Life Sciences
DNA
Deoxyribonucleic acid – the molecule that determines the characteristics of living things
Nucleotides – The smaller molecules that make up DNA
Nucleotide molecules have three parts:
 Phosphate group
 Deoxyribose sugar
 One of four nitrogen-rich bases (or just called bases)
Nucleotides are organised in a way that makes DNA a double helix. The shape of a double helix is like a twisted rope ladder.
The nitrogen-rich bases pair up to form the rungs (horizontal support on a ladder). The four
bases adenine (A), thymine (T), guanine (G) and cytosine (C) all complement each other.
This is called complementary base pairing.
Adenine (A) – Thymine (T)
Cytosine (C) – Guanine (G)
Chromosomes
Chromosomes – long thin, thread-like structures in the nuclei of all cells in the human body
that contain a nucleus.
Each chromosome is one very long DNA molecule wrapped around proteins.
Most human cells have:
 46 chromosomes (half from your father and half from your mother)
 2 metres of DNA
 6 billion DNA bases (A, T, C, G)
 Around 20 000 genes
Of the 46 chromosomes in your cells, two are sex chromosomes – the ones that determine whether you are male or female. They
are called ‘X’ and ‘Y’ chromosomes.
 In females, the sex chromosomes are a pair of X chromosomes (XX).
 In males, the sex chromosomes are one X and one Y chromosome (XY).
The other 44 chromosomes are not sex chromosomes and are known as autosomes.
The autosomes in your cells are grouped into 22 pairs. The chromosomes in each pair are homologous (the same).
Genes – are sections of DNA arranged along the chromosomes.
The number of genes on a chromosome ranges from around 200 to 2000.
The difference between one gene and the next is the:
 order of bases along the DNA strand
 number of bases in that section of DNA
The order of the bases along the DNA strand is the genetic code. Each
gene codes (contains instructions) for a specific protein. These proteins
create the
structures and perform the actions needed for your cells to survive, grow and function.
Proteins may be:
 structural, e.g. collagen (found in tendons and ligaments) and keratin (found in skin, hair, nails, scales and feathers)
 enzymes, e.g. amylase (which helps digest starch) and lactase (the sugar found in milk that helps digest lactose)
 regulatory, e.g. growth hormone (stimulates growth and cell reproduction) and insulin (which controls blood glucose
levels)
Replication
1. The strands of the double helix separate from each other
in much the same way as a zip opens. The bases are then
exposed.
2. Within the nucleus there are individual nucleotides that
are not yet part of a DNA chain. These nucleotides pair
up with the exposed bases following the rules of
complementary base pairing. The sugar and phosphate
molecules bond with neighbouring nucleotides and new
strands of DNA are formed. Replication makes two
identical DNA molecules, each one a double helix. They
are also identical to the original parent DNA.
Each replicated chromosome has the two identical DNA molecules which are now called chromatids.
Cell division

Mitosis produces two daughter cells that are
identical to parent cell. This is the type of cell
division involved in growth and repair of the body.
Chromosome number
Diploid number (2n) – the number of chromosomes in your body. Two sets of autosomes and sex chromosomes.
Haploid number (n) – the number of chromosomes that gametes (eggs and sperm) have, which is only one copy of each
chromosome.
characteristics or traits of the individual.