dna structure

DNA STRUCTURE AND FUNCTION

DNA Overview

  • Definition: Deoxyribonucleic acid (DNA) is the hereditary molecule found in living organisms, serving as the instruction manual for building an organism.

  • Location: DNA is located in the nucleus of eukaryotic cells.

  • Structure:

    • Coiled around proteins into discrete structures known as chromosomes.

    • Human Chromosomes: Humans have 23 pairs of chromosomes, with one chromosome from each parent per pair. The 23rd chromosome pair, XY, determines a person's sex.

  • Cell Count: The human body contains trillions of cells, each housing DNA that instructs on cell structure and function.

Chromosomes

  • Each chromosome comprises a single, long DNA molecule wrapped around proteins.

  • If the DNA of a single chromosome is stretched out, it would measure about 1 to 3 meters long.

MOLECULAR STRUCTURE OF DNA

Structure of DNA

  • Form: DNA is a double-stranded molecule.

  • Components: Each strand consists of a chain of subunits called nucleotides. Four nucleotide types are:

    • Adenine (A)

    • Thymine (T)

    • Guanine (G)

    • Cytosine (C)

  • Bonding: Two DNA strands are linked by hydrogen bonds, allowing base pairing:

    • A pairs with T (2 hydrogen bonds)

    • C pairs with G (3 hydrogen bonds)

Nucleotides

  • Structure of a Nucleotide:

    • Composed of:

    • A sugar

    • A phosphate group

    • A nitrogenous base (A, T, C, or G)

  • Nucleotides are bonded end to end to form a single strand of DNA, and the sequence of the nucleotides varies, contributing to genetic uniqueness.

DNA REPLICATION

Overview of DNA Replication Process

  • Purpose: DNA replication is necessary for cellular reproduction, allowing each new cell to inherit a copy of the original DNA.

  • Enzymes Involved: The process is facilitated by various enzymes:

    1. Helicase: Unwinds and unzips the DNA helix by breaking hydrogen bonds.

    2. DNA Polymerase: Synthesizes new DNA strands by adding complementary nucleotides based on parent strand sequences.

  • Process Details:

    • The helicase unwinds the helix, and DNA polymerase adds nucleotides according to base pairing rules (A with T and G with C).

    • Results in two identical double-stranded DNA molecules; each is composed of one original strand and one newly synthesized strand.

DNA TO PROTEIN TRANSFER

DNA CODE FOR PROTEINS

  • Protein Formation: Proteins are chains of amino acids coded by the DNA sequence.

  • Codon Structure: A codon is a sequence of three bases on the mRNA that specifies a single amino acid (triplet code).

Gene Expression

  • Definition: Gene expression refers to the process through which a gene produces its product, typically a protein.

  • Components: Each gene consists of two primary parts:

    • Regulatory Sequence: Controls when and where the gene is expressed.

    • Coding Sequence: Contains the actual nucleotide sequence that translates into the protein.

  • Site of Protein Synthesis: Proteins are synthesized on ribosomes located in the cytoplasm.

Steps of Gene Expression

  1. Transcription:

    • Occurs in the nucleus where the gene's DNA is copied into messenger RNA (mRNA) by RNA polymerase.

    • The mRNA sequence is complementary to the DNA coding sequence, and adenine (A) is paired with uracil (U), not thymine.

    • Resulting mRNA exits the nucleus to the cytoplasm.

  2. Translation:

    • The ribosome reads the mRNA and assembles amino acids into a polypeptide chain (protein).

    • Transfer RNA (tRNA) brings the appropriate amino acids to the ribosome using anticodons to match mRNA codons.

    • Each specific codon corresponds to a specific amino acid, facilitating the creation of a protein.

CODONS AND GENE EXPRESSION

Codons

  • Definition: Codons are three-nucleotide sequences on mRNA that specify particular amino acids.

  • Universal Code: 64 possible codons represent 20 different amino acids.

    • Some codons have multiple meanings (redundancy), ensuring that mutations may not always result in changes to the final protein.

  • Start and Stop Codons: Specific codons signal the beginning (start codon) and end (stop codons) of the protein synthesis process.

Gene Expression Steps

  • First Step - Transcription:

    1. DNA unwinds, exposing the coding sequence.

    2. RNA polymerase synthesizes mRNA by following the complementary base pairing from DNA.

    3. Completed mRNA detaches and exits the nucleus, allowing DNA to recoil.

  • Second Step - Translation:

    1. Ribosome binds mRNA and reads codons in sequences of three.

    2. tRNA matches with codons, adding specific amino acids to the growing peptide chain.

    3. The protein grows until a stop codon is reached, at which point the polypeptide is released and folds into its functional form.

MUTATIONS AND THEIR EFFECTS

Types of Mutations

  • Frameshift Mutations: Insertions or deletions of nucleotides that alter the reading frame.

  • Point Mutations: A single nucleotide change that may alter amino acid coding.

Consequences of Mutations

  • Cystic Fibrosis Example: Caused by a deletion in the CFTR gene on chromosome 7, leading to a nonfunctional protein due to a missing amino acid from the sequence.

  • Sickle Cell Disease Example: Results from a point mutation in the beta-globin gene, altering hemoglobin and affecting oxygen transport in the blood.

DNA REPAIR AND VERSATILITY

DNA Repair Mechanisms

  • Proofreading: Enzymes correct errors during DNA replication, ensuring fidelity of genetic information.

    • Mutation Rates:

    • With proofreading: 1 in a billion nucleotide pairs.

    • Without proofreading: 1 in ten thousand nucleotide pairs.

  • Implications of Mutations: While most mutations are neutral or harmful, beneficial mutations can lead to new alleles and characteristics important for evolution.