Msc.biology

1. DNA structure and chromosome
organization
PINSET ISLAABAD

2. Introduction
• Chromosomes are the structures that contain
the genetic material
• They are complexes of DNA and proteins
• The genome comprises all the genetic material
that an organism possesses
• In bacteria, it is typically a single circular
chromosome
• In eukaryotes, it refers to one complete set of
nuclear chromosomes
• Eukaryotes possess a mitochondrial genome
• Plants also have a chloroplast genome

3. Introduction
• The main function of the genetic material is to store
information required to produce an organism
• The DNA molecule does that through its base
sequence
• DNA sequences are necessary for
1. Synthesis of RNA and cellular proteins
2. Proper segregation of chromosomes
3. Replication of chromosomes
4. Compaction of chromosomes
So they can fit within living cells

4. Frederic meisher 1860
• Pus cells from bandages
• Pepsin from pigs stomach
• Centrifuge
• Pellet results in isolation of an organelle
• Nucleus

6. Conclusions
• DNA carries the genetic information
• DNA controls the synthesis of specific products

7. Life cycle of Phage Virus

8. Fig. 12.4, p.189
tail
fiber
DNA
inside
protein
coat
hollow
sheath

9. Fig. 12.4, p.189

10. Hershey and chase experiment

11. Conclusions
• Only the phage DNA enters the bacterial cells
which means that the cells DNA are responsible
for the division and multiplication of the viruses

12. Discovery of DNA Structure
• DNA consists of two strands of nucleotides,
coiled into a double helix
• Each nucleotide has
▫ A five-carbon sugar (deoxyribose)
▫ A phosphate group
▫ A nitrogen-containing base (adenine, thymine,
guanine, or cytosine)

13. DNA Nucleotides

21. Base Pairing
• Bases of two DNA strands pair in only one way
▫ Adenine with thymine (A-T)
▫ Guanine with cytosine (G-C)
• The DNA sequence (order of bases) varies
among species and individuals

22. The DNA Double Helix

23. 2-nanometer diameter overall
0.34-nanometer distance
between each pair of bases
3.4-nanometer
length of each
full twist of the
double helix
In all respects shown here, the
Watson–Crick model for DNA
structure is consistent with the
known biochemical and x-ray
diffraction data.
The pattern of base
pairing (A with T,
and G with C) is
consistent with the
known composition
of DNA (A = T,
and G = C).

24. THE DNA DOUBLE HELIX
• A DNA molecule consists of two chains of
nucleotides, hydrogen-bonded together along their
length and coiled into a double helix
• Four kinds of nucleotides make up the chains:
adenine, thymine, guanine, and cytosine
• The order in which one kind of nucleotide base
follows the next along a DNA strand encodes
heritable information
• The order in some regions of DNA is unique for each
species

25. Watson, Crick, and Franklin
• Rosalind Franklin’s research
produced x-ray diffraction
images of DNA
▫ Helped Watson and Crick
build their DNA model, for
which they received the Nobel
Prize

26. THE FRANKLIN FOOTNOTE
• Like any race, the one that led to the discovery
of DNA’s structure had its winners—and its
losers

28. Genomic Organization And Function in
the Cell Nucleus
Interphase Nucleus Mitotic Chromosomes
Chromosome territories

30. DNA Replication and Repair
• A cell replicates its DNA before dividing
▫ Enzymes unwind the double helix
▫ DNA polymerases assemble complementary DNA
strands on templates from free nucleotides
▫ DNA ligase seals gaps in new DNA strands
• Two double-stranded DNA molecules result
▫ One strand of each is new

31. Semiconservative DNA Replication

32. new
old new
old

33. Details of DNA Replication

35. Part of a parent
DNA molecule, with two
complementary strands of
base-paired nucleotides.
Replication starts.
The strands are unwound
at many sites along
the molecule’s length.
Each of the two parent
strands guides the assembly
of new DNA strands from free
nucleotides, according to
base-pairing rules.
Any gaps between bases
of the “new” DNA are joined
to form a continuous strand.
The base sequence of each
half-old, half-new DNA
molecule is identical
to that of the parent.

37. direction of
unwinding
New DNA is assembled
continuously on only one
of the two parent template
strands. It is assembled on
the other parent template
strand in short fragments.
DNA ligase seals the gaps
between the fragments.
Why discontinuous assembly? DNA
synthesis occurs only in the 5’ to 3’
direction. Free nucleotides can be
added only to the —OH group at
the 3’ end of a growing strand.
direction
of
synthesis

38. Part of a parent
DNA molecule, with two
complementary strands of
base-paired nucleotides.
Replication starts.
The strands are unwound
at many sites along
the molecule’s length.
Each of the two parent
strands guides the assembly
of new DNA strands from free
nucleotides, according to
base-pairing rules.
Any gaps between bases
of the “new” DNA are joined
to form a continuous strand.
The base sequence of each
half-old, half-new DNA
molecule is identical
to that of the parent.

39. Replication Errors
• DNA repair mechanisms fix DNA damaged by
chemicals or radiation
• Proofreading by DNA polymerases corrects most
base-pairing errors
• Uncorrected errors are mutations

40. HOW CELLS DUPLICATE THEIR DNA
• Before a cell divides, enzymes and other proteins
copy its DNA
• Newly forming DNA strands are monitored for
errors, most of which are corrected
• Uncorrected errors are mutations

41. Cloning
• Clones
▫ Genetically identical individuals
▫ Produced by artificial twinning, nuclear transfers
• To clone an adult animal
▫ Cell’s DNA must be reprogrammed to function
like an embryonic cell and direct development

42. Cloning Methods

43. Nuclear Transfer

44. Thanks 