Genetic materials, DNA replication, Central dogma of Life

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GENETIC MATERIAL

▢ Main Genetic Material:

Table 1: Genetic material

Condition	Genetic material
Most organisms	DNA
Some viruses	RNA

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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

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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	• Smallest RNA • Contains anticodon • 2D shape → clover leaf-like • 3D shape → L-shaped

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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:

Q1.

Gene is

Q2.

Which term can be used as a gene unit?

▢ 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: \(\text{Recombination frequency} = \frac{\begin{subarray}{c} \text{No. of individuals} \\ \text{showing recombination} \end{subarray}}{\text{Total number of offsprings}}\)

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CENTRAL DOGMA OF LIFE

▢ Definition: Central dogma explains the flow of genetic information from DNA to RNA and from RNA to protein.

▢ Proposed By: Francis Crick

▢ Pattern:

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Transcription

▢ Definition: Process of formation of RNA from a DNA template.

▢ Pattern: \(DNA \xrightarrow{Transcription} RNA\)

▢ Enzyme: RNA polymerase

▢ Site:

Table 1: Site of transcription

Cell Type	Site
Prokaryotic cell	Cytoplasm / nucleoid region
Eukaryotic cell	Nucleus

▢ Important Point: During transcription, information present in DNA is copied into RNA, mainly mRNA for protein synthesis.

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Translation

▢ Definition: Process by which the genetic information present in mRNA is used to synthesize a protein.

▢ Pattern: \(mRNA \xrightarrow{Translation} Protein\)

▢ Site: Ribosome in cytoplasm

▢ Requirements:

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mRNA carries genetic information
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tRNA brings specific amino acids
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Ribosome provides the site for protein synthesis
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Amino acids are joined to form a polypeptide chain

▢ Important Point: The nucleotide sequence of mRNA determines the amino acid sequence of the protein.

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Genetic Code

▢ Codon:

❖ Definition: A codon is a sequence of three nitrogenous bases or nucleotides in mRNA that specifies an amino acid or a termination signal during protein synthesis.

❖ Triplet Nature: Each codon consists of 3 nucleotides.

❖ Total Number of Codons: 64

❖ Types of Codons:

Table 1: Types of codons

Type	Number	Function
Sense codons	61	Code for amino acids
Stop / Non-sense codons	3	Do not code for amino acids; terminate translation

▢ Initiation Codon:

❖ Main Initiation Codon: \(AUG\)

❖ Codes For: Methionine

❖ Important Point: AUG acts as the normal initiation or start codon and also codes for methionine.

❖ Prokaryotic Exception: \(GUG\) can sometimes act as an initiation codon in prokaryotes, although it normally codes for valine.

▢ Termination Codons:

❖ Other Names:

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Stop codons
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Non-sense codons
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Termination signals

❖ Codons:

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\(UAA\)
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\(UAG\)
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\(UGA\)

❖ Function: They do not code for amino acids and signal termination of translation.

❖ Memory Trick: UAA, UAG and UGA are the three stop codons.

▢ Characteristics of Genetic Code:

❖ Triplet: Each codon consists of three nucleotides.

❖ Degenerate:

Most amino acids are coded by more than one codon.
Also called as Redundant genetic code
Leucine is coded by multiple different codons.

❖ Commaless: Codons are read continuously without gaps or punctuation between successive codons.

❖ Non-overlapping: Each nucleotide normally belongs to only one codon in a given reading frame.

❖ Unambiguous: A particular codon specifies only one particular amino acid.

❖ Nearly Universal:

In almost all organisms, a particular codon specifies the same amino acid.
Some mitochondrial genetic codes differ from the standard genetic code.

▢ Summary:

Table 1: Important genetic code facts

Feature	Key Point
Total codons	64
Sense codons	61
Stop codons	3
Start codon	AUG
AUG codes for	Methionine
Stop codons	UAA, UAG, UGA

▢ MCQ Points:

Q1.

A codon is composed of:

Q2.

Total number of codons in the genetic code is:

Q3.

How many sense codons are present in the genetic code?

Q4.

The initiation codon is:

Q5.

Which of the following is not a stop codon?

Q6.

Degeneracy of genetic code means:

Q7.

Genetic code is unambiguous because:

Q1.

Central dogma of molecular biology is:

Q2.

Formation of RNA from DNA is called:

Q3.

Formation of protein according to information present in mRNA is called:

6Genetic materials, DNA replication, Central dogma of Life