Genetic materials | CEE Academy

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GENETIC MATERIAL

▢ Main Genetic Material:

Table 1: Genetic material

Condition	Genetic material
Most organisms	DNA
Some viruses	RNA

▢ DNA Structure:

❖ Nucleotide:

Table 1: Nucleotide vs nucleoside

Unit	Composition
Nucleotide	Nitrogenous base + pentose sugar + phosphate group
Nucleoside	Nitrogenous base + pentose sugar
DNA sugar	Deoxyribose
RNA sugar	Ribose

❖ Bonds:

Table 1: Bonds in nucleic acids

Bond	Location / Role
Hydrogen bond	Between complementary nitrogen bases
Glycosidic bond	Base + sugar
Phosphodiester bond	Sugar-phosphate backbone

▢ RNA:

Table 1: Types of RNA

Feature	r-RNA	m-RNA	t-RNA
Stability	Most stable; long-lived	Least stable; short-lived	Stable
Abundance	Most abundant; \(80\%\)	\(5-10\%\); least	\(10-15\%\)
Special points	Ribosomal RNA	Messenger RNA	◉ Points: • Smallest RNA • Contains anticodon • 2D shape → clover leaf-like • 3D shape → L-shaped

▢ Some Terms:

❖ Gene:

▢ Definition: Smallest DNA segment specified for particular function

▢ Mendel Term: Factor

▢ Gene Units:

Table 1: Gene units

Unit	Meaning
Cistron	Functional unit of gene; specific for one protein
Muton	Smallest gene segment undergoing mutation
Recon	Smallest gene segment undergoing recombination

▢ MCQ Points:

❖ Transposons / Jumping Genes: DNA segments capable of changing position within genome

❖ Retroposon:

◉ Definition: DNA segment formed from RNA by reverse transcription

◉ Flow: \(RNA \xrightarrow{Reverse\ transcription} DNA\)

❖ Alleles / Allelomorphs:

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Alternative forms of same gene
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Present at identical locus of homologous chromosomes
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One gene with 2 alleles → biallelism
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One gene with >2 alleles → multiple allelism

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SEMI-CONSERVATIVE DNA REPLICATION

▢ Proof: Experimentally proved by Meselson and Stahl

▢ Replication Pattern:

Table 1: DNA replication pattern

Organism	Replication structure
Prokaryotes	\(\theta\)-loop
Eukaryotes	Y-fork / replication fork

▢ Replication Enzymes / Proteins:

Table 1: DNA replication enzymes and functions

Enzyme / Protein	Present in	Function
Helicase	Both	Breaks H-bonds → DNA unwinding
Topoisomerase-I	Both	Cuts + reseals single DNA strand; cut in front of replication fork; relaxes coiling
Topoisomerase-II	Both	Cuts + reseals double DNA strand; cut behind replication fork; relaxes supercoiling
DNA gyrase	Prokaryotes	Type of topoisomerase; relieves supercoiling
Single-stranded binding protein / SSB protein	Both	Prevents annealing / renaturation
Primase	Both	Synthesizes RNA primer
DNA polymerase	Both	Synthesis of new DNA strands
DNA polymerase-I / Kornberg enzyme	Prokaryotes	Proofreading; elongation of Okazaki fragment; removal of RNA primer
DNA polymerase-II	Prokaryotes	Acts in absence of DNA polymerase-I and DNA polymerase-III
DNA polymerase-III	Prokaryotes	Main enzyme of replication
DNA polymerase-\(\alpha\)	Eukaryotes	Synthesizes lagging strand
DNA polymerase-\(\delta\)	Eukaryotes	Synthesizes leading strand
DNA ligase	Both	Joins Okazaki fragments

▢ Key Events:

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Helicase → unwinding
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SSB protein → strand stabilization
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Primase → RNA primer
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DNA polymerase → strand synthesis
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DNA ligase → Okazaki fragment joining

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CROSSING OVER

▢ Definition: Exchange of genetic material between non-sister chromatids of homologous chromosomes → new combination

▢ Key Points:

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Crossing over ∝ distance between linked genes
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Single/double/multiple crossing over → depends on number of chiasmata
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Reported in Drosophila by T. H. Morgan
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Most accepted theory → precocity/strain theory by Darlington
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More in females than males
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Increases at high + low temperature
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Occurs at tetrad stage of pachytene
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Well demonstrated in Neurospora

▢ Types:

Table 1: Types of crossing over

Type	Site	Occurrence / Examples
Germinal crossing over	Germinal cells	During gamete formation
Somatic crossing over	Somatic cells	Maize, Aspergillus, Drosophila / MAD

▢ Significance:

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Basis of linkage maps / genetic maps
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Produces recombination
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Strong proof for linear arrangement of genes on chromosome

▢ Recombination Frequency: \(Recombination\ frequency = \frac{No.\ of\ individuals\ showing\ recombination}{Total\ number\ of\ offsprings}\)

6Genetic materials