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KS4 Biology
Chromosomes,
Genes and DNA
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Contents
Chromosomes, Genes and DNA
Chromosomes and gender
DNA – the molecule of life
How DNA copies itself
DNA and the genetic code
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Why are cells different?
An oak tree has about 30 different types of cells.
How many different types of cells are humans made of?
Our bodies have more than 300 different types of cells.
Why do cells have different characteristics?
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What are proteins made of?
Proteins are long molecules made from chemical units
called amino acids.
protein
molecule
amino acid
What happens if amino acids are combined in a different
order?
Different combinations of amino acids make different proteins.
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Connecting proteins and genes
Consider the following two statements…
Genes carry the
instructions for
inherited characteristics.
Cells have different
characteristics because
they make different
types of proteins.
What is the connection between genes and proteins?
Genes contain the instructions
for making proteins.
What molecule has the instructions for making proteins?
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How do genes make proteins?
Genes are made of DNA. Proteins are made of amino acids.
Each amino acid is coded for by its own special sequence
of three bases called a codon:
codon
amino acid
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How do genes make proteins?
The order of codons in a gene determines the sequence
of amino acids.
The amino acids join together to form a protein molecule.
Each gene contains the sequence of bases for one protein.
Why is the sequence of bases in DNA called the genetic code?
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How do the genes make proteins?
The genetic code is the order of DNA bases which
determines the sequence of amino acids in a protein.
How many codons code for a protein of 20 amino acids?
1 amino acid = 1 codon
20 amino acids = 20 codons
How many bases code for a protein of 20 amino acids?
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How does DNA synthesize protein?
• Transcription – DNA makes a copy of itself that
can leave the nucleus. This copy is called
messenger RNA (mRNA). It is exactly the same
as DNA except for one thing; instead of the
nitrogenous base thymine, all RNA has the
nitrogenous base uracil (U).
• The newly created mRNA then leaves the
nucleus in search of a ribosome. Why does it
need to find a ribosome?
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How does DNA synthesize protein?
• Translation – Once it reaches a ribosome
(rRNA), it waits for its partner transfer RNA
(tRNA) to bring in the appropriate amino
acid.
• The mRNA knows which amino acid to
synthesize because its codon binds with
an anticodon on the tRNA molecule.
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Table of Anticodons
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Build your own protein molecule
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2004
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Genetic code quiz
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