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Overview of Biology Topic Four

May 8, 2025

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Biology Lecture: Topic Four Overview

Introduction

  • Presented by Miss Estrich, biology teacher and tutor with over 10 years of experience.
  • Focus on AQA A-Level Biology.
  • Provides revision resources and study aids.

DNA in Cells

DNA in Eukaryotic and Prokaryotic Cells

  • Similarities:
    • Made of DNA nucleotides (deoxyribose, phosphate, nitrogenous base).
    • Nucleotides joined by phosphodiester bonds.
  • Differences:
    • Eukaryotic DNA is longer and linear.
    • Prokaryotic DNA is circular and lacks histones.
    • Eukaryotic DNA is associated with histones.

DNA in Mitochondria and Chloroplasts

  • Similar to prokaryotic DNA: short, circular, not associated with histones.
  • Allows transcription and translation of enzymes for photosynthesis (chloroplasts) and respiration (mitochondria).

Genes and Genetic Code

Genes

  • Sequence of DNA coding for amino acid sequence of a polypeptide or functional RNA.
  • Locus: location of a gene on a chromosome.

Genetic Code Features

  • Triplet: sequence of three bases coding for an amino acid.
  • Degenerate: multiple triplets code for the same amino acid.
  • Universal: same triplet codes for the same amino acid across all organisms.
  • Non-overlapping: each base participates in one triplet only.

Introns and Exons

  • Introns: non-coding DNA sequences.
  • Exons: coding sequences for amino acids.

RNA and Protein Synthesis

Types of RNA

  • mRNA: short, single-stranded, found in nucleus and cytoplasm, has codons.
  • tRNA: cloverleaf shape, found in cytoplasm, has anticodon and amino acid binding site.

Protein Synthesis Steps

  • Transcription: DNA to mRNA in nucleus involving RNA polymerase.
    • Splicing: removal of introns to form mature mRNA.
  • Translation: mRNA to polypeptide chain at ribosomes using tRNA.
    • Involves start and stop codons.

Genetic Variation

Gene Mutations

  • Changes in DNA sequence during replication.
  • Mutagenic agents: radiation, chemicals.
  • Types: substitution and deletion (can cause frameshift).

Chromosome Mutations

  • Changes in chromosome number due to non-disjunction.
    • Polyploidy: whole set changes.
    • Aneuploidy: individual chromosome changes (e.g., Down syndrome).

Meiosis and Variation

  • Independent Segregation: random orientation of homologous chromosomes.
  • Crossing Over: exchange of genetic material between chromatids.

Natural Selection and Evolution

Natural Selection

  • Process leading to evolution (change in allele frequency).
  • Involves adaptation to environment.

Types of Selection

  • Directional: extreme traits favored (e.g., antibiotic resistance).
  • Stabilizing: intermediate traits favored (e.g., birth weights).

Species and Taxonomy

Species and Courtship Behavior

  • Definition: ability to produce fertile offspring.
  • Courtship Behavior: aids in species recognition and successful mating.

Classification

  • Phylogenetic: based on evolutionary history.
  • Hierarchy: domains, kingdoms, etc., no overlap.
  • Binomial System: genus and species names.

Biodiversity

Measuring Biodiversity

  • Includes habitat range, genetic variety, and species diversity.
  • Species Diversity: species richness and individual numbers.
  • Index of Diversity: formula-based measurement.

Genetic Diversity Measurement

  • Compare DNA, mRNA, and protein sequences.

This concludes topic four. Please give the video a thumbs up if it was helpful.

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