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DNA
o A polynucleotide
o Contains carbon, hydrogen, oxygen, nitrogen and phosphorus
o Made up of sub-units called nucleotides nucleotides are the building
blocks of DNA
...
o Deoxyribonucleic acid (DNA) is the macromolecule that carries the genetic
code
Nucleotide Structure
A mononucleotide (one nucleotide) consists of:
o Phosphate group
o A Pentose sugar (ribose C5H10O5 or deoxyribose C5H10O4)
o Organic base - belonging to one of 2 different groups
- pyrimidines
cytosine, thymine = single-ring bases
- purines
adenine, guanine = double-ring bases
- make up the longer bases
(NB although only 1 of 4 kinds of base can be used in a nucleotide, uracil is found
only in RNA, replacing thymine
...
o Base attached to carbon 1
...
)
- Formed when a phosphoric acid & a base are
chemically bonded to a sugar molecule
...
The continued linking of mononucleotides in this way forms a long
chain known as a polynucleotide
...
DNA
deoxyribonucleotides
is a sequence of bases, but
o Has a sugar-phosphate backbone
...
o DNA strands have a 5’ end (ends with carbon
5) and a 3’ end (ends with carbon 3)
...
o Strands linked by hydrogen bonds between pairs of organic bases
...
o Strands are anti-parallel – run in opposite
directions
o Strands are complementary, sequence in one
determines sequence in the other
...
e
...
Figure: Basic Structure of DNA
...
They form the structural backbone of the DNA molecule for each complete turn of
this helix, there are ten base pairs
...
Also called the antisense
strand
...
Also called the sense strand
...
The hydrogen
bonds between the bases are easily disrupted and reformed allowing them to
separate during DNA replication & protein synthesis
...
- It can also be copied onto RNA for protein synthesis
...
- It contains 10s of thousands of genes
...
o The set of polypeptides coded for in an individual’s DNA determines its nature
and development
...
- Thus it can pass unchanged from generation to generation
...
The Triplet code
A sequence of amino acids
that determines the
properties of a protein
...
- This position is fixed on a particular strand of DNA
...
o Genes code for polypeptide chains – 1 gene = 1 polypeptide
...
o A triplet of bases codes for one amino acid
o Genetic code = the relationship between DNA nucleotide bases and
amino acids
...
- The code is degenerate – some amino acids are coded for by more
than one codon most amino acids have more than one triplet
code
...
on mRNA
- NB if this first methionine molecule does not form part of the final
polypeptide, it is later removed
...
o The triplet code is non-overlapping, i
...
each base in the sequence is read
only once
...
A particular sequence of bases therefore codes for a specific amino acid sequence
...
When the polypeptide chain is complete, it forms
all or part of a protein which performs a vital role in the organism
...
o Introns:
- Regions of DNA on a gene that do not code for amino acids
...
o Exons:
- Transcribed and translated part of the gene i
...
Eukaryotic and prokaryotic DNA
Prokaryotic
DNA molecules are small and circular
...
Therefore, they have no
chromosomes
...
Found in Chromosomes
Eukaryotic DNA
Prior to cell division, DNA condenses to form chromosomes
...
This coil forms loops
...
At around the same time as this, the DNA replicates itself, and chromosomes appear
as two identical threads called sister chromatids
...
Sister chromatids end up in different daughter
cells
...
The
supercoiled
chromosomes can be moved around the
cell
Supercoiled chromosomes can be stained
and seen under the light microscope –
500nm thick
Chromosomes:
The number of chromosomes is
characteristic of a species
...
- Therefore, chromosomes always exist in pairs (except in gametes)
...
- Human cells have 23 pairs of homologous chromosomes
...
Alleles
Allele - a different version of a gene
...
Formation of new alleles
Changes may occur in the genetic material in a cell a
mutation
...
o
Mutations can occur in normal cells and in gametes
...
Mutations can result a change in the base sequence of a
gene
...
o The code produces a different polypeptide
...
o Many proteins are enzymes
...
o Therefore, mutations are normally fatal to the cell, or cells or an individual
developing from it