucleus; the nucleus contains chromosomes; each chromosome contains one long DNA molecule, wound around proteins; a gene is a short length of that DNA molecule.
Putting them in order of size
Exam questions often ask you to order these structures. Build the order from the inside out:
- A gene is only a part of a DNA molecule, so a gene is the smallest.
- A DNA molecule is made of many genes joined end to end.
- A chromosome is that one DNA molecule, wound around proteins — so a chromosome is a little larger than its DNA molecule.
- A nucleus holds many chromosomes.
- A cell holds the nucleus.
- A tissue is a group of many cells.
Two traps: a gene is never larger than a chromosome, and there is one DNA molecule in each chromosome (not "many chromosomes make one DNA molecule").
Complete the sentences using words from the list. Each word may be used once.
cell chromosome DNA gene nucleus protein
"The ______ of a cell contains thread-like structures called ______s. Each one is made of a long molecule of ______. A short length of this molecule that codes for one ______ is called a ______."
Show full working
- 1
First blank: the structure that holds the chromosomes is the nucleus.
The sentence is about something "of a cell" that contains other structures, so it must be the largest one left after "cell".
- 2
Second blank: the thread-like structures are chromosomes.
"Thread-like structures" is the phrase examiners use for chromosomes.
- 3
Third blank: each chromosome is made of a long molecule of DNA.
One chromosome contains one long DNA molecule.
- 4
Fourth blank: a length of DNA codes for one protein.
Genes are instructions for making proteins — not for making carbohydrates or fats.
- 5
Fifth blank: that short length of DNA is a gene.
This is the definition of a gene. "Cell" is the word left over; it is not needed.
nucleus; chromosomes; DNA; protein; gene.
Say the chain out loud: nucleus → chromosomes → DNA → genes → proteins.
What is the correct order for the sizes of the pieces of genetic material shown in the table, from smallest to largest?
Options
| smallest → largest | |||
|---|---|---|---|
| A | gene | molecule of DNA | chromosome |
| B | molecule of DNA | gene | chromosome |
| C | chromosome | molecule of DNA | gene |
| D | gene | chromosome | molecule of DNA |
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- 1
Start with the smallest: a gene is a short length of a DNA molecule, so the gene comes first.
A part is always smaller than the thing it is part of.
- 2
Next: the DNA molecule. Last: the chromosome, which is that DNA molecule wound around proteins.
The chromosome contains the DNA molecule, so it is the largest of the three.
- 3
B is wrong: it puts the whole DNA molecule before one of its own genes.
A gene cannot be larger than the molecule it is part of.
- 4
C is wrong: it is the order from largest to smallest — backwards.
Always check which way round the question wants the order.
- 5
D is wrong: a chromosome contains its DNA molecule, so the chromosome cannot be smaller.
This is the tempting option if you think DNA is "bigger" because it is a long molecule. The chromosome is the DNA plus the proteins it is wound around.
A
gene < DNA molecule < chromosome. Learn it as "part < whole < whole wound up".
Starting with the smallest, and ending with the largest, what is the correct sequence of these parts of an organism?
Options
A chromosome → gene → nucleus → cell → tissue
B chromosome → gene → nucleus → tissue → cell
C gene → chromosome → nucleus → cell → tissue
D gene → chromosome → nucleus → tissue → cell
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- 1
The first item must be the gene, because a gene is part of a chromosome. That removes A and B.
Deal with the start of the sequence first — it removes two options at once.
- 2
The last two: a tissue is a group of cells, so the cell comes before the tissue. That removes D.
D puts tissue before cell, but a tissue is made of many cells.
- 3
C is left: gene → chromosome → nucleus → cell → tissue.
Check the middle as well: chromosomes are inside the nucleus, and the nucleus is inside the cell.
C
Each structure is inside the next one: a gene is in a chromosome, a chromosome is in a nucleus, a nucleus is in a cell, and cells make up a tissue.
Chromosomes, DNA molecules and genes are pieces of inherited material with different sizes.
Which statement describes the relationship between these pieces of inherited material?
Options
A A chromosome is larger than a gene.
B A gene is larger than a chromosome.
C Multiple chromosomes make up a DNA molecule.
D Multiple DNA molecules make up a gene.
Show full working
- 1
Use the order gene < DNA molecule < chromosome and test each statement.
Each option is a claim about size or about "what is made of what". The order answers all of them.
- 2
A: a chromosome contains a DNA molecule, which contains many genes. So a chromosome is larger than a gene — correct.
This matches the order.
- 3
B is wrong: it is the reverse of A.
A gene is only a short part of the DNA in a chromosome.
- 4
C is wrong: it is the other way round — each chromosome contains one DNA molecule.
Many chromosomes do not join to make one DNA molecule.
- 5
D is wrong: a gene is a short length of one DNA molecule; many genes make up a DNA molecule, not the reverse.
C and D both swap the part and the whole.
A
If an option says "multiple X make up Y", check that X really is the smaller one.
Your turn
- 15090/22 M/J 2019 Q3(b)(i)1 mark
Name the part of a cell that contains the genes.
Show solution
- 1
Genes are lengths of DNA, and the DNA is in the chromosomes, which are in the nucleus.
Follow the chain outwards from the gene.
- 2
The part of the cell is the nucleus. (The mark scheme also accepts chromosome or DNA.)
"Part of a cell" most naturally means an organelle, so nucleus is the safest answer.
AnswerThe nucleus (chromosomes / DNA also accepted).
- 1
- 21 mark
Put these in order of size, starting with the smallest: nucleus, gene, tissue, DNA molecule, cell, chromosome.
Show solution
- 1
Smallest: gene — it is a short part of a DNA molecule.
Always start from the part that is inside everything else.
- 2
Then: DNA molecule, then chromosome (the DNA molecule wound around proteins).
One chromosome contains one DNA molecule, so the chromosome is slightly larger.
- 3
Then: nucleus (holds the chromosomes), cell (holds the nucleus), tissue (a group of cells).
Each one is inside the next.
Answergene → DNA molecule → chromosome → nucleus → cell → tissue.
- 1
- 32 marks
A student writes: "A gene is a long molecule of DNA that is made of many chromosomes." Correct the student's sentence.
Show solution
- 1
A gene is a short length of DNA (part of a DNA molecule), not a whole long molecule.
The student has swapped "part" and "whole".
- 2
A chromosome contains one long DNA molecule, which is made of many genes — genes are not made of chromosomes.
The size order gene < DNA molecule < chromosome fixes both errors.
AnswerA gene is a short length of DNA that codes for a protein. A chromosome contains one long DNA molecule, which carries many genes.
- 1
The rest of this note
Can you do all of these?
State that chromosomes contain DNA, which carries genetic information as genes, and order gene < DNA molecule < chromosome < nucleus < cell < tissue
Describe haploid (one set) and diploid (two sets, a pair of each type) nuclei; state that a human diploid cell has 23 pairs (46 chromosomes); say which cells are haploid (gametes) and which are diploid (body cells)
Work out the haploid number, the diploid number or the number of pairs from any one of them, using the species' own numbers
Describe mitosis (genetically identical cells, chromosome number maintained); explain that chromosomes are copied first, and recognise the mitosis shape of a DNA graph (1 → 2 → 1)
Outline the four roles of mitosis — growth, repair of damaged tissues, replacement of dying cells, asexual reproduction — with an example of each
Describe stem cells: unspecialised, divide by mitosis, daughter cells can become specialised; state that they are diploid
State that cancers form as a result of uncontrolled cell division, forming a tumour
Describe meiosis (reduction division, diploid → haploid, genetically different cells, involved in making gametes); compare it with mitosis point by point
Describe asexual reproduction (genetically identical offspring from one parent) and identify examples: binary fission, tubers, runners, bulbs, suckers, plantlets, cuttings, tissue culture; calculate bacterial numbers by doubling
Describe sexual reproduction (fusion of haploid nuclei at fertilisation, diploid zygote, genetically different offspring); trace chromosome numbers and find meiosis, fertilisation, mitosis and haploid stages on a life-cycle diagram
Discuss the advantages and disadvantages of asexual and sexual reproduction, linking each to identical offspring or variation