The Cellular Basis of Reproduction and Inheritance

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Chapter 8: The Cellular Basis of Reproduction and Inheritance

Introduction to Cell Division and Cancer

Cell division is a fundamental process in all living organisms, essential for growth, development, and maintenance. While normal cell division is tightly regulated, mutations can disrupt this control, leading to cancer. Cancer cells divide uncontrollably, invade other tissues, and can ultimately be fatal.

Cell Division and Reproduction

Asexual reproduction produces offspring that are genetically identical to the parent (clones).
Sexual reproduction generates genetic diversity among offspring.
Cell division is central to both types of reproduction, ensuring continuity of life.

Chapter 8: Big Ideas - Cell Division and Reproduction, The Eukaryotic Cell Cycle and Mitosis, Meiosis and Crossing Over, Alterations of Chromosome Number and Structure

Prokaryotic Cell Division: Binary Fission

Prokaryotes reproduce asexually by binary fission, a process in which a single cell divides into two genetically identical daughter cells.

Most prokaryotic genes are carried on a single circular DNA molecule (chromosome).
During binary fission:
1. The chromosome is duplicated and the copies move apart.
2. The cell elongates and the plasma membrane pinches inward.
3. A new cell wall forms, dividing the cell into two.

Binary fission in prokaryotes

The Eukaryotic Cell Cycle and Mitosis

Chromosomes and Chromatin

Eukaryotic cells contain multiple, linear chromosomes housed in the nucleus. Each chromosome consists of a long DNA molecule and associated proteins. Chromosomes are only visible during cell division; otherwise, they exist as loosely packed chromatin.

Before division, chromosomes duplicate, forming sister chromatids joined at a centromere.
During division, sister chromatids separate, ensuring each daughter cell receives an identical set of chromosomes.

Chromosome duplication and separation

The Cell Cycle

The cell cycle is an ordered sequence of events from the formation of a cell to its own division. It consists of:

Interphase: Includes G1 (first gap), S (DNA synthesis), and G2 (second gap) phases.
Mitotic (M) phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

The cell cycle diagram

Mitosis: Phases and Dynamics

Mitosis is the process by which duplicated chromosomes are distributed into two daughter nuclei. It consists of several phases:

Prophase: Chromosomes condense, spindle forms.
Prometaphase: Nuclear envelope fragments, spindle microtubules attach to kinetochores.
Metaphase: Chromosomes align at the metaphase plate.
Anaphase: Sister chromatids separate and move to opposite poles.
Telophase: Nuclear envelopes reform, chromosomes decondense.

Phases of mitosis: Interphase, Prophase, Prometaphase Phases of mitosis: Metaphase, Anaphase, Telophase and Cytokinesis

Cytokinesis

Cytokinesis is the division of the cytoplasm, overlapping with the end of mitosis. The process differs between animal and plant cells:

Animal cells: A cleavage furrow forms, pinching the cell in two.
Plant cells: A cell plate forms, eventually developing into a new cell wall.

Cytokinesis in animal cells Cytokinesis in plant cells

Regulation of the Cell Cycle

The cell cycle is regulated by a set of proteins that control progression through critical checkpoints (G1, G2, and M). Growth factors, signaling molecules from outside the cell, are often required for a cell to pass these checkpoints and divide.

Cell cycle control system with checkpoints Growth factor signaling and cell cycle control

Cancer and Cell Division

Cancer results from the loss of cell cycle control, leading to excessive cell division and the formation of tumors. Malignant tumors can invade other tissues (metastasis). Treatments like radiation and chemotherapy target rapidly dividing cells.

Tumor growth, invasion, and metastasis Cancer mortality and survival rates by age and ethnicity

Meiosis and Crossing Over

Homologous Chromosomes

Somatic cells contain pairs of homologous chromosomes, each carrying genes for the same traits at the same loci. Humans have 46 chromosomes (23 pairs). Not all chromosomes are fully homologous (e.g., sex chromosomes in males).

Homologous chromosomes structure

Haploid and Diploid Cells

Diploid cells (2n) have two sets of chromosomes; haploid cells (n), such as gametes (egg and sperm), have one set. Sexual life cycles alternate between haploid and diploid stages.

Human life cycle: alternation of haploid and diploid stages

Meiosis: Reduction Division

Meiosis reduces the chromosome number from diploid to haploid, producing four genetically unique gametes. It involves two consecutive divisions:

Meiosis I: Homologous chromosomes pair and separate.
Meiosis II: Sister chromatids separate, similar to mitosis.
Crossing over occurs during prophase I, exchanging genetic material between homologous chromosomes.

Meiosis: alternation of haploid and diploid stages Overview of meiosis: Interphase, Meiosis I, Meiosis II Meiosis I: Homologous chromosomes separate

Comparing Mitosis and Meiosis

Both processes begin with chromosome duplication in diploid cells, but their outcomes differ:

Mitosis: Two genetically identical diploid cells (for growth, repair, asexual reproduction).
Meiosis: Four genetically unique haploid cells (for sexual reproduction).

Comparison of mitosis and meiosis Summary: mitosis vs. meiosis outcomes

Genetic Variation: Independent Assortment and Crossing Over

Genetic diversity arises from:

Independent orientation of homologous chromosomes during metaphase I, leading to many possible combinations in gametes.
Random fertilization further increases variation.
Crossing over exchanges segments between nonsister chromatids, producing new allele combinations.

Independent assortment of chromosomes Crossing over between homologous chromosomes

Alterations of Chromosome Number and Structure

Nondisjunction and Chromosome Number Abnormalities

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers. This can lead to disorders such as Down syndrome (trisomy 21).

Nondisjunction during meiosis Consequences of nondisjunction

Karyotypes

A karyotype is a photographic inventory of an individual's chromosomes, arranged in pairs. It is used to detect chromosomal abnormalities.

Karyotype preparation and analysis Karyotype example

Sex Chromosome Abnormalities

Nondisjunction of sex chromosomes can result in syndromes such as Klinefelter (XXY), Turner (XO), and others. Some abnormalities have significant effects, while others may not affect phenotype.

Sex Chromosomes	Syndrome	Origin of Nondisjunction	Symptoms
XXY	Klinefelter syndrome (male)	Meiosis in egg or sperm formation	Sterile; underdeveloped testes; secondary female characteristics
XYY	None (normal male)	Meiosis in sperm formation	None
XXX	None (normal female)	Meiosis in egg or sperm formation	Slightly taller than average
XO	Turner syndrome (female)	Meiosis in egg or sperm formation	Sterile; immature sex organs

Polyploidy and Evolution

Errors in cell division can produce polyploid organisms (with extra sets of chromosomes), which is an important mechanism in the evolution of new species, especially in plants.

Alterations of Chromosome Structure

Chromosome breakage can lead to structural changes such as deletions, duplications, inversions, and translocations. These alterations can cause genetic disorders or contribute to cancer if they occur in somatic cells.