Nucleic Acids Overview

Nucleic Acids

• Nucleic acids, discovered in 1869 by Friedrich Miescher, are fundamental constituents of living cells.
• They are found throughout the cell, not just in the nucleus.
• Nucleic acids, especially DNA, store and transmit genetic information.
• The DNA of a cell contains a genetic program for an organism’s activities.
• DNA carries directions for metabolic actions within cells.

Composition of Nucleic Acids

• Nucleic acids are composed of nucleotides.
• Nucleotides consist of a nitrogenous base, a five-carbon sugar (ribose or deoxyribose), and a phosphate group
• Bases include purines (Adenine, Guanine) and pyrimidines (Cytosine, Uracil, Thymine).
• Sugars are ribose in RNA and deoxyribose in DNA.

Nucleosides and Nucleotides

• Nucleosides are formed by combining a base and a sugar.
• Nucleotides are formed by adding a phosphate group to a nucleoside.
• Numbering of atoms in nucleosides uses a prime symbol (').

Types of Nucleotides

• Various nucleotides exist, including nucleoside monophosphates, diphosphates, and triphosphates.
• Examples include AMP, ADP, ATP, GMP, GDP, GTP, CMP, CDP, CTP, UMP, UDP, UTP, TMP, TDP.

Polynucleotides

• Nucleotides join to form polynucleotides by phosphodiester bonds (between the 3' and 5' carbons of the sugar).
• The order and arrangement of nucleotides in polynucleotides determine the genetic code.

DNA Structure and Properties

• DNA is a double helix with two antiparallel strands.
• Complementary base-pairing (A with T, G with C) holds the DNA strands together.
• DNA has a uniform diameter, dictated by base-pairing.
• The sugar-phosphate backbone forms the exterior of the double helix.
• The bases project inwards.
• DNA strands are antiparallel (5' to 3' direction).
• DNA can exist in different conformations like A-DNA, B-DNA, and Z-DNA.

DNA Replication

• DNA replication is semiconservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand.
• The process involves unwinding the double helix and using each strand as a template to synthesize a new complementary strand.
• DNA polymerase is the key enzyme in DNA replication.
• Okazaki fragments are short segments of DNA synthesized on the lagging strand during replication. They are later joined by an enzyme called ligase.

Transcription

• Transcription is the process of copying DNA information into an RNA molecule, usually mRNA.
• Only one strand of the DNA is used as a template.
• RNA polymerase is the enzyme involved in transcription.
• RNA contains uracil instead of thymine.
• RNA is single-stranded.

Post-transcriptional Modifications

• mRNA undergoes processing, including:
  • Addition of a 5' cap
  • Addition of a 3' poly(A) tail
  • Splicing out introns (non-coding regions)
• These modifications ensure the stability and proper translation of mRNA.

Translation

• Translation is the process of converting the mRNA sequence into a protein.
• mRNA is read in codons (three-base sequences).
• Each codon specifies a particular amino acid.
• tRNA molecules carry amino acids to the ribosomes, matching the codons on mRNA.
• Ribosomes are the sites of protein synthesis.
• Ribosomes position tRNA molecules with the correct amino acid, to create a polypeptide chain, until a stop codon is reached.

Genetic Code

• The genetic code is a set of rules relating particular three-base sequences (codons) on mRNA to specific amino acids on a polypeptide.
• The code is nearly universal (with some exceptions).
• Multiple codons can specify the same amino acid (degenerate code).
• One codon (AUG) signals the start of protein synthesis.
• Three codons signal the stop of protein synthesis.

Mutation

• Mutations are changes in the DNA sequence.
• Mutations can be spontaneous (due to errors in replication or repair) or induced (due to external agents like radiation or chemicals).
• Point mutations are changes in a single nucleotide.
• Frameshift mutations involve the addition or deletion of nucleotides which alter the reading frame.
• Mutations can lead to changes in protein structure and function.

Cancer

• Cancer arises from uncontrolled cell growth and division.
• Carcinogens, chemicals and even radiation, can induce mutations, leading to cancerous development.

DNA Repair

• DNA repair mechanisms correct mutations and other DNA damage.
• Various pathways of DNA repair have evolved to maintain the accuracy of the genome.

Genetic Engineering and Cloning

• Techniques allow insertion of foreign genes into organisms to produce specific protein products.
• Cloning creates identical copies of genetic material or an entire organism.