Introduction to Genetics and the Cell Cycle - Aug 27

Study Guide - Smart Notes

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Introduction to Genetics

Key Genetic Terms

Understanding genetics begins with mastering foundational vocabulary. These terms are essential for describing genetic information and inheritance.

Allele: A variant form of a gene. Each individual inherits two alleles for each gene, one from each parent.
Genotype: The genetic makeup of an organism; the combination of alleles present at a given locus.
Gene: A segment of DNA that encodes a functional product, typically a protein.
Genome: The complete set of DNA, including all genes and noncoding regions, necessary for the development and function of an organism.
Phenotype: The observable characteristics or traits of an organism, resulting from the interaction of its genotype with the environment.

Example: In pea plants, the gene for seed color may have two alleles: yellow and green. The genotype determines which color is expressed as the phenotype.

The Genome

Definition and Components

The genome is the complete set of genetic material in an organism. In humans, this includes both nuclear DNA and mitochondrial DNA.

Haploid genome: Refers to a single set of chromosomes (n), as found in gametes.
Diploid genome: Refers to two sets of chromosomes (2n), as found in somatic cells.
Nuclear genome: DNA contained within the nucleus, organized into chromosomes.
Mitochondrial genome: Small circular DNA found in mitochondria, inherited maternally and independent from nuclear DNA.

Example: Humans are diploid, possessing two copies of each nuclear chromosome and multiple copies of mitochondrial DNA.

Additional info: The human nuclear genome consists of 23 pairs of chromosomes, while the mitochondrial genome contains 35 genes.

Chromosomes

Structure and Types

Chromosomes are thread-like structures composed of DNA and proteins. They carry genetic information and are visible during cell division.

Diploid (2n): Eukaryotic organisms typically have two sets of chromosomes, one from each parent.
Karyotype: The complete set of chromosomes in a cell, often visualized during metaphase.
Homologous chromosomes: Chromosome pairs with the same genes but possibly different alleles.
Sister chromatids: Identical copies of a chromosome, joined at the centromere, produced during DNA replication.
Non-sister chromatids: Chromatids from homologous chromosomes; not identical.

Example: Human karyotypes show 22 pairs of autosomes and one pair of sex chromosomes.

Chromosome Classification by Centromere Position

Chromosomes are classified based on the location of their centromere:

Centromere Location	Designation	Metaphase Shape	Anaphase Shape
Middle	Metacentric	V-shaped	Even migration
Between middle and end	Submetacentric	L-shaped	Uneven migration
Close to end	Acrocentric	J-shaped	Uneven migration
At end	Telocentric	I-shaped	Migration from end

Additional info: Sister chromatids are duplicated sets that will later separate during cell division.

Mitochondrial DNA

Features and Inheritance

Mitochondrial DNA (mtDNA) is distinct from nuclear DNA and plays a crucial role in cellular energy production.

Location: Found in mitochondria, not in the nucleus.
Replication, transcription, and translation: Occur independently from nuclear DNA.
Gene content: Human mtDNA encodes 35 genes, including those for respiratory chain proteins.
Inheritance: Maternally inherited; all mitochondria in an embryo come from the egg cell.

Example: Mutations in mitochondrial DNA can cause inherited metabolic disorders.

The Cell Cycle

Phases of the Cell Cycle

The cell cycle describes the sequence of events in the life of a cell, including growth, DNA replication, and division.

Interphase: Period of cell growth and DNA replication; consists of G1, S, and G2 phases.
Mitosis (M phase): Division of the nucleus and distribution of duplicated chromosomes to daughter cells.
Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

Equation:

Example: Onion root tip cells are commonly used to study the cell cycle due to their rapid division.

Mitosis

Stages of Mitosis

Mitosis is the process by which a eukaryotic cell divides to produce two genetically identical daughter cells. It ensures equal distribution of chromosomes.

Prophase: Chromosomes condense, centrioles migrate, spindle fibers form, nuclear envelope disappears.
Metaphase: Chromosomes align at the metaphase plate; kinetochores attach to spindle fibers.
Anaphase: Sister chromatids separate and move to opposite poles; now called daughter chromosomes.
Telophase: Chromosomes decondense, nuclear envelope reforms, spindle fibers disappear.
Cytokinesis: Cytoplasm divides, forming two daughter cells.

Example: In animal cells, cytokinesis occurs via cleavage furrow; in plant cells, a cell plate forms.

Comparison: Animal vs. Plant Cytokinesis

Feature	Animal Cells	Plant Cells
Mechanism	Cleavage furrow	Cell plate formation
Result	Pinching in of membrane	New cell wall between daughter cells

Additional info: Plant cells require the formation of a cell plate due to the rigid cell wall.

Transfer of Genetic Material: Mitosis vs. Meiosis

Key Differences

Genetic material is transferred in two main ways: mitosis and meiosis.

Mitosis: Produces two diploid (2n) daughter cells identical to the parent cell; used for growth and repair.
Meiosis: Produces four haploid (n) gametes, each genetically unique; used for sexual reproduction.

Equation:

Example: Human somatic cells divide by mitosis, while gametes (sperm and egg) are produced by meiosis.

Cell Structures in Mitosis

Centrioles and Spindle Fibers

Centrioles and spindle fibers are essential for chromosome movement during cell division.

Centrioles: Microtubule structures that organize spindle fibers.
Spindle fibers: Microtubule arrays that attach to chromosomes via kinetochores and pull sister chromatids apart.
Kinetochores: Protein complexes at the centromere that connect chromosomes to spindle fibers.

Example: Errors in spindle fiber attachment can lead to aneuploidy (abnormal chromosome number).

Summary Table: Stages of Mitosis

Stage	Main Events
Interphase	DNA replication, cell growth, no visible chromosomes
Prophase	Chromosomes condense, spindle forms, nuclear envelope disappears
Metaphase	Chromosomes align at metaphase plate, kinetochores attach
Anaphase	Sister chromatids separate, migrate to poles
Telophase	Chromosomes decondense, nuclear envelope reforms
Cytokinesis	Cytoplasm divides, two daughter cells form

Visual Example: Onion Root Tip Cells

Onion root tip cells are used to observe different stages of the cell cycle under a microscope. Each stage can be identified by the appearance and arrangement of chromosomes.

Interphase: Chromatin is uncoiled and not visible as distinct chromosomes.
Prophase: Chromosomes condense and become visible.
Metaphase: Chromosomes align at the cell's equator.
Anaphase: Chromatids separate and move to opposite poles.
Telophase: Chromosomes decondense, nuclear envelope reforms.

Further Study

For more information, refer to Chapter 2, p. 20 (Telophase section) in your textbook and review the provided video on Mendelian genetics for upcoming topics.