Cell and Nuclear Division Notes

Generation of New Cells by Cell Division

• Cell division is how new cells are generated in living organisms.
• Even single-celled organisms like paramecium and chlamydomonas reproduce through cell division.
• The nucleus divides via mitosis before the cell reproduces.

Reproduction: Asexual vs. Sexual

• Reproduction can be asexual or sexual.
• Sexual reproduction produces genetically unique offspring, increasing genetic variation.
• Meiosis is cell division where a cell divides its genetic material among four daughter cells during gamete formation.
• Asexual reproduction yields offspring genetically identical to the parent.
• Binary fission and mitosis are mechanisms where a parent cell divides into two identical daughter cells.

Cell Theory: Cells Arise from Pre-existing Cells

• Cells are formed by division of pre-existing cells.
• We can trace the origin of all cells in our body back to the zygote formed by sperm and egg fertilization.
• An adult human body contains 30-40 trillion human cells, plus 40-50 trillion symbiotic prokaryotic cells.
• Cells in multicellular organisms specialize for specific functions through differentiation.
• The origin of all cells can be traced back to LUCA (Last Universal Common Ancestor), approximately 3.5 to 3.8 billion years ago.
• Organisms are classified into three domains: bacteria, archaea, and eukarya.
• Cladograms show the probable sequence of divergence between groups evolved from a common ancestor.
• Earth's history includes major geological and biological changes.
• Life emerged at least 3.8 billion years ago.
• The first cells likely had a membrane surrounding a self-replicating molecule like RNA.

Cytokinesis: Splitting of Cytoplasm

• Cytokinesis is the division of cytoplasm in a parent cell between daughter cells.
• The cell cycle includes interphase, mitosis, and cytokinesis.
• Mitosis is the division of the nucleus and DNA, while cytokinesis is the division of the cytoplasm and organelles.
• Mitosis and cytokinesis can occur simultaneously.

Cytokinesis in Plant vs. Animal Cells

• Plant Cells: Golgi vesicles fuse to create a cell plate, which extends and fuses with the parent cell sides, separating the daughter cells. Cellulose is released by exocytosis to build the cell wall.
• Animal Cells: Actin and myosin filaments form a ring at the cell equator, contracting to form a cleavage furrow that pinches the cell into two.

Equal and Unequal Cytokinesis

• Cytokinesis is the division of the cytoplasm and organelles into two daughter cells.
• In equal cytokinesis, the cytoplasm and organelles are equally partitioned.
• Each daughter cell must receive essential cell components, including organelles.
• Organelles like the nucleus, endoplasmic reticulum, and Golgi apparatus are disassembled and reformed during cytokinesis.
• Mitochondria and chloroplasts undergo their own division to repopulate the new daughter cells.
• In unequal cytokinesis, cytoplasm is divided unequally.
• Yeast Budding: An asymmetric division where the daughter cell receives a small portion of cytoplasm and remains attached to the parent.
• Oogenesis: Cytoplasm divides unevenly to produce a large egg cell and three small polar bodies, providing organelles and energy to the developing embryo.

Roles of Mitosis and Meiosis

• Eukaryotic cell division includes nuclear division (mitosis or meiosis) and cytoplasmic division (cytokinesis).
• Nuclei must divide before cytokinesis to ensure each daughter cell has a nucleus containing DNA.
• Mitosis is a single nuclear division resulting in two genetically identical nuclei.
• Meiosis includes two nuclear divisions, resulting in four genetically diverse daughter cells with half the chromosome number.
• If cytokinesis occurs without prior nuclear division, one daughter cell will lack a nucleus (anucleate).
• Anucleate cells cannot synthesize proteins, grow, or maintain structure without DNA.

DNA Replication: Prerequisite for Mitosis and Meiosis

• Before cell division (mitosis or meiosis), DNA must be replicated to create identical DNA strands.
• DNA replication ensures each daughter cell has a complete copy of genetic material.
• DNA replication occurs in the S phase of interphase.
• After replication, identical DNA strands are called sister chromatids, held together by the centromere and cohesin.
• The centromere adheres sister chromatids and serves as the site for kinetochore and microtubule attachment.
• Cohesin protein complex holds sister chromatids together until anaphase.
• Cohesin is removed by the start of anaphase, allowing sister chromatids to split and move to opposite poles.

Chromosome Condensation and Movement

• DNA is packaged into nucleosomes, which coil to form chromatin.
• Chromatin condenses during mitosis and meiosis to form chromosomes.
• During interphase, DNA is in chromatin form to be accessible for transcription.
• Each chromosome consists of coiled DNA around proteins, containing a portion of the $6,400,000,000$ base pairs that make up DNA.
• During prophase, replicated DNA in chromatin form condenses into chromosomes with two sister chromatids.
• DNA condenses by supercoiling for easier movement to the cell poles without tangling or breaking.
• Chromosome movement during anaphase in mitosis and meiosis is controlled by the kinetochore and microtubules.
• The kinetochore is a protein complex at the centromere, linking chromatids to microtubules.
• The mitotic spindle is made of microtubules, polymers of tubulin.
• At anaphase, sister chromatids separate, and motor proteins of kinetochores drive their movement along microtubules to the poles.
• Motor proteins like kinesin pull chromosomes toward the poles.

DNA Packaging

• Unreplicated Chromatin: DNA double helix wrapped around histones to form nucleosomes during interphase G1.
• Replicated Chromatin: DNA replicated in S phase, with two sister chromatids in chromatin form during interphase G2.
• Replicated Chromosome: DNA condensed by supercoiling in prophase, with two sister chromatids in chromosome form during metaphase.
• Unreplicated Chromosome: Sister chromatids separated, each as a single supercoiled chromosome during anaphase.

Phases of Mitosis

• Mitosis includes interphase, mitosis, and cytokinesis.
• Mitosis has four major phases: prophase, metaphase, anaphase, telophase.

Mitosis Phases

• Prophase: Replicated DNA condenses into chromosomes, each with sister chromatids joined at the centromere. Kinetochore attaches to the centromere, microtubules form the mitotic spindle, and the nuclear membrane breaks apart.
• Metaphase: Chromatids are attached to each other with cohesin. Microtubules attach to the kinetochore, and chromosomes align equidistant from the poles at the metaphase plate.
• Anaphase: Cohesin is removed, separating sister chromatids into individual daughter chromosomes. Motor proteins pull daughter chromosomes along microtubules towards the poles.
• Telophase: Chromosomes are pulled into tight groups at each pole, nuclear membrane reforms around each set of daughter chromosomes, chromosomes decondense to chromatin, and microtubules break down. Cytokinesis occurs simultaneously.
• Progression through mitosis is regulated by proteins called cyclins.
• Cancer can result when mitosis proceeds inappropriately.

Identifying Mitosis Phases in Micrographs

• Identifying cells in mitosis is required for calculating a mitotic index.
• Interphase: Nuclei are rounded or oval; DNA is granular (chromatin); chromosomes not present; microtubules distributed around the nucleus.
• Early Prophase: Nuclei are rounded or oval; DNA begins to condense into loose chromosomes; microtubules radiate from organizing centers near the nucleus; nuclear membrane intact.
• Late Prophase (Prometaphase): Nuclear membrane breaks down; chromosomes are more condensed; microtubules attach to chromosomes at the kinetochore; mitotic apparatus takes shape.
• 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 irregular, elongated spindle.
• Telophase: Nuclei may start to reform in pairs smaller than interphase cells; DNA is dense; microtubules are seen in a dense bundle; animal cells may take on a dumbbell shape during cytokinesis.

Meiosis as a Reduction Division

• A chromosome is a DNA molecule supercoiled around histone proteins.
• Genes are sections of DNA containing genetic information for traits.
• Transcription is RNA synthesis using a DNA template; translation is polypeptide synthesis from mRNA.
• Homologous chromosomes are chromosome pairs, one from each parent.
• Homologous chromosomes have the same size, centromere location, and genes in the same order but can have different alleles.
• Diploid cells/organisms contain homologous pairs of each chromosome (2n); haploid cells have a single copy of each chromosome (n).
• Humans: diploid (2n = 46), haploid (n = 23).
• Sexual reproduction combines genetic information from two parents.
• Meiosis reduces chromosome number from diploid to haploid in forming eggs and sperm.
• Meiosis occurs in gametogenesis (sperm and egg formation) and has two rounds of division.
• Meiosis I splits a diploid cell into two haploid cells; meiosis II separates sister chromatids, forming four haploid gametes.
• Cells go through prophase, metaphase, anaphase, and telophase during each round of division.

Meiosis Stages

• Interphase: Cell is diploid (2n); DNA in chromatin form; DNA replicates in S phase.
• Prophase I: Cell is diploid (2n); nuclear membrane breaks down; DNA condenses into chromosomes; homologous chromosomes pair up (synapsis); crossing over occurs.
• Metaphase I: Cell is diploid (2n); homologous chromosomes align independently at the cell equator.
• Anaphase I: Cell is diploid (2n); homologous pairs separate, moving to opposite poles; sister chromatids do not separate.
• Telophase I and Cytokinesis: New nuclei form; DNA uncoils to chromatin; spindle fibers break apart; two haploid daughter cells are produced.
• Prophase II: Cells are haploid (n); DNA condenses into chromosomes; nuclear membrane breaks down.
• Metaphase II: Cells are haploid (n); chromosomes align at the cell equator.
• Anaphase II: Cells are haploid (n); sister chromatids pull apart and move to opposite poles.
• Telophase II and Cytokinesis: Cells are haploid (n); spindle fibers break apart; DNA uncoils, forming chromatin; four haploid daughter cells are produced.

Mitosis vs. Meiosis Comparison

• Mitosis: one division, two identical daughter cells.
• Meiosis: two divisions, four genetically unique daughter cells.
• Both mitosis and meiosis are preceded by interphase.

Down Syndrome and Nondisjunction

• Normal meiosis results in four haploid daughter cells with separated homologous chromosomes and chromatids.
• Nondisjunction is the failure of chromosomes to separate during anaphase I or II of meiosis, leading to gametes with an incorrect chromosome number.

Types of Nondisjunction

• Anaphase I: Homologous chromosomes fail to separate; one gamete receives two of the same chromosome, and another receives none.
• Anaphase II: Sister chromatids fail to separate; some gametes have an incorrect chromosome number, while others are unaffected.
• A zygote with an extra chromosome results from a gamete with an extra chromosome fusing with a normal gamete.
• Down Syndrome results from an extra chromosome #21.
• The risk of Down syndrome increases with maternal age due to decreased levels of proteins like cohesin, leading to instability in homologous chromosome pairs.

Meiosis as a Source of Variation

• Variation is a defining feature of life; intraspecies variation is genetic and inheritable.
• Genetic variation enables some organisms to survive better, reproduce, and pass on beneficial variations, driving natural selection and evolution.

Sources of Genetic Variation

• Mutation
• Gene flow
• Meiosis
• Sexual reproduction

Meiosis Generates Genetic Diversity

• Crossing over between non-sister chromatids during prophase I creates new combinations of alleles on the same chromosome.
• Random orientation and independent assortment of bivalents during metaphase I creates gametes with new combinations of alleles on different chromosomes.

Crossing Over

• Homologous chromosomes pair up and align gene by gene during prophase I.
• DNA strands of non-sister chromatids are cut and rejoined, exchanging DNA.
• The location of the switch is called a chiasma.
• Crossing over produces recombinant chromosomes with new allele combinations.

Random Orientation and Independent Assortment

• Homologous chromosome pairs form bivalents on the cell equator during metaphase I.
• Each chromosome of the bivalent moves to a different pole during anaphase I randomly.
• Orientation of one bivalent does not affect others (independent assortment), resulting in gametes with varied maternal and paternal chromosome combinations.
• The number of possible chromosome combinations in gametes is $2^n$, where n is the haploid number of chromosomes.
• Humans have $2^{23} = 8,388,608$ possible combinations of maternal and paternal chromosomes.