General Biology: Cell Structure, Genetics, and Domains of Life Study Notes

Cell Theory and Cellular Structure

All Living Things are Composed of Cells

All living organisms are made up of cells, which are the basic unit of life. This concept is central to the cell theory in biology.

• Cell Theory: States that all living things are composed of cells, cells are the basic units of structure and function, and all cells come from pre-existing cells.

• Common Features of All Cells: All cells have a plasma membrane, cytoplasm, genetic material (DNA), and ribosomes.

• Example: Both Escherichia coli (a bacterium) and human skin cells are composed of a cell membrane and contain DNA.

Cell Membrane Structure

Basic Components and Arrangement

The cell membrane is a selectively permeable barrier that separates the interior of the cell from its external environment.

• Phospholipid Bilayer: The membrane consists of two layers of phospholipids. Each phospholipid has a polar (hydrophilic) head and a non-polar (hydrophobic) tail.

• Arrangement: The hydrophilic heads face outward toward the water inside and outside the cell, while the hydrophobic tails face inward, away from water.

• Additional Components: Proteins, cholesterol, and carbohydrates are embedded within or attached to the bilayer.

• Example: The plasma membrane of a red blood cell maintains its shape and allows selective transport of substances.

Prokaryotic vs. Eukaryotic Cells

Key Differences

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

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. DNA is located in the nucleoid region.

• Eukaryotic Cells: Have a true nucleus enclosed by a nuclear membrane and possess membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

• Additional info: Prokaryotes include bacteria and archaea; eukaryotes include animals, plants, fungi, and protists.

• Example: Streptococcus (bacterium) is prokaryotic; human liver cell is eukaryotic.

DNA Structure and Base Pairing

Double-Stranded DNA and Nucleotide Bases

DNA is the hereditary material in almost all living organisms and is composed of two strands forming a double helix.

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

• Base Pairing: A pairs with T, and C pairs with G via hydrogen bonds.

• Base Pairing Rule: $A = T$ and $C = G$

• Example: The sequence ATCG on one strand pairs with TAGC on the complementary strand.

N-Bases and Protein Structure

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

• Genetic Code: Sets of three bases (codons) code for specific amino acids.

• Central Dogma: DNA → RNA → Protein

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

Chromosomes and Genes

Chromosome Number and Gene Copies

Human somatic cells typically contain 46 chromosomes (23 pairs), with two copies of each chromosome and gene.

• Diploid Cells: Have two sets of chromosomes (2n), one from each parent.

• Gene Copies: Each gene is present in two copies (alleles) in diploid cells, except for genes on sex chromosomes in males (XY).

• Exception: Haploid cells (gametes) have only one copy of each chromosome.

• Example: Sperm and egg cells are haploid (n = 23).

Cell Division: Mitosis and Meiosis

Mitosis

Mitosis is the process by which somatic cells divide to produce two genetically identical daughter cells.

• Purpose: Growth, repair, and asexual reproduction.

• Result: Two diploid cells, each with 46 chromosomes.

• Example: Skin cell regeneration after injury.

Meiosis

Meiosis is the process by which gametes (sperm and egg) are formed, reducing the chromosome number by half.

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

• Result: Four haploid cells, each with 23 chromosomes.

• Genetic Variation: Daughter cells are genetically different due to crossing over and independent assortment.

• Example: Formation of human eggs and sperm.

Genetics: PTC Tasting and Central Dogma

PTC Tasting Gene

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

• Dominant vs. Recessive: PTC tasting is typically a dominant trait.

• Central Dogma Application: The gene is transcribed and translated to produce a receptor protein that binds PTC.

• Gamete Proportion: If heterozygous (Tt), half of gametes will carry the tasting allele (T), and half will carry the non-tasting allele (t).

• Example: A person with genotype Tt will produce gametes with T and t alleles in equal proportion.

Evolutionary Biology: Gene Similarity Across Species

Conservation of Nucleotide Sequences

Genes in different species often have very similar nucleotide sequences due to shared ancestry and evolutionary conservation.

• Homologous Genes: Genes inherited from a common ancestor that retain similar sequences and functions.

• Example: The gene for cytochrome c is highly conserved across mammals, birds, and fish.

• Additional info: Evolutionary biologists use sequence similarity to infer evolutionary relationships.

Classification: Domains and Kingdoms of Life

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 animals, plants, fungi, and protists.

Kingdoms within Eukarya

Eukaryotes are further classified into kingdoms based on their characteristics.

• Largest Kingdoms: Animalia, Plantae, Fungi.

• Other Eukaryotes: Protists are eukaryotes that do not fit neatly into the other kingdoms.

• Photosynthetic Protists: Called algae (e.g., Chlamydomonas).

Comparing Fungi and Animals

Key Differences

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

• Nutrition: Fungi are heterotrophic by absorption; animals are heterotrophic by ingestion.

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

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

• Example: Mushrooms (fungi) absorb nutrients from decaying matter; dogs (animals) ingest food.

Summary Table: Cell Types and Division

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	No	Yes
Membrane-bound Organelles	No	Yes
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Process	Number of Cells Produced	Chromosome Number	Genetic Identity
Mitosis	2	Diploid (2n)	Identical
Meiosis	4	Haploid (n)	Different

Additional info: Figures 53.4, 53.5, 53.7, 53.8, 53.10, and 53.13 likely refer to textbook diagrams on population ecology, but are not covered in detail here.