IBDP Biology Unit 2.6: Cell and Nuclear Division Notes

D2.1.1 Generation of New Cells

• Cell division generates new cells in living organisms.

• Reproduction is a capability of all life, even single-celled organisms.

  • Examples include paramecium and chlamydomonas.

  • Paramecium divides its nucleus via mitosis before reproducing.

  • Chlamydomonas divides its nucleus via mitosis before reproducing.

Reproduction: Asexual vs. Sexual

• Reproduction can be asexual or sexual.

• Sexual reproduction:

  • Produces genetically unique offspring.

  • Increases genetic variation within a species.

  • Involves meiosis, where a cell divides its genetic material among four daughter cells during gamete formation.

• Asexual reproduction:

  • Produces genetically identical offspring to the parent.

  • Involves binary fission and mitosis, where a parent cell divides into two identical daughter cells.

Cell Theory and Cell Formation

• Cells are formed by the division of pre-existing cells.

• Cells do not spontaneously generate (except at the origin of life).

Implications of Cells Arising from Pre-existing Cells

• Implication #1: All cells in our body can be traced back to the zygote (fertilized egg).

  • An adult human body contains an estimated 30-40 trillion human cells, plus 40-50 trillion symbiotic prokaryotic cells.

  • Multicellular organisms' cells specialize for specific functions through differentiation.

• Implication #2: The origin of all cells can be traced back to LUCA (Last Universal Common Ancestor).

  • LUCA is the last universal common ancestor of all life on Earth.

  • Organisms are classified into three domains: bacteria, archaea, and eukarya.

  • Cladograms show the probable sequence of divergence between groups evolved from a common ancestor.

  • LUCA is estimated to have lived 3.5 to 3.8 billion years ago.

• Implication #3: There must have been a first cell that arose from non-living material.

  • Life first emerged at least 3.8 billion years ago, about 750 million years after Earth's formation.

  • How life originated and the first cell came to be is speculative, but experiments offer evidence.

  • The first cells likely had a membrane surrounding a self-replicating molecule like RNA.

D2.1.2 Cytokinesis

• Cytokinesis is the splitting of cytoplasm in a parent cell between daughter cells.

• Cytokinesis is part of the eukaryotic cell cycle, which includes:

  • Interphase

  • Mitosis

  • Cytokinesis

• The cell cycle is the events leading to cell division, producing two daughter cells.

Cytokinesis vs. Mitosis

• Mitosis: division of the nucleus and DNA into two daughter cells.

• Cytokinesis: division of the cytoplasm and organelles into two daughter cells.

• Mitosis and cytokinesis can occur simultaneously.

Cytokinesis in Plant and Animal Cells

• Plant Cells:

  • Golgi buds off vesicles that move toward the cell equator.

  • Vesicles fuse to create a disc-shaped cell plate.

  • The cell plate extends until it fuses with the parent cell sides, separating the daughter cells.

  • Daughter cells release cellulose via exocytosis into the space between membranes.

  • Cellulose builds the cell wall of each daughter cell.

• Animal Cells:

  • Actin and myosin form rings at the cell equator.

  • These proteins contract, pulling the cell membrane inward to form a cleavage furrow.

  • The cleavage furrow pinches in until the cell splits into two.

D2.1.3 Equal and Unequal Cytokinesis

• Equal Cytokinesis:

  • Cytoplasm and organelles are equally partitioned between daughter cells.

  • Each daughter cell receives essential cell components.

  • Organelles are usually in large enough numbers to be inherited.

  • Some organelles (nucleus, ER, Golgi) are disassembled and reformed during cytokinesis.

  • Other organelles (mitochondria, chloroplasts) undergo their own division to repopulate daughter cells.

• Unequal Cytokinesis:

  • Cytoplasm is sometimes divided unequally.

  • Yeast Budding:

    • An asymmetric division mechanism used by most yeasts.

    • The nucleus divides by mitosis.

    • The daughter cell receives a small portion of the cytoplasm and initially remains attached to the parent cell, eventually separating and maturing.

  • Oogenesis:

    • Production of an egg (ovum) cell.

    • Cytoplasm divides unevenly, producing one large egg cell and three small polar bodies.

    • The egg contains the cytoplasm of all four daughter cells, providing organelles and stored energy for the developing embryo.

    • Egg cell size is an aspect of specialization.

D2.1.4 Roles of Mitosis and Meiosis in Eukaryotes

• Eukaryotic cell division involves:

  • Division of the nucleus by mitosis or meiosis.

  • Division of the cytoplasm by cytokinesis.

• Nuclei must divide before cytokinesis, ensuring each daughter cell has a nucleus.

• Most cells need a nucleus because it contains DNA, essential for:

  • Expressing genes.

  • Performing cell functions.

  • Sustaining metabolism.

  • Maintaining structure.

• Mitosis vs. Meiosis:

  • Mitosis: single nuclear division, results in two genetically identical nuclei.

  • Meiosis: two nuclear divisions, results in four genetically diverse daughter cells with half the chromosomes of the parent cell.

• DNA replication, chromosome condensation, and movement are shared features of both mitosis and meiosis.

D2.1.5 DNA Replication as a Prerequisite

• There are two main phases of the cell cycle:

  • Interphase (growth period between cell divisions)

  • Cell Division

    • Mitosis (division of the nucleus)

    • Cytokinesis (division of the cytoplasm and organelles)

• DNA Replication in S Phase of Interphase:

  • Before mitosis or meiosis, cells replicate their DNA, creating two identical DNA strands.

  • Replication ensures each daughter cell has a complete copy of the genetic material.

  • DNA replication occurs during the S phase of interphase.

  • After replication, each identical DNA strand is called a sister chromatid.

• Sister Chromatids:

  • Held together by:

    • Centromere (adheres sister chromatids and is the site of kinetochore and microtubule attachment).

    • Cohesin (protein complex that holds sister chromatids together until anaphase).

  • Cohesin is established in interphase before mitosis and meiosis.

  • Cohesin is removed by the start of anaphase, allowing sister chromatids to split and move to opposite poles.

D2.1.6 Chromosome Condensation and Movement

• DNA Packaging:

  • Eukaryotic cell DNA wraps around histone proteins to form a nucleosome.

  • Nucleosomes coil and stack to form chromatin fibers.

  • Chromatin condenses during mitosis and meiosis to form chromosomes.

  • During interphase, most DNA is in chromatin form for enzyme accessibility.

• DNA Packaging (Advanced):

  • Each chromosome contains one continuous DNA molecule coiled around proteins.

  • The process starts with the assembly of a nucleosome, formed when eight histone protein subunits attach to the DNA molecule.

  • Six nucleosomes are coiled together and stack on top of each other to form a fiber of packed nucleosomes known as chromatin.

  • During prophase, replicated DNA in chromatin form condenses into a chromosome with two sister chromatids.

  • DNA condenses by supercoiling for easier movement without tangling.

• Movement of Chromosomes during Mitosis and Meiosis:

  • Controlled by the kinetochore and microtubules.

  • Kinetochore: protein complex assembling at the centromere.

  • Each sister chromatid has its own kinetochore that faces in opposite directions.

  • Kinetochores link chromatids to microtubules.

  • Mitotic spindle: made of microtubules.

  • Microtubules: polymers of tubulin, components of the cell's cytoskeleton.

• Analogy:

  • Rope = Microtubules

  • Harness = Kinetochore

  • Person = Chromosome

  • Waist = Centromere

• At anaphase, sister chromatids separate, and motor proteins of kinetochores drive movement along microtubules to the poles.

• Motor proteins within kinetochores (e.g., kinesin) pull chromosomes toward the poles.

D2.1.7 Phases of Mitosis

• Mitosis is part of the eukaryotic cell cycle.

• Mitosis is divided into four major phases:

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

• IB Biology students should identify mitosis phases in micrographs.

Mitosis Phases:

• Prophase:

  • Replicated DNA in chromatin condenses to become chromosomes.

  • Each replicated chromosome is a pair of sister chromatids joined at the centromere by cohesin.

  • Kinetochore attaches to the centromere of chromatids.

  • Microtubules form the mitotic spindle.

  • The nuclear membrane breaks apart.

• Metaphase:

  • Chromatids are still attached by cohesin.

  • Microtubules attach to the kinetochore of each chromatid.

  • Chromosomes move equidistant from the two poles at the metaphase plate.

• Anaphase:

  • Cohesin is removed, separating sister chromatids into individual daughter chromosomes.

  • Motor proteins of the kinetochore pull daughter chromosomes along microtubules towards the poles.

• Telophase:

  • Chromosomes are pulled into a tight group at each pole.

  • The nuclear membrane reforms around each set of daughter chromosomes.

  • Chromosomes decondense to form chromatin.

  • Microtubule spindle fibers break down.

  • Occurs simultaneously with cytokinesis (division of cytoplasm and organelles).

Consequences of Mitosis:

• Cancer can result when mitosis occurs when it shouldn't.

• Progression through mitosis is regulated by cyclins proteins.

D2.1.8 Identification of Mitosis Phases

• IB Biology students need to identify mitosis phases in micrographs.

• Identifying cells in mitosis is required when calculating a mitotic index.

Micrographs of Mitosis Phases:

• Interphase:

  • Rounded or oval nuclei.

  • DNA is granular (in chromatin form).

  • Chromosomes are not present.

  • Microtubules are distributed around the nucleus.

• Early Prophase:

  • Rounded or oval nuclei.

  • DNA begins to condense into loose chromosomes.

  • Microtubules radiate from organizing centers near the nucleus.

  • The nuclear membrane is intact.

• Late Prophase (Prometaphase):

  • The nuclear membrane has broken down.

  • Chromosomes are more condensed.

  • Microtubules attach to chromosomes at the kinetochore, forming a mitotic apparatus.

• Metaphase:

  • Chromosomes form a condensed bar-shaped mass across the cell center.

  • Microtubules form a spindle shape.

  • Chromosomes are completely aligned.

• Anaphase:

  • Chromosomes separate into two clusters at the poles.

  • Trailing chromosome arms point back toward the cell center.

  • Microtubules form an elongated, irregular spindle.

• Telophase:

  • Nuclei start to reform in pairs that are smaller than interphase cells.

  • DNA is dense (solid color).

  • Microtubules are seen in a dense bundle.

  • Animal cells may be dumbbell-shaped as cytokinesis occurs.

D2.1.9 Meiosis as a Reduction Division

• Genes and Chromosomes:

  • A chromosome is a DNA molecule supercoiled around nucleosome-forming histone proteins.

  • Genes are sections of DNA in the chromosome.

  • The chromosome contains hundreds to thousands of genes.

  • Each gene stores genetic information that codes for a trait.

  • Transcription synthesizes RNA using a DNA template.

  • Translation synthesizes a polypeptide from mRNA.

• Homologous Chromosomes:

  • In many eukaryotic cells, chromosomes are found as pairs, one from the mother and one from the father.

  • These pairs are homologous chromosomes.

• Features of Homologous Chromosomes:

  • Same size, centromere location, and genes in the same order.

  • Variations in nucleotide sequences within a specific gene are called alleles.

  • The chromosomes may have the same allele (homozygous) or different alleles (heterozygous).

• Diploid Cells:

  • Contain homologous pairs of each chromosome.

  • Nearly all mammals are diploid, with two sets of chromosomes in body cells.

  • Diploid cells are described as 2n.

  • In humans, 2n = 46.

• Chromosome Number:

  • The total number of chromosomes in a typical body cell.

• Ploidy Number:

  • The number of sets of chromosomes.

• Haploid Number:

  • The number of chromosomes in a single complete set. In humans, n=23.

• Haploid Cells:

  • Has a single copy of each chromosome.

  • Do not contain homologous chromosomes.

  • Eggs and sperm are haploid cells.

  • Haploid cells are described as n.

  • In humans, n = 23.

• Chromosomes in Sexual Reproduction:

  • The genetic information of two parents combines in an offspring.

  • The number of chromosomes in offspring must be maintained.

  • There must be a reduction in the number of chromosomes to form eggs and sperm, halving from diploid to haploid.

• During meiosis:

  • A diploid starting cell with 2n = 46 chromosomes divides to form haploid gametes with n = 23 chromosomes.

  • At fertilization, two haploid gametes combine to form a zygote, which again has the full complement of 2n = 46 chromosomes.

• Meiosis:

  • The type of cell division in gametogenesis (formation of sperm and egg).

  • There are two rounds of division in meiosis.

  • Meiosis I splits a diploid cell (2n) into two haploid cells (n).

  • Meiosis II separates sister chromatids, forming four new haploid gametes.

  • Meiosis is the reduction division because it reduces chromosome number by half.

Phases of Meiosis:

• Cell division occurs twice during meiosis.

• Each round includes prophase, metaphase, anaphase, and telophase stages.

• Interphase:

  • Cell is diploid (2n).

  • DNA is in chromatin form.

  • G1: metabolically active period where growth involves synthesis of cell components, proteins, and DNA.

  • S phase: DNA replicates, creating two identical strands held together at the centromere.

  • G2: Cell prepares for meiosis.

• Prophase I:

  • Cell is diploid (2n).

  • The nuclear membrane breaks down.

  • Spindle fibers form.

  • DNA condenses into chromosomes.

  • Homologous chromosomes pair up in synapsis.

  • Crossing over occurs, where chromosomes swap pieces of DNA, increasing genetic variation.

• Metaphase I:

  • Cell is diploid (2n).

  • Homologous chromosomes align independently at the cell equator.

  • The orientation of each homologous pair is random (independent assortment).

• Anaphase I:

  • Cell is diploid (2n).

  • Homologous pairs separate, and chromosomes move towards opposite poles.

  • Sister chromatids are NOT separated.

• Telophase I and Cytokinesis:

  • Telophase occurs simultaneously as cytokinesis.

  • New nuclei form around chromosomes at each pole.

  • DNA uncoils into chromatin.

  • Spindle fibers break apart.

  • By the end, there are TWO HAPLOID daughter cells.

• Prophase II:

  • Cells are haploid (n).

  • DNA condenses into chromosomes.

  • The nuclear membrane breaks down.

  • Spindle fibers form.

• Metaphase II:

  • Cells are haploid (n).

  • Chromosomes are aligned at the cell equator.

• Anaphase II:

  • Cells are haploid (n).

  • Sister chromatids pull apart and move towards opposite poles.

• Telophase II and Cytokinesis:

  • Telophase occurs simultaneously as cytokinesis.

  • Cells are haploid (n).

  • Spindle fibers break apart, and DNA uncoils into chromatin.

  • At the end, there are FOUR HAPLOID daughter cells.

• Mitosis vs. Meiosis Summary:

  • Mitosis: one division, two genetically identical daughter cells, growth, cell replacement, tissue repair.

  • Meiosis: two divisions, four genetically unique daughter cells, half as much DNA, production of sperm and egg cells.

  • Both mitosis and meiosis are preceded by interphase.

D2.1.10 Down Syndrome and Nondisjunction

• Normal Meiosis:

  • Four haploid daughter cells are produced.

  • Homologous chromosomes separate in meiosis I, halving the chromosome number.

  • Chromatids of each replicated chromosome separate in meiosis II.

• Errors of Meiosis:

  • Nondisjunction is the failure of chromosomes to separate during anaphase I or II of meiosis.

  • Results in gamete cells with an incorrect number of chromosomes.

• Nondisjunction in Anaphase I:

  • Homologous chromosomes fail to separate.

  • One gamete receives two of the same chromosome, and the other receives none.

  • All resulting gametes have an incorrect number of chromosomes.

• Nondisjunction in Anaphase II:

  • Sister chromatids fail to separate.

  • Half of the gametes have an incorrect chromosome number.

• Consequence of Nondisjunction:

  • A gamete with an extra chromosome fuses with a normal gamete, resulting in a zygote with three copies of that chromosome.

  • Most events of nondisjunction result in non-functioning egg and/or sperm cells.

  • In nearly all cases, a zygote or early embryo with the incorrect number of chromosomes will not survive.

• Down Syndrome:

  • The most common chromosomal abnormality.

  • People with Down Syndrome have an extra chromosome #21.

  • The risk increases with maternal age due to decreased cohesin proteins in older eggs.

D2.1.11 Meiosis as a Source of Variation

• Variation:

  • A defining feature of life.

  • Variation within a species is called intraspecies variation.

  • Genetic variation within a species is inheritable.

• Variation drives evolution:

  • Genetic variation in a population allows some organisms to survive better in their environment.

  • Differential survival and reproduction are part of natural selection.

• Sources of genetic variation:

  • Mutation: changes in the sequences of genes in DNA.

  • Gene flow: the movement of genes between different groups of organisms.

  • Meiosis: formation of egg and sperm, leading to new combinations of genes.

  • Sexual reproduction: random fertilization between egg and sperm.

• Meiosis as a Source of Genetic Diversity:

  • Crossing over between non-sister chromatids of homologous chromosomes during prophase I.

  • Random orientation and independent assortment of bivalents during metaphase I.

• Crossing Over:

  • During prophase I, homologous chromosomes pair up and align gene by gene.

  • The DNA strands of two non-sister chromatids are cut and rejoined to exchange DNA.

  • The location where fragments switch is called a chiasma.

  • Produces recombinant chromosomes with new combinations of alleles.

• Random Orientation of Bivalents:

  • Each homologous pair forms a bivalent on the equator of the cell during metaphase I.

  • Each chromosome of the bivalent moves to a different pole during anaphase I randomly.

  • Gametes formed at the end of meiosis will have different alleles from each other.

• Independent Assortment:

  • The orientation of one bivalent does not affect other bivalents.

  • Results in gametes with a wide variety of maternal and paternal chromosome combinations.

• Independent Assortment and Variation:

  • There are 2^n possible ways chromosomes can independently assort into gametes, where n is the number of chromosomes in a haploid cell.

  • In humans, 2^{23} = 8,388,608 possible combinations of maternal and paternal chromosomes.