Chromosomes, genes and DNA Cambridge IGCSE Co-ordinated Sciences (9–1) revision
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In plain words
Inside the nucleus of nearly every cell is a full set of instructions for building you, written in a chemical code called DNA. The DNA is packed into chromosomes, and each instruction on it, a gene, tells the cell how to make one protein.
Three things to know
- A chromosome is a long molecule of DNA. A gene is a length of DNA that codes for a protein. An allele is one of the alternative versions of a gene.
- Human body cells have 23 pairs of chromosomes (46). Sex is decided by one pair: XX is female, XY is male.
- DNA is a double helix: two strands held together by pairs of bases. A always pairs with T, and C with G. The order of the bases is the code.
Tips and tricks
- Size order, biggest first: nucleus, chromosome, gene, base.
- Gene and allele: everyone has the gene for eye colour, but you might have the allele for brown or the allele for blue.
It lands in your notebook with its questions as flashcards.
Chromosomes, genes and DNA: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is a gene?
Each chromosome carries many genes.
How many chromosomes are there in a human body cell?
They are in 23 pairs.
Which sex chromosomes does a human male have?
The Y chromosome is inherited from the father.
In DNA, which base pairs with C?
C pairs with G, and A pairs with T.
What is an allele?
Different alleles give different forms of a characteristic.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Chromosomes, genes and DNA
Cambridge IGCSE Co-ordinated Sciences (9–1) 0973 · 7 marks · papermunch.org
Name ______________________________ Date ______________
State what is meant by a chromosome, a gene and an allele.[3]
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A chromosome is a (thread-like) structure made of DNA. A gene is a length of DNA that codes for a protein. An allele is an alternative form of a gene.
State the sex chromosomes of a human male and a human female.[2]
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Male: XY. Female: XX.
One strand of DNA has the bases A T G C C. Write the bases on the other strand.[2]
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T A C G G.
Answers: Chromosomes, genes and DNA
- 1. A chromosome is a (thread-like) structure made of DNA. A gene is a length of DNA that codes for a protein. An allele is an alternative form of a gene.
- 2. Male: XY. Female: XX.
- 3. T A C G G.