General Biology: Cell Structure, Genetics, and Diversity Study Notes

Cell Theory and Cellular Structure

All Living Things are Composed of Cells

Cells are the fundamental units of life. All living organisms, from bacteria to plants and animals, are made up of one or more cells.

• Cell Theory: States that all living things are composed of cells, and all cells arise from pre-existing cells.

• Common Features of All Cells:

  • Plasma membrane

  • Cytoplasm

  • Genetic material (DNA)

  • Ribosomes

• Example: Both Escherichia coli (a bacterium) and human skin cells have a plasma membrane and DNA.

Cell Membranes

Basic Components and Arrangement

Cell membranes are essential for maintaining the internal environment of the cell and mediating interactions with the external environment.

• Two Basic Components:

  • Phospholipids: Each has a polar (hydrophilic) head and a non-polar (hydrophobic) tail.

  • Proteins: Embedded within or attached to the lipid bilayer, serving various functions (transport, signaling, etc.).

• Arrangement: Phospholipids form a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward toward the aqueous environment.

• Additional info: Cholesterol and carbohydrates are also present in animal cell membranes, contributing to fluidity and cell recognition.

Prokaryotic vs. Eukaryotic Cells

Key Differences

Cells are classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotic Cells:

  • No membrane-bound nucleus; DNA is in the nucleoid region.

  • Lack membrane-bound organelles.

  • Examples: Bacteria and Archaea.

• Eukaryotic Cells:

  • Have a true nucleus enclosed by a nuclear membrane.

  • Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

  • Examples: Plants, animals, fungi, and protists.

• Additional info: Eukaryotic cells are generally larger and more complex than prokaryotic cells.

DNA Structure and Base Pairing

Double-Stranded DNA and Nucleotide Bases

DNA (deoxyribonucleic acid) stores genetic information in all living organisms.

• Structure: DNA is a double helix composed of two strands of nucleotides.

• Nucleotide Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

• Base Pairing: Bases pair specifically:

  • A pairs with T

  • C pairs with G

  Equation: $\text{A} = \text{T}$ $\text{C} = \text{G}$

• Hydrogen Bonds: Hold the base pairs together (A-T has 2, C-G has 3 hydrogen bonds).

N-Bases and Protein Structure

The sequence of nitrogenous bases in DNA determines the sequence of amino acids in proteins.

• Genetic Code: Triplets of bases (codons) specify amino acids.

• Central Dogma: DNA → RNA → Protein

• Example: The codon ATG codes for the amino acid methionine.

Chromosomes, Genes, and Ploidy

Chromosome and Gene Copies in Human Cells

Humans have 46 chromosomes in most cells, organized as 23 pairs.

• Diploid Cells (2n): Contain two copies of each chromosome (one from each parent).

• Gene Copies: Two copies (alleles) of each gene, except for genes on sex chromosomes in males (XY).

• Exceptions: In males, genes on the X and Y chromosomes may be present in only one copy.

Haploid Cells

Haploid cells contain only one copy of each chromosome.

• Gametes: Sperm and egg cells are haploid (n), produced by meiosis.

• Fertilization: Fusion of two haploid gametes restores diploidy.

Cell Division: Mitosis and Meiosis

Mitosis

Mitosis is the process by which most cells divide, producing two genetically identical daughter cells.

• Purpose: Growth, repair, and asexual reproduction.

• Result: Two diploid (2n) cells, identical to the parent cell.

Meiosis

Meiosis is the process that produces gametes (sperm and eggs) with half the number of chromosomes.

• Location: Occurs in the gonads (testes and ovaries).

• Result: Four haploid (n) cells, each genetically unique.

• Chromosome Number: Each gamete has one copy of each chromosome.

• Genetic Variation: Introduced through crossing over and independent assortment.

Genetics: PTC Tasting and Central Dogma

PTC Tasting Gene and Inheritance

The ability to taste phenylthiocarbamide (PTC) is determined by a gene on chromosome 7.

• Central Dogma: DNA is transcribed to RNA, which is translated to protein. The protein determines the ability to taste PTC.

• Dominant vs. Recessive: PTC tasting is a dominant trait (T), non-tasting is recessive (t).

• Heterozygote Gametes: If genotype is Tt, half of the gametes will carry the T allele, half will carry t.

• Proportion: 50% of gametes will contain the PTC tasting allele.

Evolutionary Biology: Gene Similarity Across Species

Conservation of Nucleotide Sequences

Genes in different species can have very similar nucleotide sequences due to shared ancestry.

• Homologous Genes: Genes inherited from a common ancestor.

• Conservation: Essential genes change little over time due to selective pressure to maintain function.

• Example: The gene for cytochrome c is highly conserved among eukaryotes.

Classification of Life

The Three Domains of Life

All living organisms are classified into three domains based on genetic and cellular differences.

• Bacteria: Prokaryotic, unicellular organisms.

• Archaea: Prokaryotic, often found in extreme environments.

• Eukarya: Eukaryotic organisms, including plants, animals, fungi, and protists.

Kingdoms within Eukarya

Eukaryotes are further divided into kingdoms based on major differences in structure and function.

• Largest Kingdoms:

  • Animalia (animals)

  • Plantae (plants)

  • Fungi (fungi)

• Other Eukaryotes: Organisms that do not fit into these kingdoms are called protists.

• Photosynthetic Protists: Called algae.

Fungi vs. Animals

Key Differences

Fungi and animals are both eukaryotic, but they differ in several important ways.

• Nutrition: Fungi are heterotrophs that absorb nutrients from decomposing matter; animals ingest and digest food internally.

• Cell Wall: Fungi have cell walls made of chitin; animal cells lack cell walls.

• Mobility: Most animals are motile at some stage; fungi are generally non-motile.

Summary Table: Prokaryotes vs. Eukaryotes

Feature	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Membrane-bound Organelles	No	Yes
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists
Cell Size	Small (1-10 μm)	Larger (10-100 μm)

Additional info:

• Figures 53.4, 53.5, 53.7, 53.8, 53.10, and 53.13 are referenced for further study, likely from a biology textbook chapter on population ecology or diversity.