Cell Cycle and Cell Division

Binary Fission

Binary fission is the process by which prokaryotic cells, such as bacteria, divide. It is a simple mechanism due to the absence of a nucleus and involves the replication and segregation of a single circular chromosome.

• Process: The DNA is copied, the two copies move apart, the cell elongates, and then pinches into two identical cells.

• Key Feature: No nucleus; division is straightforward.

Diploid vs. Haploid

Chromosome number is a fundamental concept in cell division and genetics.

• Diploid (2n): Two sets of chromosomes, one from each parent. Human somatic cells have 46 chromosomes.

• Haploid (n): One set of chromosomes. Human gametes (egg and sperm) have 23 chromosomes.

Homologous Chromosomes vs. Sister Chromatids

• Homologous Chromosomes: A pair of chromosomes (one maternal, one paternal) carrying the same genes but possibly different alleles.

• Sister Chromatids: Two identical copies of a single chromosome, joined at the centromere, produced during DNA replication.

Centromere vs. Kinetochore

• Centromere: The region of DNA where sister chromatids are joined.

• Kinetochore: A protein complex assembled on the centromere; spindle fibers attach here during mitosis/meiosis.

Histones and Chromatin Structure

• Histones: Positively charged proteins around which negatively charged DNA wraps.

• Nucleosome: DNA wrapped around a group of 8 histones; resembles beads on a string.

• Function: Allows DNA to be compacted to fit inside the nucleus.

Interphase

Interphase is the preparatory phase before cell division.

• G1: Cell growth and normal function.

• S: DNA synthesis (replication).

• G2: Preparation for division; checks DNA and synthesizes division proteins.

Mitosis (PMAT)

Mitosis is the process by which somatic cells divide, producing two genetically identical daughter cells.

• Prophase: Chromosomes condense, spindle forms, nucleolus disappears.

• Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

Cytokinesis

• Animals: Cleavage furrow forms via a contractile ring.

• Plants: Cell plate forms, developing into a new cell wall.

Cell Cycle Regulation

• MPF (Mitosis Promoting Factor): Composed of CDK (cyclin-dependent kinase) and cyclin. CDK is always present; cyclin levels fluctuate. When cyclin binds CDK, MPF is active and triggers mitosis.

• Checkpoints:

  • G1: Cell size, DNA integrity, environmental conditions.

  • G2: DNA replication accuracy.

  • M: Chromosome attachment to spindle.

Cancer and Cell Cycle

• Proto-oncogenes: Promote cell division; mutations create oncogenes (overactive).

• Tumor Suppressor Genes: Inhibit cell division; mutations disable these brakes (e.g., p53).

Meiosis

Meiosis produces gametes (egg and sperm) and reduces chromosome number by half.

• Meiosis I: Homologous chromosomes separate, resulting in haploid cells.

  • Prophase I: Synapsis and crossing over (recombination at chiasmata).

  • Metaphase I: Homologous pairs align at the equator.

  • Anaphase I: Homologs separate; sister chromatids remain together.

• Meiosis II: Sister chromatids separate, similar to mitosis. Result: four haploid cells.

Gametogenesis

• Spermatogenesis: One cell produces four sperm; continuous from puberty.

• Oogenesis: One cell produces one egg and three polar bodies; eggs pause at prophase I (before birth) and metaphase II (until fertilization).

Mitosis vs. Meiosis Comparison

Nondisjunction

• Definition: Failure of chromosomes to separate properly during meiosis.

• Result: Gametes with extra (n+1) or missing (n−1) chromosomes.

• Example: Trisomy 21 (Down syndrome) – three copies of chromosome 21.

Classical Genetics

Basic Terms

• Gene: DNA sequence coding for a trait.

• Locus: Location of a gene on a chromosome.

• Allele: Variant form of a gene.

• Homozygous: Two identical alleles (AA or aa).

• Heterozygous: Two different alleles (Aa).

• Dominant: Expressed when present.

• Recessive: Expressed only when homozygous.

Monohybrid and Dihybrid Crosses

• Monohybrid Cross: Aa × Aa

  • Genotype ratio: 1 AA : 2 Aa : 1 aa

  • Phenotype ratio: 3 dominant : 1 recessive

• Dihybrid Cross: AaBb × AaBb

  • Phenotype ratio: 9 : 3 : 3 : 1

  • Genotype ratio: 1:2:1:2:4:2:1:2:1

Punnett Square Tips

• Arrange gametes from each parent along the top and side; fill in boxes to predict offspring genotypes.

• For dihybrid crosses, gametes are AB, Ab, aB, ab.

Mendel's Laws and Meiosis

• Law of Segregation: Alleles separate during gamete formation (anaphase I).

• Law of Independent Assortment: Genes on different chromosomes assort independently (random alignment in metaphase I).

Test Cross

• Cross a dominant phenotype with a homozygous recessive to determine genotype.

• All dominant offspring = homozygous dominant; 1:1 ratio = heterozygous.

Non-Mendelian Patterns

• Polygenic Inheritance: Multiple genes contribute to a trait (e.g., height, skin color).

• Multiple Alleles: More than two alleles exist (e.g., ABO blood types: IA, IB, i).

• Codominance: Both alleles are expressed (e.g., AB blood type).

• Incomplete Dominance: Heterozygote is intermediate (e.g., pink flowers from red × white).

• Sex-linked Traits: Genes on X chromosome; males (XY) show recessive traits more often (e.g., color blindness).

DNA Replication

DNA Structure

• Nucleotide: Sugar (deoxyribose), phosphate, and base (A, T, G, C).

• Base Pairing: A–T (2 hydrogen bonds), G–C (3 hydrogen bonds).

• Antiparallel Strands: One runs 5'→3', the other 3'→5'.

Semiconservative Replication

• Each new DNA molecule contains one old strand and one new strand.

Replication Proteins and Their Functions

Leading vs. Lagging Strand

• Leading Strand: Synthesized continuously toward replication fork.

• Lagging Strand: Synthesized discontinuously in Okazaki fragments, away from fork; fragments joined by ligase.

Eukaryotic Replication

• Multiple origins of replication per chromosome.

• More types of DNA polymerase.

• Histones must be managed during replication.

Telomeres and Telomerase

• Telomeres: Repetitive DNA at chromosome ends; protect from degradation.

• Telomerase: Enzyme that extends telomeres; active in stem cells, germ cells, and cancer cells.

Nucleotide Excision Repair

• Damaged DNA (e.g., thymine dimers) is excised, replaced, and sealed.

• Example: Xeroderma pigmentosum – lack of repair leads to skin cancer.

Gene Expression

Central Dogma

• Information flows: DNA → RNA → Protein.

• Transcription: DNA is copied into RNA (in nucleus).

• Translation: RNA is decoded to build protein (at ribosome).

• Genotype: Genetic makeup.

• Phenotype: Observable traits.

Strands and Base Pairing

• Template Strand: Read by RNA polymerase.

• Coding Strand: Matches mRNA sequence (except T→U).

• Example: Template DNA 3'-TAC GGT-5' → mRNA 5'-AUG CCA-3'.

Genetic Code

• Codon: Three mRNA bases code for one amino acid; 64 codons.

• Start Codon: AUG (methionine).

• Stop Codons: UAA, UAG, UGA.

• Degeneracy: Multiple codons for same amino acid.

• Universality: Nearly all organisms use the same code.

RNA Polymerase and mRNA Processing

• RNA polymerase synthesizes RNA from DNA template; starts at promoter.

• Eukaryotic mRNA is processed: 5' cap, poly-A tail, intron removal (splicing).

Types of RNA

• mRNA: Messenger RNA; carries genetic information.

• tRNA: Transfer RNA; brings amino acids to ribosome.

• rRNA: Ribosomal RNA; forms ribosome structure.

DNA vs. RNA

• RNA has ribose sugar, uracil (U) instead of thymine (T), and is usually single-stranded.

Mutations

• Silent: No effect (same amino acid).

• Missense: Changes amino acid.

• Nonsense: Creates stop codon; protein truncated.

Translation

• Ribosome: Site of protein synthesis; has large and small subunits.

• tRNA: Anticodon pairs with mRNA codon; carries specific amino acid.

• Ribosome Sites:

  • A site: Incoming tRNA.

  • P site: Holds growing polypeptide.

  • E site: Exit for empty tRNA.

• Stages:

  • Initiation: Ribosome assembles at start codon.

  • Elongation: tRNAs bring amino acids; chain grows.

  • Termination: Stop codon reached; protein released.

Post-Translational Processing

• Proteins are folded (chaperones), may be cut, and modified (e.g., glycosylation, phosphorylation).

Biotechnology

DNA Sequencing

• Sanger Sequencing: Uses chain-terminating nucleotides (ddNTPs) labeled with colors; produces fragments of varying lengths; accurate for short sequences.

• Nanopore Sequencing: DNA passes through a protein pore; each base alters electric current; reads long sequences in real time.

Recombinant DNA and GMOs

• Combining DNA from different sources; transgenic organisms carry genes from other species.

Gene Cloning into Plasmids

1. Plasmid (circular DNA vector) with antibiotic resistance and insertion site.

2. Restriction enzymes cut DNA at specific sequences, creating sticky or blunt ends.

3. Gene and plasmid cut with same enzyme; ligase joins them.

4. Plasmid inserted into bacteria; only bacteria with plasmid survive on antibiotic plates.

PCR (Polymerase Chain Reaction)

• Amplifies DNA; each cycle doubles the amount.

• Steps:

  1. Denaturation (~95°C): DNA strands separate.

  2. Annealing (~50–60°C): Primers bind.

  3. Extension (~72°C): Taq polymerase synthesizes new DNA.

Gel Electrophoresis

• Separates DNA fragments by size; DNA moves toward positive electrode; smaller fragments travel farther.

• DNA ladder used for size comparison.

Genetic Engineering Examples

• Human Insulin: Human gene inserted into bacteria; bacteria produce insulin.

• Bt Crops: Bacterial insecticide gene inserted into plants; plants produce pest-killing toxin.

RT-PCR and CRISPR

• RT-PCR: Reverse transcriptase converts RNA to cDNA; PCR amplifies it. Used to detect gene expression or RNA viruses.

• CRISPR: Guide RNA directs Cas9 to specific DNA; Cas9 cuts DNA; cell repairs or inserts new sequence. Used in gene therapy.

Additional info:

• Practice drawing mitosis and meiosis stages, Punnett squares, and transcription/translation exercises for exam preparation.