DNA is the genetic material of :
(in 1928-Frederick Griffith showed this
subject by transformation)
Viruses (in 1952,result of an experiment by
Alfred Hershey and Martha Chase)
In some viruses, the genetic material is RNA.
DNA-is a molecule that all
living organisms carry in
every cell in their body.
DNA- Contains information
needed to caryy out cell
activities, it is the genetic
→every person’s DNA is
unique result DNA can serve
as an individual identifier.
In the1900-1950:a series of experiments revealed two
important features of DNA.
→ DNA are passed down from
parent to offspring
→ the instructions on how to create a
body and control its growth and
development are encoded in the DNA
Edwin Chargaff at Columbia university had measured the
base composition of nucleic acid .
The amount of adenine
nearly always equalled the
amount of thymine .
The amount of cytosine
nearly always equalled the
amount of guanine .
In 1951- X-ray diffraction
studies Maurice Wilkins showed
a diffraction pattern of DNA at a
scientific meeting in Napales
In 1952- Rosalin Franklin
produced X-ray pictures of DNA
that were critical to decoding its
In 1953:Francis Crick and
James Watson,deduce the
exact structure of DNA
Nucleic acids are made from repeating units of nucleotides
1) A phosphate group
Two major forms of nucleic acid polymers DNA and RNA
The physical structure of DNA is frequently described
as a “double helix.”
What exactly is a double helix?
imagine long ladder twisted around like a spiral
the backbone of DNA molecule- each is made from
alternating molecules : a sugar, then a phosphate,
then a sugar, then a phosphate, and so on.
the backbone of DNA molecule-
nucleotides are connected to
each other to from a long chain
Fromed between the phosphate
group of one nucleotide and 3´-
OH of the next nucleotide.
The rungs of the ladder-
Attached to each sugar,
and protruding like half
of a rung on the ladder,
is one of the nitrogen-
There are four types of DNA nucleotides, each
differing in their nitrogen base only
1) Adenine (double ring= purine) 2) Thymine (singlering=prymidine)
3)Guanine (double ring=purine) 4)Cytosine(single ring=pyrimidine)
These nitrogen bases
are held together by :
The two strands of nucleotides are anti-parallel to each
other .one is oriented 5 to 3 , the other 3 to 5 the two strand
warp around each other to create the helical of the molecule
In the cell, the most commonly seen from of DNA
double helix is called the B form or the Watson-
The double helix can also exist in an A form which is
shorter and wider than the B form with the bases at
an angle rather than perpendicular to the helix axis
The A form is seen in RNA double helices and in
RNA-DNA hybrid helix structures observed in
transcription and RNA processing
structural variation in DNA reflects three things :
The different possible conformation of the deoxyribose
Rotation about the contiguous bonds that make up the
Free rotation about the C-1´- N glycosyl bond
-The Watson-Crick structure is
also referred to as B form DNA
-The B form is the most stable
structure and standard point of
reference in DNA study.
-A and Z forms have been
characterized.(Z form is very
The major and minor grooves are lined by sequence-
specific hydrogen bonding group
- DNA in B form has a major groove and minor groove
- The presence of the grooves allows access to the
hydrogen-bonding capabilities of the exposed bases.
- The hydrogen-bonding capabilities provides a
mean of sequence specific interactions between
DNA and the molecules is most interact with in the
process of replication and transcription
Supercoiling-The double helix can also wind
around itself to change the overall conformation or
topology of the DNA in space.
supercoiling creates tension
in the DNA , and thus can
only occur in the DNA has
no free ends
Mutation change the sequence of DNA
Spontaneous mutations-all organism suffer a certain number
of mutations as the result of normal cellular operations or
random interaction with the environment.
Induced mutation-the occurrence of mutations can be
increased by treatment with certain compounds ,these are
A point mutation-changes only a single base pair and can
be caused by either of two types of event :
chemical modification of DNA directly changes one base into
a different base.
An error during the replication of DNA cause by the wrong
base to be inserted into a polynucleotide
A point mutation
Hot spots - some sites gain far more than the number of
mutations ,expected from a random distribution thay may
have ×10 or even × 100 more mutation than predicted by
Genome-The full set of DNA present in an
The genome incloudes:
- Choromosal DNA
- DNA in plasmids
- Organellar DNA as found in
chloroplasts ( in eukaryotes)
In prokaryotest the information contained within
circular pieces of DNA ( such as all bacteria)
In eukaryotes this information is laid out in long
linear strands of DNA in the nucleus.(such as humans)
The genome in viruses
than being one super-long
DNA strand, eukaryotic
DNA exists as many
smaller, more manageable
Gene - a sequence of bases in a DNA molecule that
carries the information necessary for producing a
functional product, usually a protein or RNA molecule.
alternative versions of a
gene that code for the
Is the size of an organism’s genome related to its
- Comparing the amount of DNA present in various
species, in terms of both
numbers of chromosomes and numbers of base
pairs, reveals a paradox: there
does not seem to be any relationship between the
size of an organism’s genome and the organism’s
Junk DNA-a huge proportion of base sequences in DNA
do not code for anything and have no obvious purpose
In what types of organisms do we find the most “junk
- Bacteria and viruses tend to have very little noncoding
DNA; genes make up 90% or more of their DNA.
It is in the eukaryotes (with the exception of yeasts) that
we see an explosion in the amount of non-coding DNA,
about 25% of which occurs within genes and 75%
Non-coding regions of DNA sometimes:
- take the form of short sequences that are repeated
thousands of times
- slightly longer repeated sequences
- consists of gene fragments, duplicate versions of
genes,and pseudogenes (sequences very similar to actual genes
butwith a few slight alterations that make them lose their
- occur both within genes in which case they are called
introns and between genes
All the extra DNA may serve some
purpose. Perhaps it is a reservoir of
potentially useful sequences. Or it may
have some function in regulating when
genes are turned on or off.
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