Cell Division and Reproduction

Asexual and Sexual Reproduction

Cells reproduce to maintain life, growth, and development. There are two main types of reproduction: asexual and sexual.

• Asexual reproduction: A single cell divides to produce two genetically identical daughter cells. This process is called mitosis.

• Sexual reproduction: Genetic material from two parent cells combines, resulting in daughter cells that are genetically distinct from the parents. This process involves meiosis.

Examples: Skin cells reproduce asexually, while gametes (egg and sperm) are produced sexually.

Mitosis: Cell Division in Eukaryotes

Mitosis is the process by which eukaryotic cells divide to produce two identical daughter cells. It is essential for growth, repair, and replacement of cells.

• Functions of mitosis:

  • Replacement of damaged or dead cells

  • Growth (e.g., baby growing in mother's womb)

  • Repair of tissues

  • Development of multicellular organisms

• Cancer: Occurs when mitosis goes wrong, leading to uncontrolled cell division. Cancer cells can bypass cell cycle checkpoints and metastasize (spread to other tissues).

Phases of Mitosis

Mitosis consists of four main phases, each with distinct events:

• Prophase: Nuclear envelope disappears, chromosomes condense and become visible, centrioles move to opposite sides of the cell.

• Metaphase: Chromosomes line up at the middle (equator) of the cell.

• Anaphase: Chromosomes separate and move to opposite poles; centromeres are responsible for this separation.

• Telophase: Nuclear envelope reforms, cell begins to split into two daughter cells. Cytokinesis (division of cytoplasm) occurs, forming two separate cells.

Additional info: In animal cells, cytokinesis forms a cleavage furrow; in plant cells, a cell plate forms due to the presence of a rigid cell wall.

Cell Cycle and Checkpoints

The cell cycle is a repeating pattern of growth and division. Checkpoints ensure the integrity of DNA and proper cell division.

• Stages of Interphase:

  • G1 (growth 1)

  • S (synthesis): DNA replication

  • G2 (growth 2)

• Checkpoints:

  • G1: Is the DNA intact and ready for replication?

  • G2: Has the DNA been completely and accurately replicated?

  • Metaphase to anaphase: Are chromosomes properly attached to the spindle and aligned?

Environmental factors can affect the function of checkpoints, potentially leading to diseases like cancer.

Chromosomes and DNA Structure

Chromosomes are compacted DNA molecules that carry genetic information. Each DNA molecule consists of a long chain of nucleotides.

• Nucleotide: The building block of DNA, composed of a sugar (deoxyribose), a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, or thymine).

• Sugar-phosphate backbone: Linked by dehydration synthesis, forms the structural framework of DNA.

• Genes: Segments of DNA that encode specific proteins.

Stem Cells

Stem cells are undifferentiated cells with the ability to self-renew and differentiate into various cell types.

• Self-renewal: Ability to divide and produce more stem cells.

• Potency: Ability to differentiate into specialized cell types.

Prokaryotic vs. Eukaryotic Cell Division

Binary Fission in Prokaryotes

Prokaryotic cells (e.g., bacteria) reproduce by binary fission, a simpler process than mitosis.

• Cell grows and replicates DNA

• Cell divides, producing two identical daughter cells

Key Differences

• Prokaryotic chromosomes are circular; eukaryotic chromosomes are linear.

• Eukaryotic cell cycle includes mitosis and multiple checkpoints; prokaryotic binary fission is simpler.

Telomeres and Centromeres

• Telomeres: Protective caps at the ends of chromosomes, prevent loss of genetic material during replication and keep chromosomes from unraveling.

• Centromere: Region where sister chromatids are held together until separation during cell division.

Genetics and Inheritance

Genetic Variability

Genetic variability refers to differences among individuals due to variations in their genes.

• Sexual reproduction increases genetic variability by combining genetic material from two parents.

• Mutations, crossing over, and independent assortment contribute to genetic diversity.

Chromosomes and Karyotypes

• Genetic information is packaged in chromosomes, which occur in pairs in eukaryotes.

• Karyotype: The set of chromosomes in a cell, typically 23 pairs (46 chromosomes) in humans.

• Autosomes: Chromosomes 1-22; Sex chromosomes: X and Y, determine gender.

• Homologous chromosomes: Chromosome pairs with similar appearance and genetic information.

Meiosis: Formation of Gametes

Meiosis is a type of cell division that produces haploid gametes (egg and sperm) from diploid cells, increasing genetic variability.

• Meiosis I: Homologous chromosomes separate; crossing over occurs, exchanging genetic material between chromatids.

• Meiosis II: Sister chromatids separate, resulting in four haploid cells.

Key processes:

• Crossing over: Exchange of genetic material between homologous chromosomes, forming chiasmata.

• Genetic recombination: New combinations of alleles in offspring.

• Fusion of gametes: Further increases genetic variability.

Additional info: Every gamete is genetically unique; the chance of identical gametes is about 1 in 64 trillion.

Chromosomal Disorders

Errors in meiosis can lead to inherited disorders due to abnormal chromosome numbers.

• Nondisjunction: Failure of chromosomes to separate properly.

• Examples:

  • Turner syndrome (XO): Female, infertile, does not undergo puberty.

  • Klinefelter syndrome (XXY): Male, reduced testosterone, taller than average.

  • Trisomy 21 (Down syndrome): Three copies of chromosome 21, causes developmental and health issues.

Mendelian Genetics

Principles of Inheritance

Inheritance is the process by which traits are passed from parents to offspring. Gregor Mendel established the foundational principles through experiments with pea plants.

• Gene: Segment of DNA at a specific locus encoding a trait.

• Allele: Alternative forms of a gene (e.g., blue eyes vs. brown eyes).

• Mutation: Source of new alleles; changes the nucleotide sequence.

Mendel's Laws

• Law of Segregation: Each gamete receives only one allele of each gene.

• Law of Independent Assortment: Alleles of different genes are distributed independently to gametes.

Genotype and Phenotype

• Genotype: Genetic makeup (combination of alleles).

• Phenotype: Observable characteristics.

• Dominant allele: Expressed when present (uppercase letter).

• Recessive allele: Expressed only when two copies are present (lowercase letter).

• Homozygous: Two identical alleles (e.g., PP or pp).

• Heterozygous: Two different alleles (e.g., Pp).

Punnett Squares and Test Crosses

Punnett squares predict the genotypes and phenotypes of offspring from genetic crosses.

• Assign letters to alleles (uppercase for dominant, lowercase for recessive).

• Test cross: Breeding a dominant phenotype individual with a homozygous recessive to determine genotype.

Complex Patterns of Inheritance

• Multiple alleles: Some genes have more than two alleles (e.g., blood type).

• Pleiotropy: Single gene affects multiple traits.

• Polygenic inheritance: Multiple genes influence a single trait (e.g., skin color).

• Environmental influence: Environment can affect gene expression and phenotype.

Sex-Linked Traits and Pedigrees

• Sex chromosomes: X and Y chromosomes determine sex; some traits are linked to these chromosomes.

• Pedigree: Diagram showing genetic relationships and inheritance patterns in families.

• Carrier: Heterozygous individual for a recessive disorder.

Human Genetic Disorders

• Recessive disorders: Sickle cell anemia (mutation in hemoglobin gene).

• Dominant disorders: Huntington's disease (neurodegenerative disorder).

• Sex-linked disorders: Hemophilia (mutation on X chromosome).

DNA Structure and Replication

DNA Structure

DNA is composed of four nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). The structure is a double helix with a sugar-phosphate backbone.

• Complementary base pairing: A pairs with T, G pairs with C via hydrogen bonds.

• Genome: Complete set of genetic information in an organism.

DNA Replication

DNA replication is the process of copying the DNA molecule before cell division.

• Semiconservative replication: Each new DNA molecule consists of one original strand and one new strand.

• DNA polymerase: Enzyme that synthesizes new DNA strands by adding nucleotides.

Equation:

Mutations

• Substitution: One base pair is replaced by another.

• Insertion: One or more base pairs are added.

• Deletion: One or more base pairs are removed.

• Inversion: A segment of DNA is reversed.

• Translocation: A segment of DNA is moved to another location.

Gene Expression and Regulation

Transcription and Translation

Gene expression involves converting genetic information in DNA into functional proteins via RNA intermediates.

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

• Translation: mRNA is decoded by ribosomes to assemble amino acids into proteins.

Equation:

Types of RNA

• Messenger RNA (mRNA): Carries genetic code from DNA to ribosomes.

• Transfer RNA (tRNA): Brings amino acids to ribosomes during protein synthesis.

• Ribosomal RNA (rRNA): Combines with proteins to form ribosomes.

Genetic Code

• Codon: Sequence of three nucleotides specifying an amino acid.

• Start codon: AUG (methionine)

• Stop codons: UAG, UGA, UAA

Gene Regulation

• Gene expression is regulated at multiple levels (transcription, translation, protein stability).

• Epigenetic controls: Changes in gene expression without altering DNA sequence.

Biotechnology

Genetic Engineering and Recombinant DNA

Biotechnology involves manipulating organisms or their components to produce useful products.

• Recombinant DNA: DNA containing genes from two or more organisms.

• Transgenic organisms: Organisms with recombinant DNA from different species.

• Transformation: Uptake of foreign DNA by a cell.

• CRISPR-Cas9: Technology for targeted gene editing.

Polymerase Chain Reaction (PCR)

PCR is a technique used to amplify specific DNA sequences.

• Uses primers to target DNA regions

• Produces millions of copies of DNA

Equation:

Applications of Biotechnology

• Genetically modified crops (e.g., soybeans, corn)

• Gene therapy: Treating diseases by altering genes in patient cells

• Forensic analysis: DNA profiling using short tandem repeats (STRs)

HTML Table: Comparison of Mitosis and Meiosis