An Introduction to the DNA Structure

1. DNA & Gene Structure
Elham lasemi

2. DNA is the genetic material of :
Bacteria
(in 1928-Frederick Griffith showed this
subject by transformation)
Viruses (in 1952,result of an experiment by
Alfred Hershey and Martha Chase)
Eukaryotic
cell
In some viruses, the genetic material is RNA.

3. 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
material.
→every person’s DNA is
unique result DNA can serve
as an individual identifier.

4. 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
molecule.

5. Chargaff's rules:
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 .

6. 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
shape.

7. In 1953:Francis Crick and
James Watson,deduce the
exact structure of DNA

8. DNA-Deoxyribonucleic Acid
Nucleic acids are made from repeating units of nucleotides
1) A phosphate group
Two major forms of nucleic acid polymers DNA and RNA
Nucleotide contain:

9. 2) A sugar
3)A nitrogenous base

10. The nucleotide structure
Glycoside bond

11. The physical structure of DNA is frequently described
as a “double helix.”

12. What exactly is a double helix?
imagine long ladder twisted around like a spiral
staircase
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.

13. the backbone of DNA molecule-
nucleotides are connected to
each other to from a long chain
phosohodiester bond:
Fromed between the phosphate
group of one nucleotide and 3´-
OH of the next nucleotide.

14. The rungs of the ladder-
Attached to each sugar,
and protruding like half
of a rung on the ladder,
is one of the nitrogen-
containing bases.

15. 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)

16. These nitrogen bases
are held together by :
Hydrogen bonds

17. 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

18. the three helical forms of DNA ( RNA)

19. In the cell, the most commonly seen from of DNA
double helix is called the B form or the Watson-
Crick helix
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




20. structural variation in DNA reflects three things :
The different possible conformation of the deoxyribose
Rotation about the contiguous bonds that make up the
phospho-deoxyribose backbone
Free rotation about the C-1´- N glycosyl bond
-The Watson-Crick structure is
also referred to as B form DNA
or B-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
unusal)

21. The major and minor grooves are lined by sequence-
specific hydrogen bonding group

22. - 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

23. Major and minor groove

24. 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

25. Two kind of supercoiling

26. ∆L=∆W+∆T
L
(Linking number)
W= writhing number
T=Twisting number

27. DNA replication

28. 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
called mutagens.

29. 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
Transition
Transversion

30. 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
random hits.

31. Difference between DNA and RNA

32. Genome-The full set of DNA present in an
individual organism
The genome incloudes:
- Choromosal DNA
- DNA in plasmids
- Organellar DNA as found in
mitochondria and
chloroplasts ( in eukaryotes)

33. In prokaryotest the information contained within
circular pieces of DNA ( such as all bacteria)

34. In eukaryotes this information is laid out in long
linear strands of DNA in the nucleus.(such as humans)

35. The genome in viruses
DNA
RNA
dsDNA
SSDNA
dsRNA
SSRNA

36. Chromosomes- Rather
than being one super-long
DNA strand, eukaryotic
DNA exists as many
smaller, more manageable
pieces.

37. 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.

38. Alleles-These
alternative versions of a
gene that code for the
same feature

39. Is the size of an organism’s genome related to its
complexity?
- 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
complexity.

41. 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
DNA”?
- 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%
between genes

43. 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
proteincoding ability)
- occur both within genes in which case they are called
introns and between genes

45. 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.

46. thanks for your attention