Mitosis and Meiosis: Cellular Basis of Genetic Inheritance

Chapter 2: Mitosis and Meiosis

Introduction

Genetic material in living organisms is composed of DNA, which is organized into chromosomes. In eukaryotes, the transmission of genetic material occurs through the processes of mitosis and meiosis. Mitosis produces genetically identical cells, while meiosis generates gametes or spores, introducing genetic variation.

2.1 Cell Structure Is Closely Tied to Genetic Function

Types of Cells

• Prokaryotic cells: Include bacteria and archaea; lack a membrane-bound nucleus.

• Eukaryotic cells: Include protists, plants, fungi, and animals; possess a membrane-bound nucleus and organelles.

• All cells share common features: plasma membrane, DNA, and ribosomes.

Plasma Membrane and Cell Wall

• The plasma membrane surrounds all cells, delimiting the cell from its environment.

• Plant cells have a cell wall composed mainly of cellulose; bacterial cell walls contain peptidoglycan.

Glycocalyx

• The glycocalyx (cell coat) is a covering on the plasma membrane of animal cells, made of glycoproteins and polysaccharides.

• Functions in biochemical identity and cell signaling via receptor molecules.

Nucleus and Nucleoid

• Nucleus: Found in eukaryotes, membrane-bound, houses DNA. Contains the nucleolus, where rRNA is synthesized.

• Nucleoid: Found in prokaryotes, not membrane-bound, contains genetic material.

Cytoplasm and Cytoskeleton

• Cytoplasm: Includes organelles and cytosol (the colloidal material surrounding organelles).

• Cytoskeleton: Network of microtubules (tubulin) and microfilaments (actin) providing structural support and facilitating movement.

Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with ribosomes; site of protein synthesis.

• Smooth ER (SER): Lacks ribosomes; site of fatty acid and phospholipid synthesis.

Mitochondria and Chloroplasts

• Mitochondria: Site of oxidative phosphorylation and ATP production in both plant and animal cells.

• Chloroplasts: Site of photosynthesis in plants, algae, and some protozoans.

Centrioles

• Located in the centrosome of animal and some plant cells.

• Organize spindle fibers for chromosome movement during cell division.

2.2 Chromosomes Exist in Homologous Pairs in Diploid Organisms

Homologous Chromosomes

• Chromosomes in diploid organisms exist in homologous pairs, carrying genes for the same traits but possibly different alleles.

• Humans have 46 chromosomes (23 pairs); the diploid number is denoted as 2n.

• Each gene is located at a specific locus on a chromosome.

Centromere and Chromosome Morphology

• The centromere is a constricted region that determines chromosome shape and is essential for proper segregation during cell division.

• Types of chromosomes based on centromere position:

  • Metacentric: Centromere in the middle

  • Submetacentric: Centromere between middle and end

  • Acrocentric: Centromere close to end

  • Telocentric: Centromere at the end

Centromere location	Designation	Metaphase shape	Anaphase shape
Middle	Metacentric	Sister chromatids with centromere in the middle	Migration to poles
Between middle and end	Submetacentric	p arm (short), q arm (long)	Migration to poles
Close to end	Acrocentric	Centromere near end	Migration to poles
At end	Telocentric	Centromere at end	Migration to poles

Karyotype

• A karyotype is a visual representation of the complete set of chromosomes in a cell, arranged in homologous pairs.

Genome and Alleles

• The genome is the complete set of genetic information in a haploid set of chromosomes.

• Alleles are alternative forms of the same gene found at the same locus on homologous chromosomes.

• Biparental inheritance: Diploid organisms inherit one chromosome of each pair from each parent.

Sex Chromosomes

• Sex-determining chromosomes (e.g., X and Y in humans) are not homologous in size or genetic content but behave as homologs during meiosis.

2.3 Mitosis Partitions Chromosomes into Dividing Cells

Cell Cycle Overview

• The cell cycle consists of interphase (G1, S, G2) and mitosis.

• G0 phase: Nondividing but metabolically active state.

• Karyokinesis: Division of the nucleus; cytokinesis: division of the cytoplasm.

Interphase

• During interphase, chromosomes are not visible and exist as chromatin.

• DNA replication occurs in the S phase.

Mitosis Stages

• Prophase: Chromosomes condense, centrioles move to poles, nuclear envelope breaks down.

• Prometaphase: Chromosomes move to the metaphase plate, spindle fibers form.

• 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, cytokinesis occurs.

Key Proteins in Chromosome Segregation

• Cohesin: Protein complex holding sister chromatids together.

• Separase: Enzyme that degrades cohesin to allow chromatid separation.

• Shugoshin: Protects cohesin from premature degradation.

Cell Cycle Regulation

• Regulated by cyclin-dependent kinases (CDKs) and cyclins.

• Checkpoints monitor for errors and ensure proper division.

2.4 Meiosis Creates Haploid Gametes and Spores and Enhances Genetic Variation in Species

Overview of Meiosis

• Meiosis reduces the chromosome number by half, producing haploid gametes or spores.

• Consists of two sequential divisions: Meiosis I (reductional) and Meiosis II (equational).

• DNA replication occurs only once, before Meiosis I.

Genetic Variation: Crossing Over

• During prophase I, homologous chromosomes pair (synapsis) and exchange genetic material (crossing over) at chiasmata, increasing genetic diversity.

Stages of Meiosis

• Each meiotic division includes prophase, metaphase, anaphase, and telophase.

• Prophase I: Homologous chromosomes pair to form bivalents (tetrads); crossing over occurs.

• Metaphase I: Homologous pairs align at the metaphase plate.

• Anaphase I: Homologous chromosomes separate; sister chromatids remain attached.

• Telophase I: Two haploid cells form; nuclear membranes may reform.

• Meiosis II: Similar to mitosis; sister chromatids separate, resulting in four haploid cells.

Comparison: Mitosis vs. Meiosis

• Mitosis: Produces two genetically identical diploid cells for growth and repair.

• Meiosis: Produces four genetically unique haploid cells for sexual reproduction.

Summary

• Mitosis and meiosis are essential for genetic continuity and variation.

• Proper regulation and execution of these processes ensure healthy development and inheritance.