Cell Cycle (Mitosis)

Cell Reproduction

• Overview: Understanding the mechanisms of cell reproduction is crucial for bioscience education.

Prokaryote Reproduction

Prokaryote DNA

• Prokaryotic chromosomes are simpler than eukaryotic chromosomes.

• DNA in prokaryotes consists of a single circular chromosome attached to the cell membrane.

• Unlike eukaryotes, prokaryotes lack a nucleus containing their DNA.

Cell Division in Prokaryotes

• Prokaryotes reproduce via binary fission.

• The process involves copying the DNA which produces two identical chromosomes.

• A new cell membrane develops between the two DNA copies.

• Binary fission can occur rapidly, from 35 minutes to 11 hours.

Chromosomes

Structure and Function

• Chromosomes are rod-shaped structures made of DNA and protein.

• DNA wraps tightly around proteins called histones, which maintain chromosome shape and compact the DNA.

• A chromatid is one half of a chromosome that forms when DNA copies itself prior to cell division.

Chromosome Structure

Key Features

• Chromosomes consist of two chromatids joined at a centromere, which holds them together until cell division separates them.

• During interphase, chromosomes uncoil into chromatin for cellular activity and gene expression.

Chromosome Numbers

Organization in Animals

• Animal chromosomes are classified into sexes and autosomes.

• Sex chromosomes determine the organism's sex, typically two types: X and Y.

• Females have XX and males have XY.

• Humans possess 44 autosomes, categorized in pairs 1-22.

Chromosome Number

Homologous Pairs

• Every sexually reproducing organism inherits two homologous chromosomes for each autosome (one from each parent).

• Typical humans have 46 total chromosomes (44 autosomes + 2 sex chromosomes).

• A karyotype displays the chromosomes of a normally diving human cell.

Diploid vs Haploid

Definitions

• Somatic Cells: Body cells possessing two homologous chromosomes - termed diploid (2n).

• These include skin, blood, and muscle cells.

• Haploid Cells: Cells with one copy of chromosomes (gametes - egg and sperm).

Cell Division in Eukaryotes

Types

• Eukaryotic cell division consists of two types, both examples of asexual reproduction:

  • Mitosis: Involves growth, repair, development.

  • Meiosis: Produces gametes (sperm and egg).

The Cell Cycle

• The cell cycle includes growth and division phases:

  • Major phases: Interphase (95% of cycle duration) and either Mitosis or Meiosis.

• Interphase consists of:

  • G1 (Gap 1): Cell growth.

  • S Phase: DNA synthesis.

  • G2 (Gap 2): Preparation for division.

  • G0 Phase: Resting state when cells exit the cycle.

Cell Cycle Summary

Mitosis Phases

Overview

1. Prophase: Longest phase, nucleus disappears, chromatids form.

2. Metaphase: Shortest phase, chromosomes align at equator.

3. Anaphase: Centromeres split, chromatids separate.

4. Telophase: Chromosomes reach poles, cytokinesis occurs, resulting in two identical cells.

Cell Cycle Checkpoints

Importance

• G2 Checkpoint: DNA repair enzymes check replication.

• Mitosis Checkpoint: Ensures spindle fibers attach before progression.

• Cell Growth Checkpoint: Controls whether the cell continues to divide or stop.

Cancer and Cell Cycle Control

Overview

• Enzymes monitor phase transitions in the cell cycle.

• Dysfunction can lead to uncontrolled cell division - cancer.

• Contact Inhibition: Cells stop growing upon touching unless they are cancerous.

• External factors can contribute to cancer development.

Cancer Mechanism

• Cancer is linked to gene mutations affecting cell cycle enzymes.

• Tumors can deprive normal cells of nutrients and may metastasize.

Meiosis and Gamete Formation

Process Overview

• Meiosis reduces chromosome number by half involved in sexual reproduction.

• Begins with a germ cell (2n) and transitions to haploid gametes (n).

Meiosis I vs Mitosis

Key Differences

• Both undergo interphase (G1, S, G2), yet result in genetic variation through synapsis and crossing-over during prophase I.

Meiosis I Phases

1. Prophase I: Homologous chromosomes pair up (synapsis), forming tetrads, leading to genetic recombination.

2. Metaphase I: Tetrads line up at midline, spindle fibers attach.

3. Anaphase I: Homologous chromosomes move to opposite poles (independent assortment).

4. Telophase I: Cytokinesis forms two haploid cells with homologous pairs.

Meiosis II Overview

• Meiosis II occurs without interphase.

1. Prophase II: Chromosomes move to the midline.

2. Metaphase II: Chromatids align at the midline.

3. Anaphase II: Sister chromatids separate.

4. Telophase II: Nuclear membranes form around four new cells, each with half the original chromosome number.

Spermatogenesis

• Spermatogenesis is the sperm cell production process.

Oogenesis

• Oogenesis refers to mature egg cell formation, where only one viable egg is produced during meiosis, and the rest, polar bodies, degenerate.

Sexual Reproduction

• The fusion of sperm and egg produces genetically diverse offspring, essential for evolutionary adaptation.