Untitled

BIO130: Genes and Cellular Functioning

DNA Structure and Function

  • Deoxyribonucleic acid (DNA):
    • Description: Long, thread-like molecule with a uniform diameter and varied length.
    • Human Cells: Contains 46 DNA molecules (chromosomes) in the nucleus.
    • Average Length: An average human DNA molecule is about 2 inches long.

DNA and Nucleic Acids Structure

  • Nucleotides: DNA and other nucleic acids are polymers of nucleotides.
    • Each nucleotide consists of three components:
    • Sugar: Deoxyribose
    • Phosphate Group
    • Nitrogenous Base:
      • DNA Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)
      • Purines: Adenine (A), Guanine (G) - have double-ringed structure
      • Pyrimidines: Cytosine (C), Thymine (T), Uracil (U) - have single-ringed structure.

The Genetic Code

  • DNA Alphabet: A, T, C, G
  • Triplets: Within a gene, groups of 3 nucleotides in the template strand of DNA form "words" known as triplets.
    • Example triplets: ATG, GCG, TCA, GGT, CAT. There are 64 different possible combinations.
    • Each triplet codes for a specific amino acid in the protein encoded by the gene.
    • Gene containing 3,000 nucleotides (1,000 triplets) codes for a protein of 1,000 amino acids.

Structure of DNA

  • Double Helix Shape: Resembles a spiral staircase.
    • Backbones: Comprised of alternating phosphate groups and deoxyribose.
    • Base Pairings: Step-like connections between backbones formed by nitrogenous bases united by hydrogen bonds:
    • Purines pair with pyrimidines: A with T (connected by 2 hydrogen bonds), G with C (connected by 3 hydrogen bonds).
    • Law of Complementary Base Pairing: Sequence of one strand governs the sequence of the other.

DNA Function

  • Essential Function: DNA carries instructions (genes) for synthesizing proteins.
  • Gene: A segment of DNA coding for the synthesis of a specific protein.
  • Gene Count: Humans possess approximately 20,000 genes, constituting about 2% of total DNA while 98% is noncoding (plays roles in chromosome structure and regulating gene activity).
  • Chromatin: Fine filamentous DNA material complexed with proteins known as histones, organized into 46 chromosomes in most cells.

Chromatin Structure

  • Chromatin is dynamic within nondividing cells; structure and location change according to genetic activity.
    • Genes can be turned on or off.
  • During Cell Division: Cell duplicates all nuclear DNA, resulting in each chromosome consisting of two identical filaments called sister chromatids joined at a constricted centromere. Kinetochores are protein plaques on each side of the centromere that assist in cell division.

RNA Structure and Function

  • Ribonucleic Acids (RNAs): Various forms and diverse functions.
    • General structure includes sugar ribose with bases A, U, G, C (with uracil (U) replacing thymine).
    • Primarily exists as a single nucleotide chain in the cytoplasm.
  • Types of RNA important for protein synthesis:
    • Messenger RNA (mRNA): Carry codes from the nucleus to the cytoplasm.
    • Ribosomal RNA (rRNA): Forms part of ribosomal structure.
    • Transfer RNA (tRNA): Delivers specific amino acids to ribosomes.

Definition of a Gene

  • Gene: An information-containing segment of DNA coding for the production of an RNA molecule, often involved in synthesizing proteins.
  • Each protein's amino acid sequence is determined by nucleotide sequences in DNA.
  • Human chromosomes count: 46 chromosomes paired in sets of 23 from each parent, totaling 3.1 billion nucleotide pairs.
  • Genomics: The study of the entire genome.

The Genetic Code and Protein Synthesis

  • The human body can produce millions of different proteins from 20 amino acids encoded by genes made from 4 nucleotides (A, T, C, G).
  • Genetic Code: Represents the system for encoding amino acid sequences using nucleotide triplets.
    • Base Triplet: A sequence of three DNA nucleotides representing one amino acid.
    • Codon: A three-base sequence in mRNA; there are 64 possible codons:
    • 61 codons code for amino acids; 3 are stop codons.
    • Start Codon: AUG coding for methionine initiates protein synthesis.
    • Stop Codons: UAG, UGA, UAA signal the end of protein synthesis.

Protein Synthesis Overview

  • All body cells (except sex cells and some immune cells) contain identical genes; however, different genes are activated in different cells.
  • The process of protein synthesis follows:
    • From DNA to RNA: Transcription processes to make mRNA from DNA.
    • From RNA to Protein: Translation processes to synthesize a protein from mRNA.
    • Transcription: The copying of genetic instructions from DNA to mRNA, utilizing the enzyme RNA polymerase to create a complementary RNA strand.

Translation of mRNA to Protein

  • Translation Mechanism: Converts nucleotide language into amino acid language.
    • Major Components Involved:
    • mRNA: Carries genetic information from nucleus to cytoplasm.
      • Contains a protein cap for ribosome recognition.
    • tRNA: Delivers amino acids to the ribosome; contains an anticodon corresponding to the mRNA codon.
    • Ribosomes: Read mRNA and build peptide chains with a large subunit and a small subunit, located free in cytosol or on rough ER.
    • Three Sites in Ribosomes: E site, P site, and A site, all play roles during translation.

Steps of Translation

  1. Initiation: Ribosome assembles with mRNA in cytosol, binding starts at the leader sequence, initiating synthesis at the AUG start codon. tRNA brings methionine.
  2. Elongation: tRNA carrying amino acids binds at the A site; peptide bond formation between amino acids; ribosome shifts down codons, continuing elongation of the peptide chain.
  3. Termination: Ribosome reaches a stop codon, binding a release factor, resulting in ribosome disassembly and cessation of protein synthesis.
  • Proteins destined for lysosomes or secretion are further modified by the ER and packaged into transport vesicles for delivery.

Protein Processing and Secretion

  • Final Assembly: Proteins made in ribosomes in the cytoplasm later undergo the processing and secretion stages.
  • Mechanism: Polyribosomes facilitate production efficiency.

Synthesis of Compounds Other Than Proteins

  • Cells can synthesize compounds such as glycogen, fat, steroids, phospholipids, and pigments without having direct genes for them.
  • These processes are regulated by enzymatic reactions, which are themselves encoded by genes.
    • Example: Testosterone synthesis involves converting cholesterol within the testes, a process dependent on the activation of enzyme-related genes.

DNA Replication and Cell Cycle Overview

  • Before cell division, DNA must replicate to ensure each daughter cell receives a complete set of genetic information.
  • The process is highly accurate, relying on the law of complementary base pairing for prediction during replication.

DNA Replication Process

  • Enzymatic Role: DNA Polymerase enzymes:
    • Unwind the helical DNA.
    • Break hydrogen bonds between complementary strands creating a replication fork.
    • Read nucleotide sequences to synthesize new complementary strands from available free nucleotides.
  • At the end of replication, each cell has 46 pairs of DNA molecules (totaling 96 single-stranded).

Semiconservative DNA Replication

  • This replication method is called semiconservative because resulting DNA molecules consist of one original strand and one newly synthesized strand.

Errors and Mutations in DNA Replication

  • DNA polymerase can make errors during replication.
    • DNA Damage Response (DDR): Mechanisms exist to correct replication errors with a typical error rate of one mistake per billion bases.
    • Mutations: structural changes in DNA caused by errors or environmental factors such as radiation or chemicals.
    • They can lead to benign effects, cell death, cancer, or genetic defects over generations.
  • Genetic Mosaicism: Variation within an individual’s genome due to mutations or replication errors.

The Cell Cycle

  • The cell cycle includes interphase and the mitotic phase:
    • Interphase Subphases:
    • G1 (First Gap Phase): Normal cellular function, prepares for DNA replication.
    • S (Synthesis Phase): DNA replication and centriole duplication.
    • G2 (Second Gap Phase): Cell repairs, grows, and synthesizes enzymes for division.
    • Mitotic Phase consists of:
    • Prophase
    • Metaphase
    • Anaphase
    • Telophase
    • Cytokinesis (cytoplasmic division occurs).
  • G0 Phase: Describes cells that have exited the cell cycle and cease dividing for extended periods or permanently.

Mitosis Overview

  • Mitosis: Cell division resulting in two genetically identical daughter cells.
  • Key Functions:
    • Development from fertilized egg to trillions of cells.
    • Tissue growth, repair, and cell replacement post-birth.
  • Four Phases of Mitosis:
    1. Prophase: Chromatin condenses into chromosomes, centrioles form spindle fibers, and the nuclear envelope disintegrates.
    2. Metaphase: Chromosomes align along the cell equator; spindle apparatus forms.
    3. Anaphase: Sister chromatids are separated and move to opposite cell poles.
    4. Telophase: Distinct nuclear envelopes form around each cluster of chromosomes; chromosomes decondense.
  • Cytokinesis: Division of the cytoplasm that follows mitotic division, creating two separate daughter cells.
  • Regulation of division relies on the presence of cytoplasmic volume, nutrient levels, presence of growth factors, and cell contact inhibition to halt divisions when not necessary.