This PPT file includes the outline of the principles of genetics suitable for higher secondary and university level students.

1. Principles of Genetics
Mohammad Rashed
Lecturer
Department of Fisheries Biology and Genetics

2. Lecture 1
• Objective of the Course
• Genetics
• Gene
• Branches of Genetics
• Three great milestones
of genetics
• Scope and significance
of genetics
Lecture 1
• Objective of the Course
• Genetics
• Gene
• Branches of Genetics
• Three great milestones
of genetics
• Scope and significance
of genetics

3. Objective
• To know the importance of studying genetics
• To know background of genetics
• To understand basic principles of genetics
• Applications of genetics
• To know gene expression
• Mutation and so on

4. Genetics
The word genetics derived from the Greek root gen means to
become or to grow into.
it was first coined by William Bateson in 1906 for the study
of physiology of heredity and variations.
The biological science which deals with the phenomena of
heredity, (i.e. transmission of traits from one generation to
another) and variation (the study of the laws governing
similarities and differences between individuals related by
descents) is called genetics.

5. Gene
A gene is the basic physical and functional unit of heredity
Genes are made up of DNA
Some genes act as instructions to make molecules called
proteins
However, many genes do not code for proteins

7. Microbial Genetics
Mycogenetics
Plant Genetics
Animal Genetics
Human Genetics
Population Genetics
Cytogenetics
Biochemical Genetics
Molecular Genetics
Clinical Genetics
Developmental Genetics
Radiation Genetics
Quantitative or
biometric Genetics
Ecological Genetics

8. Three great milestones of genetics
Mendel: Genes and the rules of inheritance (1866)
Watson and Crick: The structure of DNA (1953)
The Human Genome Project: Sequencing DNA and
Cataloguing Genes (1990)

12. Scope and significance of genetics

13. Lecture 2

14. DNA (Deoxyribonucleic Acid)
Adenine (A)
Guanine (G)
Thymine (T)
Cytosine (C)

15. Nucleoside vs Nucleotide
Nucleoside = Sugar + Base
Nucleotide = Sugar + Base + Phosphate

16. Antiparallel orientation

17. Complementary base pairs

19. The important features of Watson –
Crick Model or double helix model
of DNA are as follows-
1. The DNA molecule consists of two polynucleotide
chains or strands that spirally twisted around each other
and coiled around a common axis to form a right-handed
double-helix.
2. The two strands are antiparallel i.e. they ran in
opposite directions so that the 3′ end of one chain facing
the 5′ end of the other.
3. The sugar-phosphate backbones remain on the
outside, while the core of the helix contains the purine
and pyrimidine bases.

20. 4. The two strands are held together by hydrogen bonds
between the purine and pyrimidine bases of the opposite
strands.
5. Adenine (A) always pairs with thymine (T) by two
hydrogen bonds and guanine (G) always pairs with
cytosine (C) by three hydrogen bonds. This
complimentarily is known as the base pairing rule. Thus,
the two stands are complementary to one another.
6. The base sequence along a polynucleotide chain is
variable and a specific sequence of bases carries the
genetic information.
Continued

21. Continued
7. The base compositions of DNA obey Chargaff s rules
(E.E. Chargaff, 1950) according to which A=T and G=C;
as a corollary ∑ purines (A+G) = 2 pyrimidines (C+T);
also (A+C) = (G+T). It also states that ratio of (A+T)
and (G+C) is constant for a species (range 0.4 to 1.9)

22. 8. The diameter of DNA
is 2.0 nm or 20 A.
Adjacent bases are
separated 0.34 nm or by
3.4 A along the axis. The
length of a complete turn
of helix is 3.4 nm or 34 A
i.e. there are 10bp per
turn. (B- DNA-Watson
rick DNA)
9. The DNA helix has a
shallow groove called
minor groove (1.2nm)
and a deep groove called
major groove (2.2nm)
across.
Continued

23. DNA replication
Molecular mechanism of DNA
replication
(Roles of DNA polymerases and other
replication enzymes. Leading and lagging
strands and Okazaki fragments)

24. Key points:
 DNA replication is semiconservative. Each strand in the
double helix acts as a template for synthesis of a new,
complementary strand.
 New DNA is made by enzymes called DNA polymerases,
which require a template and a primer (starter) and
synthesize DNA in the 5' to 3' direction.

25. Key points (Continued):
 During DNA replication, one new strand (the leading
strand) is made as a continuous piece. The other (the
lagging strand) is made in small pieces.
 DNA replication requires other enzymes in addition to DNA
polymerase, including DNA primase, DNA helicase,
DNA ligase, and topoisomerase.

27. DNA polymerase

28. Key features of DNA polymerases
o They always need a template
o They can only add nucleotides to the 3' end of a DNA strand
o They can't start making a DNA chain from scratch, but
require a pre-existing chain or short stretch of nucleotides
called a primer
o They proofread, or check their work, removing the vast
majority of "wrong" nucleotides that are accidentally added
to the chain

29. *The addition of nucleotides requires energy
In prokaryotes such as E. coli, there are two
main DNA polymerases involved in DNA
replication:
DNA pol III (the major DNA-maker),
and
DNA pol I, which plays a crucial
supporting role

30. Starting DNA replication
Origins of replication
replication forks
replication bubble

31. Helicase
single-strand
binding
proteins
Primers and primase
RNA primer

32. Leading and lagging strands

34. The maintenance and cleanup crew
Sliding clamp Topoisomerase

35. DNA polymerase I
DNA ligase

36. o Helicase opens up the DNA at the replication fork.
o Single-strand binding proteins coat the DNA around the
replication fork to prevent rewinding of the DNA.
o Topoisomerase works at the region ahead of the replication fork to
prevent supercoiling.
o Primase synthesizes RNA primers complementary to the DNA
strand.
o DNA polymerase III extends the primers, adding on to the 3' end,
to make the bulk of the new DNA.
o RNA primers are removed and replaced with DNA by DNA
polymerase I.
o The gaps between DNA fragments are sealed by DNA ligase.

39. DNA Repair Mechanism

40. Proofreading
DNA polymerases are the enzymes that build DNA in cells
During DNA replication (copying), most DNA polymerases
can “check their work” with each base that they add.
This process is called proofreading
If the polymerase detects that a wrong (incorrectly paired)
nucleotide has been added, it will remove and replace the
nucleotide right away, before continuing with DNA synthesis

42. Direct reversal
Single-strand damage
Base excision repair (BER)
Nucleotide excision repair (NER)
Mismatch repair
Double-strand breaks
Non-homologous end joining (NHEJ)
Microhomology-mediated end joining
(MMEJ)
Homologous recombination (HR)

43. Direct reversal
 The formation of pyrimidine dimers upon irradiation with
UV light results in an abnormal covalent bond between
adjacent pyrimidine bases.
Such direct reversal mechanisms are specific to the type of
damage incurred and do not involve breakage of the
phosphodiester backbone

45.  Photolyase, an old enzyme present in bacteria, fungi,
and most animals no longer functions in humans, who
instead use nucleotide excision repair to repair damage
from UV irradiation.
 The photoreactivation process directly reverses this
damage by the action of the enzyme photolyase, whose
activation is obligately dependent on energy absorbed
from blue/UV light (300–500 nm wavelength) to promote
catalysis.
Direct reversal

46. o Another type of damage, methylation of guanine bases, is
directly reversed by the protein methyl guanine methyl
transferase (MGMT), the bacterial equivalent of which is
called ogt.
o This is an expensive process because each MGMT molecule
can be used only once; that is, the reaction is stoichiometric
rather than catalytic.
Direct reversal

49. Single-strand damage
When only one of the two strands of a double helix has a
defect, the other strand can be used as a template to guide
the correction of the damaged strand.
excision repair mechanisms

50. Base excision repair (BER)
 Base excision repair (BER) repairs damage to a single
nitrogenous base by deploying enzymes called glycosylases.
 These enzymes remove a single nitrogenous base to create an
apurinic or apyrimidinic site (AP site).
 Enzymes called AP endonucleases nick the damaged DNA
backbone at the AP site.
 DNA polymerase then removes the damaged region using its
5’ to 3’ exonuclease activity and correctly synthesizes the new
strand using the complementary strand as a template.

52. Nucleotide excision repair (NER)
 Nucleotide excision repair (NER) repairs damaged DNA which
commonly consists of bulky, helix-distorting damage, such as
pyrimidine dimerization caused by UV light.
 Damaged regions are removed in 12–24 nucleotide-long strands
in a three-step process which consists of recognition of damage,
excision of damaged DNA both upstream and downstream of
damage by endonucleases, and resynthesis of removed DNA
region.

54. o NER is a highly evolutionarily conserved repair
mechanism and is used in nearly all eukaryotic and
prokaryotic cells.
o In prokaryotes, NER is mediated by Uvr proteins.
o In eukaryotes, many more proteins are involved,
although the general strategy is the same.
Nucleotide excision repair (NER) (continued)

55. Mismatch repair
 These systems consist of at least two proteins.
 One detects the mismatch, and the other recruits an
endonuclease that cleaves the newly synthesized DNA
strand close to the region of damage.
 In E. coli, the proteins involved are the MUT class
proteins. This is followed by removal of damaged region
by an exonuclease, resynthesis by DNA polymerase,
and nick sealing by DNA ligase.

57. Double-strand breaks
Three mechanisms exist to repair double-
strand breaks (DSBs):
 Non-homologous end joining (NHEJ),
 Microhomology-mediated end joining
(MMEJ), and
 Homologous recombination (HR)

58. Non-homologous end joining (NHEJ)
 In NHEJ, DNA Ligase IV, a specialized DNA
ligase that forms a complex with the cofactor
XRCC4, directly joins the two ends.
 To guide accurate repair, NHEJ relies on short
homologous sequences called microhomologies
present on the single-stranded tails of the DNA
ends to be joined.

59. Homologous recombination (HR)
 Homologous recombination requires the presence of an
identical or nearly identical sequence to be used as a template
for repair of the break.
 The enzymatic machinery responsible for this repair process is
nearly identical to the machinery responsible for chromosomal
crossover during meiosis.
 This pathway allows a damaged chromosome to be repaired
using a sister chromatid (available in G2 after DNA
replication) or a homologous chromosome as a template.

64. • Unlike double-stranded DNA, RNA is a single-stranded molecule
in many of its biological roles and consists of much shorter chains
of nucleotides. However, a single RNA molecule can, by
complementary base pairing, form intrastrand double helixes, as
in tRNA.
• While the sugar-phosphate "backbone" of DNA contains
deoxyribose, RNA contains ribose instead. Ribose has a hydroxyl
group attached to the pentose ring in the 2' position, whereas
deoxyribose does not. The hydroxyl groups in the ribose
backbone make RNA more chemically labile than DNA by
lowering the activation energy of hydrolysis.
• The complementary base to adenine in DNA is thymine, whereas
in RNA, it is uracil, which is an unmethylated form of thymine.
RNA

65. Thank You