3. Chromosomes & Genes
The nucleus of living cells contain chromosomes which
are made up of a string of genes.
Different genes control the development of different
characteristics by issuing instructions to the cell.
4. Chromosomes and their genes are made of a molecule called DNA.
DNA molecules carry the code that controls what cells are made of and what they do. A
section of DNA which codes for a particular trait is called a Gene
DNA stands for
deoxyribonucleic
acid.
Each chromosome
is a very long molecule
of tightly coiled DNA.
What is DNA ?
The DNA molecule
looks like a
twisted ladder this
spiral shape is
called a
DOUBLE HELIX
5. The double helix ‘ladder’ of a DNA molecule is held together
by ‘rungs’ made from pairs of chemicals called bases.
DNA Bases
There are four bases:
A T
C G
ADENINE
CYTOSINE
THYMINE
GUANINE
6. Complimentary Base Pairs hold the strands of the DNA helix together.
Only A can pair with T
Only G can pair with C
Base Pairs
A T
C G
C G
A T
7. A Nucleotide:
Phosphate: Links
neighboring sugars
Sugar: One of two types possible: ribose
in RNA and deoxyribose in DNA
Base: Four types are possible in
DNA: adenine, guanine, cytosine
and thymine.
8. Sugar
(deoxyribose)
Phosphate
DNA Structure – Many Nucleotides
Hydrogen
bonds
(guanine)
(thymine)
(adenine)
(cytosine) • ‘rungs’ of the ladder
are made from
bases
• The backbone of
DNA is a chain of
alternating sugar-
phosphate
molecules.
