Download Genetics - DNA

0
Genetics - DNA
DNA STRUCTURE
DNA is double stranded. It consists of two long
chains of nucleotides – chemical groups that
are comprised of a phosphate, sugar and base.
The two are wound around each other giving
DNA its double helix structure. The phosphate
& sugar groups of each nucleotide are strung
together end to end to form the back-bone of
the DNA molecule. DNA contains 4 different
types of bases A (Adenine), T (Thymine), C
(Cytosine), G (Guanine). These bases can pair
with each other by forming hydrogen bonds. It
is the bonds between the bases that holds the
two nucleotide strands together.
Base pairing rule
A (Adenine) always pairs with T (Thymine)
C (Cytosine) always pairs with G (Guanine)
The specific sequence of bases within a persons
DNA codes for their characteristics. That’s why
DNA is called the ‘code of life’.
The Human Genome
Each of our cells contains 46 chromosomes. These 46
chromosomes contain our entire genome – the set of
genetic instructions that makes us who we are. We
inherit half of these chromosomes from each of our
parents.
Gametes (sex cells) contain half the normal number
of chromosomes - they are haploid (n). For instance
human egg and sperm cells each contain 23
chromosomes. During fertilisation they will fuse
together to form a zygote – a single cell with the
normal number of chromosomes. This single cell will
grow and divide many times, copying it’s set of
chromosomes each time. Eventually it will develop
into an embryo. Each cell within the embryo will
contain its own copy of the 46 chromosomes – these
cells are diploid (2n).
The chromosomes inside one of our body cells they
can be arranged into 23 homologous pairs. Our cells
ultimately contain two versions of every chromosome
because we inherited one from our mother and the
other from our father. A photo of the chromosomes in
a cell, arranged from biggest to smallest in their
homologous pairs, is called a karyogram or karyotype.
Mutations
Pair of
alleles at a
gene locus
(location)
A mutation is any permanent change to the sequence of bases in an individual’s
genome. Small changes to the bases sequence of a gene can alter the function of the
protein it codes for, affecting one of our characteristics. A mutation can be:




Silent (no effect)
Deleterious (negative effect)
Lethal (deadly)
Advantageous (positive effect)
Gametic Mutations
Pair of
alleles at a
gene locus
(location)
Somatic mutations
Homologous chromosome pairs
have alleles for the same genes
at specific loci (locations)
Chromosomes, Genes & Alleles
Each of our Chromosomes is a long piece of DNA that has been tightly coiled.
Each chromosome contains many genes. We inherit two copies of each
chromosome (one from each parent) and this is why our chromosomes can be
arranged into homologous pairs.
A Gene is a section of DNA that contains a specific sequence of bases. This
sequence codes for a chain of amino acids that folds into a protein and
determines one of our characteristics. Each of our chromosomes contains many
genes for our different traits.
Alleles are alternative forms of a gene. We all carry genes for the same
characteristics, but some individuals have slightly different versions of these
genes. These alternative versions are usually very similar and differ only by a
few bases. In the example above both individuals have a different allele
(version) of the gene for hair type. Because we inherit two copies of each
chromosome, we also inherit two versions of each gene. The two alleles
(versions of each gene) are found on each of our homologous chromosome pairs.
If a mutation occurs in a germ cell during meiosis, the resulting gametes produced may
carry the mutation. If the affected gamete is involved in fertilisation, the resulting
zygote will carry the mutation in all of its cells. However, if the mutation occurs in a
somatic (body) cell during mitosis, then only a small portion of the individual will be
affected. Tumours are cells that have undergone a somatic mutation and lost the
ability to regulate their growth.