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DNA structure and chromosomes
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A summary of the lecture on the structure and principles of DNA and chromosomes
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History What DNA needs to be: A large base (purine – A, G) pairs1868 – Friedrich Miescher isolated - Stable over time (for storage) with a small base (pyrimidine –
nuclein – now called nucleic acid. He - Be able to be faithfully C, T) giving a DNA molecule of
noticed it had a high nuclear weight, replicated so progeny cells have 2nm diameter.
contained lots of phosphorous and the same genetic information Adenine pairs with thymine via 2
was acidic - Be able to control expression of hydrogen bonds, and guanine
1944 – Avery, MacLeod & McCarthy traits/encode proteins pairs with cytosine via 3
found that pathogenic bacteria can - Be able to change in a controlled hydrogen bonds. The bonds
turn non-pathogenic bacteria way to allow for variation and themselves are fairly weak
pathogenic, and though isolating adaptation allowing the strands to be
components of the cell found that it separated. However, due to the
was DNA that was transferred DNA is formed of two antiparallel large number of bonds it takes a
1953 – Hershey and Chase labelled strands held together by hydrogen lot of energy to break up the
DNA and protein with different bonds forming a double helix. Each molecule. This leads helps it in
isotopes and showed that it was the strand is a liner polymer made up of its function.
DNA that was transferred despite it repeating units called nucleotides and
originally being thought of as too has its own directionality (5’ -> 3’) DNA is also non-symmetrical,
simple with only 4 base types. Nucleotide= sugar group + base + having both a minor and major
Rosalind Franklin showed the DNA’s phosphate groove as the backbones are
helical structure and the use of Note that a nucleoside is just the closer on one side that the other.
phosphates as a ‘backbone’ through sugar group and a base. Each of its bases are planar and
x-ray crystallography
stacked on top of each other.
Genome – All the DNA in
(the nucleus of) a cell , or of
an organelle like
DNA and The bases house the genetic
information which is classed a
mitochondria or
chloroplasts. chromosome digital and quaternary code (4
bases) which allows for easy
Autosome – non-sex
storage and analysis by a
chromosome
structure computer.
The genetic material is distributed Nucleosides:
amongst discrete units – chromosomes, Deoxy-ribose + adenine = deoxy adenosine
and if stretched out would reach 2m + guanine = deoxy guanosine
long. Therefore, the DNA must be tightly + cytosine = deoxy cytidine
packed to fit into a cell. + thymine = deoxy thymidine
DNA wraps twice around 8 core histone
proteins ( 2 x 4 different types) to form The nucleoli are regions where
nucleosomes, set out like ‘beads-on-a- chromosomes carrying rRNA cluster
string’ to form chromatin. The together, rRNAs are synthesised and
chromatin is then further packaged by ribosomes are assembled.
histone H1 which brings the
nucleosomes together to form a
chromosome fibre. The chromatin is There are two types of chromatin:
then further condensed by scaffold - Euchromatin: a less compact DNA form
proteins making the DNA 10,000-fold containing genes that are readily
shorter than at its stretched length. So, expressed
chromosomes are composed of protein - Heterochromatin: a more compact form
and nucleic acids. containing DNA that is not transcribed
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