Cell Division, Chromosome Structure, and Genetic Stability

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Cell Division and Types of Reproduction

Asexual Reproduction

Asexual reproduction is a process by which organisms produce offspring without the fusion of gametes. It results in genetically identical offspring and is common in single-celled organisms and some multicellular organisms.

Binary Fission: A type of asexual reproduction where a single cell divides into two identical daughter cells. Common in prokaryotes and some protists.
Advantages: Rapid reproduction, no need for a mate, offspring are well-adapted to stable environments.
Disadvantages: Lack of genetic diversity, risk of overpopulation.
Example: Binary fission in Amoeba.

Binary fission in Amoeba

Sexual Reproduction

Sexual reproduction involves the fusion of gametes produced by meiosis, resulting in offspring with genetic variation. This process is essential for the evolution and adaptation of species.

Gametes: Sperm and egg cells, each haploid (n).
Fertilization: Fusion of gametes forms a diploid (2n) zygote.
Advantages: Genetic diversity, increased survival in changing environments.
Disadvantages: Requires more energy and time, finding a mate can be challenging.
Example: Human life cycle showing mitosis and meiosis.

Life cycle of humans

Genetic Material: Prokaryotes vs. Eukaryotes

Prokaryotic DNA

Prokaryotes (bacteria and archaea) have DNA that is circular and not associated with proteins. It is found freely in the cytoplasm and often contains plasmids.

Circular DNA: Compact, with little repetitive DNA.
Plasmids: Small, extrachromosomal DNA molecules.

Prokaryotic DNA under microscope

Eukaryotic DNA

Eukaryotes (plants, animals, fungi, protists) have linear DNA organized into chromosomes within a nucleus. DNA is associated with histone proteins and contains large amounts of non-coding regions.

Linear DNA: Organized into multiple chromosomes.
Histones: Proteins that help package DNA.

Eukaryotic DNA under microscope

Viruses and Their Reproduction

Types of Viruses

Viruses are diverse entities that infect prokaryotic and eukaryotic cells. Their genetic material can be DNA or RNA, single- or double-stranded.

Prokaryotic Viruses: Bacteriophages, less diverse.
Eukaryotic Viruses: More diverse, include retroviruses and influenza viruses.

Lytic vs. Lysogenic Cycles

Viruses reproduce by two main cycles:

Lytic Cycle: Virus replicates inside host, causing cell lysis and release of new viruses.
Lysogenic Cycle: Viral DNA integrates into host genome and remains dormant until triggered to enter lytic cycle.

Lytic cycle vs Lysogenic cycle

Chromosome Structure and Cell Division

Chromosome, Chromatid, and Chromatin

Chromosomes are highly condensed structures of DNA visible during cell division. Each duplicated chromosome consists of two sister chromatids joined at the centromere.

Chromatin: DNA and protein complex in non-dividing cells.
Chromatid: One half of a duplicated chromosome.
Centromere: Region where sister chromatids are attached.

Chromosome duplication and mitosis Chromosome structure with centromere and kinetochore

Diploid vs. Haploid Cells

Diploid cells (2n) contain two sets of chromosomes, while haploid cells (n) contain one set. This distinction is crucial for sexual reproduction.

Somatic Cells: Diploid, 2n.
Gametes: Haploid, n.
Example: Human somatic cells have 46 chromosomes; gametes have 23.

Karyotypes

A karyotype is an organized profile of an individual's chromosomes, used to detect chromosomal abnormalities and determine sex.

Autosomes: Non-sex chromosomes.
Allosomes: Sex chromosomes (X and Y).

Human karyotype

The Cell Cycle and Mitosis

Phases of the Cell Cycle

The cell cycle consists of interphase (G1, S, G2), mitosis (M phase), and cytokinesis. Interphase is the period of growth and DNA replication, while mitosis is the process of nuclear division.

G1: Cell growth.
S: DNA synthesis.
G2: Preparation for mitosis.
M: Mitosis and cytokinesis.

Cell cycle diagram

Stages of Mitosis

Mitosis is divided into four main stages: prophase, metaphase, anaphase, and telophase, followed by cytokinesis.

Prophase: Chromosomes condense, spindle fibers form, nuclear envelope dissolves.
Metaphase: Chromosomes align at the cell's equator.
Anaphase: Sister chromatids are pulled apart to opposite poles.
Telophase: Nuclear envelope reforms, chromosomes decondense.
Cytokinesis: Cytoplasm divides, forming two identical daughter cells.

Prophase illustration Metaphase illustration Anaphase illustration Telophase illustration Cytokinesis illustration

Microscopic Evidence of Mitosis

Cell division stages can be observed in stained root tip cells under a microscope, with distinct features for each phase.

Anaphase in onion root tip Interphase in onion root tip Prophase in onion root tip Metaphase in onion root tip

Cell Cycle Regulation and Checkpoints

Cell Cycle Control System

The cell cycle is regulated by checkpoints at G1, G2, and M phases, ensuring proper division and preventing uncontrolled growth.

Checkpoints: Stop-and-go signals that monitor DNA integrity, cell size, and environmental conditions.
G1 Checkpoint: Ensures cell is ready for DNA synthesis.
G2 Checkpoint: Ensures DNA replication is complete.
M Checkpoint: Ensures chromosomes are properly aligned and attached to spindle fibers.

Cell cycle checkpoints

Cell Fate: Division, Differentiation, and Apoptosis

Cell Differentiation

Cells can specialize in structure and function, a process called differentiation. Differentiated cells produce more of their type when they divide.

Apoptosis

Apoptosis is programmed cell death, eliminating damaged or unnecessary cells. It is essential for development and preventing diseases like cancer.

Disruptions in the Cell Cycle: Cancer and Chromosomal Mutations

Cancer

Cancer results from uncontrolled cell division due to mutations in genes regulating the cell cycle. Tumors can be benign (localized) or malignant (spreading).

Causes: DNA damage from carcinogens, radiation, chemicals, viruses.
Treatment: Surgery, radiation, chemotherapy.
Angiogenesis: Tumors stimulate blood vessel growth to supply nutrients.

Chromosomal Mutations

Chromosomal mutations can occur during mitosis or meiosis, leading to genetic disorders. Only mutations in gametes are inherited.

Types: Deletion, duplication, inversion, insertion, translocation.

Mutation Type	Description
Deletion	An entire section of a chromosome is missing.
Duplication	One section of the chromosome is doubled.
Inversion	A section of a chromosome has its gene sequences reversed.
Insertion	Part of one chromosome is inserted into a different chromosome.
Translocation	Two non-homologous chromosomes exchange alleles during crossing over.

Nondisjunction and Aneuploidy

Nondisjunction is the failure of chromosomes to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers (aneuploidy).

Meiosis I: All daughter cells are aneuploid.
Meiosis II: Half of the daughter cells are aneuploid.
Examples: Down syndrome (trisomy 21), Klinefelter syndrome (XXY), Turner syndrome (X0).

Summary

Cell division is essential for growth, repair, and reproduction.
Errors in cell division can lead to cancer or genetic disorders.
Chromosomal mutations and nondisjunction are severe when they affect gametes.
Cell cycle checkpoints maintain genetic stability.

Key Equations:

Diploid chromosome number:
Haploid chromosome number:

Additional info: The notes expand on textbook content and lecture slides, providing definitions, examples, and visual aids for exam preparation.