DNA and RNA Basics

DNA and RNA

• Two classes of genetic material are responsible for transmitting information from one generation to another: DNA and RNA.
• DNA is the primary genetic material for most organisms, while some viruses use RNA.

The Nucleus

• The nucleus is a cell's control center housing all genetic instructions for protein synthesis.
• Each cell, excluding germ cells, contains a complete set of DNA.

Organization of DNA

• Genetic instructions are organized within strands of DNA, arranged in an orderly manner.
• Chromatin threads in the nucleus are composed of DNA and associated proteins.
• DNA is wrapped around histone proteins, forming nucleosomes, which resemble a beaded necklace.
• During cell division, chromatin condenses into chromosomes, ensuring safe transportation of DNA to daughter cells.
• Humans have an estimated 22,000 genes distributed across 46 chromosomes.

DNA Structure and Functions

• DNA's basic building block is a nucleotide, consisting of deoxyribose sugar, a phosphate group, and a nitrogenous base.
• Deoxyribose sugar molecules form the DNA backbone, connected by phosphodiester bonds.
• The phosphate group links nucleotides and gives DNA a negative charge.
• There are four nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G).
• DNA strands form a double helix, resembling a twisted ladder.
• The two strands run in opposite directions (antiparallel), with one strand having a 5' end (free phosphate) and a 3' end (free hydroxyl group).
• Complementary base pairing occurs between nitrogenous bases: A always pairs with T (two hydrogen bonds), and C always pairs with G (three hydrogen bonds).

DNA Replication

• DNA replication faithfully copies the entire genome of a cell.
• The process involves three stages: initiation, elongation, and termination.
• During initiation, the two DNA strands are separated by enzymes like helicase.
• In elongation, each strand serves as a template for synthesizing a new complementary strand, facilitated by DNA polymerase.
• Termination occurs when the two original strands are bound to their newly synthesized complements.
• DNA proofreading mechanisms minimize errors during replication, ensuring the accuracy of genetic information.

Gene Expression

• Gene expression describes the process of converting genetic information into functional products, primarily proteins.
• Messenger RNA (mRNA) acts as an intermediate messenger, carrying genetic code out of the nucleus for protein synthesis.

Transcription

• Transcription is the synthesis of mRNA from a DNA template.
• It involves three stages: initiation, elongation, and termination.
• Initiation begins at a promoter sequence, a specific nucleotide sequence that triggers transcription.
• During elongation, RNA polymerase unwinds the DNA segment, using one strand as a template to build a complementary mRNA strand.
• Termination occurs when a "stop" codon is reached, signaling the end of transcription.

RNA Processing

• After transcription, mRNA undergoes processing, mainly splicing, to remove non-coding regions called introns.
• Exons are segments of RNA that remain after splicing and are joined back together.
• Splicing is facilitated by a spliceosome, a complex of proteins and other molecules.

Translation

• Translation is the process of converting mRNA into a polypeptide chain (protein).
• It requires ribosomes and transfer RNA (tRNA).
• Ribosomes, composed of ribosomal RNA (rRNA) and proteins, serve as the site of protein synthesis.
• tRNA transports the correct amino acids to the ribosome and links them to the growing polypeptide chain.
• Anticodons on tRNA molecules recognize codons on mRNA, ensuring accurate amino acid placement.

Translation Stages

• Translation is divided into three stages: initiation, elongation, and termination.
• During initiation, a ribosome binds to an mRNA transcript.
• Elongation involves the recognition of the tRNA anticodon with the next mRNA codon, bringing in the corresponding amino acid for polypeptide chain extension.
• Termination occurs when a "stop" codon is reached, signaling the release of the complete protein.