DNA, genes and chromosomes

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Big idea: the instructions inside every cell

Big idea (key concept): Systems. A cell is a system of parts working together. The nucleus holds the cell's instructions, and those instructions are written in a molecule called DNA.
Related concept: Structure. The way DNA is built, as a twisted ladder, explains how it stores information and how it can be copied exactly.
Global context: Scientific and technical innovation. In 1953 two scientists built a model of DNA. Since then, scientists have read the whole human genome, solved crimes with DNA evidence and made new medicines.
DNA (short for deoxyribonucleic acid) is a very long molecule that stores the instructions for building and running an organism. Think of it as a recipe book written in a four-letter alphabet.
Almost every cell in your body contains a full copy of the recipe book. A skin cell and a nerve cell look very different, but they hold the same DNA. They are different because they use different parts of the instructions.
Scientists describe the instructions at several levels, from big to small: cell, nucleus, chromosome, gene, DNA and its bases.

Zooming in from a cell to the DNA inside it

The structure of DNA: a twisted ladder

A DNA molecule is made of two strands twisted together into a spiral called a double helix. It looks like a ladder that has been twisted.
Each strand is built from small units called nucleotides. A nucleotide has three parts: a sugar, a phosphate group and one of four bases. The sugars and phosphates join up to form the two sides of the ladder, called the backbone.
The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G). The bases stick out from the backbone and meet in the middle, forming the rungs of the ladder.
The bases pair up in a fixed way, called complementary base pairing: A always pairs with T, and C always pairs with G. Each pair is held together by weak hydrogen bonds.
Because of this rule, if you know the order of bases on one strand, you know the other. A strand reading A-T-G-C is matched by a strand reading T-A-C-G.
The double helix was worked out in 1953 by James Watson and Francis Crick, using X-ray images made by Rosalind Franklin and Maurice Wilkins. Franklin's pictures showed the spiral shape.

The DNA double helix and how its bases pair up

Genes, the genetic code and proteins

A gene is a section of DNA that carries the instructions for making one protein. Proteins do most of the work in a cell. Some are enzymes, some build structures such as hair, and some, like haemoglobin, carry oxygen.
The instructions are in the order of the bases. The cell reads them in groups of three, called triplets. Each triplet codes for one amino acid, and amino acids are the building blocks of proteins.
There are four bases, so there are 4 x 4 x 4 = 64 possible triplets. That is more than enough to code for the 20 different amino acids that make proteins. Pairs of bases would give only 4 x 4 = 16, which is not enough.
The DNA stays safe inside the nucleus, so a copy of the gene is made. This copy is a similar molecule called messenger RNA (mRNA). In RNA, the base uracil (U) is used in place of thymine, so A in the DNA is copied as U.
The mRNA leaves the nucleus through a pore and reaches a ribosome in the cytoplasm. The ribosome reads the triplets and joins the correct amino acids together, in order, to make the protein.
A change in the order of bases can change the protein, and so change a characteristic of the organism. This is how genes control characteristics such as blood group or eye colour.

How a gene is copied into mRNA and used to build a protein

Chromosomes and the nucleus

DNA molecules are extremely long. To fit inside the tiny nucleus, each DNA molecule is wound tightly around proteins and coiled up. A coiled package of DNA is a chromosome.
Each chromosome carries hundreds or thousands of genes. The genes are arranged one after another along the DNA.
Human body cells have 46 chromosomes, arranged as 23 pairs. In each pair, one chromosome came from your mother and one from your father. One of the 23 pairs is the sex chromosomes (XX or XY).
Sperm and egg cells (gametes) have only 23 chromosomes, one from each pair. At fertilisation the two gametes join, which restores the number to 46.
Different species have different numbers of chromosomes: a fruit fly has 8, a pea plant 14, a chimpanzee 48 and a dog 78. The number does not show how complex the organism is.
Before a cell divides, it copies all its DNA, so each new cell gets a full set of chromosomes.
The nucleus is surrounded by a nuclear membrane with tiny nuclear pores. Inside it, the DNA is spread out as thin threads, and a dark region called the nucleolus helps to make ribosomes.

From a cell to a chromosome to the DNA double helix

Alleles and the genome

You have two copies of most genes, one on each chromosome of a pair. The two copies may be slightly different. Different versions of the same gene are called alleles.
Both copies sit at the same position on the chromosome pair. This position is called the gene's locus. For a gene for ear lobe shape, one allele might code for attached lobes and another for free lobes.
Your genome is the complete set of DNA in a cell. The human genome has about 3 billion base pairs and about 20 000 genes. Only a small fraction of the DNA, about 1 to 2 per cent, codes for proteins.
The Human Genome Project read the order of the bases in human DNA and was declared finished in 2003. It showed that any two people have DNA that is about 99.9% the same.
Why is DNA the same shape in all organisms? Bacteria, plants, fungi and animals all use a double helix built from the same four bases, and all read the code in triplets. Most scientists take this as strong evidence that all living things share a common ancestor.
Because the code is shared, a gene from one species can often work in another. This is the basis of genetic engineering, such as bacteria that make human insulin.

Alleles at the same locus on a pair of chromosomes

Think like a scientist: extracting DNA from fruit

You can see DNA with simple equipment. Mash a ripe strawberry in a bag with a detergent and salt solution. The detergent breaks open the cell and nuclear membranes, and the salt helps the DNA clump together.
Filter the mixture to remove the lumps. Then very slowly pour ice-cold ethanol (an alcohol) down the side of the tube, so it forms a layer on top. Your teacher should handle the ethanol, because it is flammable.
DNA does not dissolve in cold ethanol. After a minute, white, stringy clumps of DNA appear where the two liquids meet, and you can lift them out with a stick.
The independent variable is the one you change on purpose, such as the type of fruit or the temperature of the solution. The dependent variable is what you measure, such as the length of the DNA clump or its mass. A fair test controls everything else: mass of fruit, volume of solution, mashing time and the volume of ethanol.
A single trial can be misleading. Repeating the experiment and calculating a mean makes the results more reliable.
Inquiry task: plan an investigation to find out which fruit, strawberry or banana, gives more DNA. State what you will change and measure, name three variables you will control, explain how you will make your results reliable, and suggest one limitation of judging the amount of DNA by eye.

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Câu hỏi luyện tập

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1.In an animal cell, where is most of the DNA found?
Easy
- AIn the cell membrane
- BIn the nucleus
- CIn the cytoplasm
- DIn the ribosomes
2.What does DNA stand for?
Easy
- ADouble nucleic atom
- BDioxygen nitrogen acid
- CDeoxyribose nuclear agent
- DDeoxyribonucleic acid
3.What shape is a DNA molecule?
Easy
- AA double helix
- BA single straight chain
- CA flat sheet of bases
- DA hollow sphere
4.Chromosomes are made of DNA.
Easy
True or false?
5.In DNA, which base pairs with adenine (A)?
Easy
- ACytosine (C)
- BGuanine (G)
- CThymine (T)
- DUracil (U)
6.In DNA, which base pairs with cytosine (C)?
Easy
- AAdenine (A)
- BThymine (T)
- CGuanine (G)
- DCytosine (C)
7.What is a gene?
Easy
- AA section of DNA that codes for a protein
- BA whole chromosome made only of protein
- CA cell that stores DNA for the body
- DA sugar that links the DNA strands
8.How many chromosomes are there in a normal human body cell?
Easy
- A23
- B44
- C92
- D46

Thêm câu hỏi

9.Complete the sentence.
Easy
The four bases found in DNA are A, T, C and ____.
10.The nucleus controls the activities of a cell because it contains the cell's DNA.
Easy
True or false?
11.Match each term to its meaning.
Easy
- Gene
- Chromosome
- Nucleus
- Base
- The part of the cell that contains the DNA
- One of the four chemicals A, T, C and G
- A section of DNA that codes for a protein
- A long, coiled package of DNA and protein
12.Put these in order from the smallest to the largest.
Easy
- Nucleus
- Base
- Cell
- Chromosome
- Gene
13.Which two scientists built the first model of the DNA double helix in 1953?
Easy
- ADarwin and Wallace
- BWatson and Crick
- CHooke and Pasteur
- DMendel and Linnaeus
14.Human body cells have 23 pairs of chromosomes.
Easy
True or false?
15.Which of these are found inside the nucleus of a human body cell? (select all that apply)
Easy
- AChromosomes
- BChloroplasts
- CDNA
- DA cell wall
- EGenes
16.Complete the sentence.
Easy
Genes carry the instructions for making ____.
17.How are genes and chromosomes related?
Easy
- AA chromosome carries many genes
- BA gene carries many chromosomes
- CA gene is the same thing as a chromosome
- DGenes are kept outside the nucleus
18.What holds the two strands of a DNA molecule together?
Easy
- AA strand of protein in the middle
- BThe cell membrane around the nucleus
- CBonds between pairs of bases
- DSugar crystals between the strands
19.Bacteria, plants and animals all use the same four bases in their DNA.
Easy
True or false?
20.One strand of DNA has the bases A-A-C-G-T. Which sequence is on the complementary strand?
Medium
- AT-T-C-G-A
- BT-T-G-C-A
- CA-A-C-G-T
- DC-C-A-T-G
21.One strand of DNA reads G-C-T-A-C. Which sequence is on the other strand?
Medium
- AC-G-T-A-C
- BG-C-T-A-C
- CT-A-G-C-A
- DC-G-A-T-G
22.In a sample of DNA, 35% of the bases are thymine. What percentage of the bases are adenine?
Medium
23.Why does adenine always pair with thymine and never with cytosine?
Medium
- AThe cell chooses the partner at random each time
- BEach base has a shape that fits only its partner
- CAdenine and cytosine are the same size so they cannot join
- DThe sugar groups only let A and T sit side by side
24.What forms the two sides of the DNA ladder?
Medium
- APairs of bases joined together
- BChains of amino acids
- CStrands of protein
- DAlternating sugar and phosphate groups
25.How many bases code for one amino acid?
Medium
- AThree
- BOne
- CTwo
- DFour
26.What decides which protein a gene makes?
Medium
- AThe number of chromosomes in the cell
- BThe colour of the nucleus in the cell
- CThe order of the bases in the gene
- DThe thickness of the cell membrane
27.Where does the cell join amino acids together to make a protein?
Medium
- AOn the chromosome in the nucleus
- BInside the nuclear membrane
- CAt a ribosome in the cytoplasm
- DIn the cell membrane
28.Why is a copy of the gene, called mRNA, sent to the ribosome instead of the DNA itself?
Medium
- AThe DNA stays safe in the nucleus while a copy does the job
- BDNA is made of amino acids that the ribosome cannot read
- CmRNA contains more genes than the chromosome does
- DRibosomes can only read molecules made of thymine
29.Which statements about the human genome are correct? (select all that apply)
Medium
- AIt contains about 3 billion base pairs
- BIt is found only in egg and sperm cells
- CIt is the complete set of DNA in a cell
- DIt is identical in humans and fruit flies
- EIt includes all of a person's genes
30.Two unrelated people have DNA that is about 99.9% the same.
Medium
True or false?
31.A human sperm cell contains 46 chromosomes.
Medium
True or false?
32.A dog has 78 chromosomes in each body cell. How many are in one dog egg cell?
Medium
- A78
- B156
- C19
- D39
33.Why does a cell copy its DNA before it divides?
Medium
- ASo that the new cells will have half the DNA
- BSo that each new cell gets a full set of chromosomes
- CSo that the nucleus can be removed first
- DSo that the cell can make more bases
34.Match each organism to the number of chromosomes in one of its body cells.
Medium
- Human
- Fruit fly
- Pea plant
- Dog
- 78
- 46
- 14
- 8
35.Put these steps of making a protein in order.
Medium
- A ribosome reads the mRNA
- A gene in the DNA is copied as mRNA
- Amino acids are joined to make a protein
- The mRNA leaves the nucleus
36.What is an allele?
Medium
- AA different version of the same gene
- BA gene found only on the X chromosome
- CA section of DNA that never codes for anything
- DA chromosome found only in sperm cells
37.A person has 23 pairs of chromosomes. Where does each pair come from?
Medium
- ABoth chromosomes from the mother
- BBoth chromosomes from the father
- COne chromosome from each parent
- DChromosomes made new in each cell
38.Complete the sentence.
Medium
The two strands of DNA are held together by weak ____ bonds between the bases.
39.Bacteria, plants and animals use the same four bases and read the code in triplets. What does this suggest?
Medium
- AAll living things have the same number of chromosomes
- BAll living things share a common ancestor
- CAll living things make exactly the same proteins
- DAll living things have identical genes
40.A sample of double-stranded DNA is found to be 22% cytosine. What percentage of its bases are adenine?
Hard
41.A section of a gene is 90 bases long and is read in triplets. Up to how many amino acids can it code for?
Hard
42.A section of DNA reads T-A-C-G-G-A. It is copied into mRNA. Which is the mRNA sequence?
Hard
- AA-T-G-C-C-T
- BU-A-C-G-G-A
- CT-A-C-G-G-A
- DA-U-G-C-C-U
43.Why do cells read the DNA code in groups of three bases rather than groups of two?
Hard
- APairs of bases cannot form hydrogen bonds with each other
- BPairs of bases give only 16 combinations, not enough for 20 amino acids
- CTriplets are the only groups that can fit inside a nucleus
- DTwo bases would code for proteins that are too small
44.
A student measured the percentage of each base in four DNA samples.
Sample A (%) T (%) G (%) C (%)
1 30 30 20 20
2 24 24 26 26
3 28 18 27 27
4 32 32 18 18
Which sample is NOT likely to be normal double-stranded DNA?
Hard
- ASample 1
- BSample 2
- CSample 4
- DSample 3
45.A student extracts DNA from strawberries using solutions at different temperatures (20, 40 and 60° Celsius) and measures the length of the DNA clump in millimetres. What is the dependent variable?
Hard
- AThe length of the DNA clump
- BThe temperature of the solution
- CThe mass of strawberry used each time
- DThe type of fruit that is tested
46.Which plan is a fair test of whether strawberries or bananas give more DNA?
Hard
- AUse 50 g of each fruit and keep the solution, mashing time and ethanol the same
- BUse 50 g of strawberry and 100 g of banana, with the same solution
- CMash the strawberries for longer because they are smaller than bananas
- DUse warm solution for the banana and cold solution for the strawberry
47.A student extracts more DNA from one strawberry than from one banana and says, 'Strawberries always give more DNA than bananas.' What is the best evaluation of this conclusion?
Hard
- AIt is strong, because strawberries are red and bananas are yellow
- BIt is weak, because one trial of each fruit is not enough to be sure
- CIt is strong, because the student saw the DNA clump with their own eyes
- DIt is weak, because bananas have no DNA at all
48.Describe how DNA, genes and chromosomes are related, starting with the nucleus.
Hard
49.Explain how complementary base pairing allows DNA to be copied accurately.
Hard

Ghi chú ôn tập

## The big idea - **DNA** stores the instructions for an organism. It is found in the **nucleus** (**Systems**, **Structure**). - Scientists describe the instructions at several levels: cell, nucleus, chromosome, gene, DNA and bases. ## The structure of DNA - A **double helix** of two strands. The backbone is sugar and phosphate; the rungs are pairs of **bases**. - **A pairs with T** and **C pairs with G**. This rule lets one strand predict the other and lets DNA be copied accurately. - Watson and Crick built the model in 1953, using X-ray images by Franklin and Wilkins. ## Genes and proteins - A **gene** is a section of DNA coding for a **protein**. Bases are read in **triplets**, and each triplet codes for an amino acid. - A gene is copied as **mRNA**, which goes to a **ribosome** where the protein is built. ## Chromosomes, alleles and the genome - Human body cells have **46 chromosomes** (23 pairs). Gametes have **23**. - **Alleles** are versions of a gene. The **genome** is all of a cell's DNA: about 3 billion base pairs, about 20 000 genes. - All life shares the same DNA structure and code, which points to a **common ancestor**. ## Investigating - DNA can be extracted from fruit. Change one variable, measure one, control the rest, repeat and take a mean.

DNA: The molecule in cells that stores the instructions for building and running an organism.
Double helix: The twisted-ladder shape of a DNA molecule, made of two strands.
Nucleotide: The unit that DNA is built from: a sugar, a phosphate group and a base.
Base: One of the four chemicals in DNA: adenine, thymine, cytosine and guanine.
Complementary base pairing: The rule that A pairs with T and C pairs with G.
Gene: A section of DNA that carries the instructions for making a protein.
Triplet: A group of three bases that codes for one amino acid.
mRNA: A copy of a gene that carries the instructions from the nucleus to a ribosome.
Chromosome: A coiled package of DNA and protein that carries many genes.
Nucleus: The part of the cell that contains the DNA.
Allele: A different version of the same gene.
Genome: The complete set of DNA in a cell.
Gamete: A sex cell (sperm or egg) with half the usual number of chromosomes.
Locus: The position of a gene on a chromosome.

Thẻ ghi nhớ

What does DNA do?

It stores the instructions for building and running an organism.

Describe the structure of DNA.

Two strands twisted into a double helix. A sugar-phosphate backbone forms the sides and pairs of bases form the rungs.

Name the four bases in DNA.

Adenine (A), thymine (T), cytosine (C) and guanine (G).

What is the base-pairing rule?

A pairs with T, and C pairs with G.

What is a nucleotide?

A unit of DNA made of one sugar, one phosphate group and one base.

What is a gene?

A section of DNA that carries the instructions for making one protein.

What is a triplet?

A group of three bases that codes for one amino acid.

Why are triplets needed, not pairs?

Four bases in pairs give only 16 codes. Triplets give 64, enough for the 20 amino acids.

What is mRNA and why is it needed?

A copy of a gene. It carries the instructions out of the nucleus, so the DNA can stay safe inside.

What is a chromosome?

A long molecule of DNA coiled around proteins. Each chromosome carries many genes.

How many chromosomes do human body cells and gametes have?

Body cells have 46 (23 pairs). Gametes have 23.

Why is DNA copied before a cell divides?

So that each new cell receives a full set of chromosomes.

What is an allele?

A different version of the same gene, found at the same locus on a chromosome pair.

What is the genome?

The complete set of DNA in a cell. The human genome has about 3 billion base pairs.

How similar is the DNA of two people?

About 99.9% the same.

What does the shared DNA structure and code of all life suggest?

That all living things share a common ancestor.

How can DNA be extracted from fruit?

Mash the fruit with detergent and salt, filter it, then add ice-cold ethanol. White clumps of DNA appear.

Who built the first DNA model, and who provided key evidence?

Watson and Crick built it in 1953, using X-ray images by Rosalind Franklin and Maurice Wilkins.

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Sample	A (%)	T (%)	G (%)	C (%)
1	30	30	20	20
2	24	24	26	26
3	28	18	27	27
4	32	32	18	18