Cell Division: Asexual vs. Sexual Reproduction

• Cell division is crucial for transmitting genetic information, ensuring offspring cells inherit identical chromosomes from the parent cell.
• It is essential for reproduction (both asexual and sexual), development, growth, and repair in organisms.

Asexual Reproduction

• A single parent produces genetically identical offspring without the fusion of gametes (sperm and egg).
• Seen in single-celled organisms (prokaryotes, yeast) via binary fission and in some multicellular organisms (sea stars, houseplants) through fragmentation.
• Offspring are clones, sharing identical genes with the parent.

Sexual Reproduction

• Requires the fusion of gametes (egg and sperm), formed through a special type of cell division in reproductive organs.
• Gametes possess half the chromosome number of the parent cell and exhibit unique gene combinations.
• Offspring are genetically distinct from parents and siblings, inheriting a unique gene combination from both parents, leading to variation.

Role in Multicellular Organisms

• Enables development from a single fertilized egg (zygote) into a mature adult.
• Facilitates continuous renewal and repair by replacing cells lost due to wear, tear, or injury.
• Millions of cells divide every second to replace damaged or dead cells.

Types of Cell Division

• Mitosis is responsible for growth, maintenance, and asexual reproduction.
• Meiosis is involved in the production of egg and sperm cells.

Terms to Know

• Chromosomes are gene-carrying structures in the nucleus, composed of DNA and proteins.
• Cell division is the process of genome duplication and cytoplasm division, resulting in cell reproduction.
• Binary fission is an asexual reproduction method, where a parent cell divides into two genetically identical daughter cells of roughly equal size.

Binary Fission in Prokaryotes

• Prokaryotes (bacteria and archaea) reproduce asexually through binary fission ("dividing in half").
• Genes are carried on a single circular DNA molecule, which forms the chromosome.
• Despite being shorter than eukaryotic chromosomes, duplicating and distributing prokaryotic chromosomes to daughter cells is challenging.
• As the chromosome replicates, each copy moves towards opposite ends as the cell elongates.
• After chromosome duplication and cell elongation, the plasma membrane pinches inward, and the cell wall forms to divide the parent cell.

Eukaryotic Chromosomes

• Eukaryotic cells have more genes than prokaryotic cells, and genes are housed in the nucleus within multiple chromosomes (excluding those in mitochondria and chloroplasts).
• Eukaryotic species possess a characteristic chromosome number in each cell nucleus (e.g., humans have 46, dogs have 78).
• Each chromosome consists of a long DNA molecule bearing genes and associated proteins.
• DNA and protein form chromatin, which exists as diffuse fibers when the cell isn't dividing.
• As a cell prepares to divide, chromatin condenses into tightly coiled chromosomes visible under a light microscope.

Chromosome Duplication and Segregation

• Before cell division, each chromosome is duplicated, and consists of two identical sister chromatids joined by proteins along their length and most closely at the centromere region.
• During cell division, sister chromatids separate, becoming individual chromosomes.
• Each daughter cell receives one chromosome copy, resulting in a complete and identical set of chromosomes; in humans, cells have 46 duplicated chromosomes, resulting in 46 single chromosomes in each daughter cell.

Terms to Know

• Chromatin is the DNA and protein complex that constitutes eukaryotic chromosomes; it exists in a diffuse form when the cell is not dividing.
• Sister chromatids are the two identical copies of a duplicated chromosome, connected at the centromere before mitosis.
• The centromere is the region where sister chromatids are joined; it's also where spindle microtubules attach during cell division.

Cell Cycle Overview

• Cell division underlies reproduction, growth, and cell replacement.
• The cell cycle includes interphase (cell growth and DNA replication) and the mitotic phase (cell division).

Interphase

• Accounts for about 90% of the cell cycle.
• The cell performs its normal functions, grows, duplicates cytoplasm, synthesizes proteins, and replicates its DNA.
• Three subphases:
  • G1 phase (first gap): Cell grows and functions normally.
  • S phase (synthesis): Chromosomes duplicate.
  • G2 phase (second gap): Cell prepares for division.

Mitotic Phase (M Phase)

• Accounts for about 10% of the cell cycle.
• Comprises mitosis (nuclear division) and cytokinesis (cytoplasmic division).
• Mitosis results in two daughter nuclei.
• Cytokinesis divides the cytoplasm, yielding two genetically identical daughter cells.
• The daughter cells then proceed through G1 and repeat the cycle.

Mitosis Accuracy

• Ensures each daughter cell receives identical chromosomes.
• Errors in chromosome distribution are rare, showcasing the accuracy of the process.

The Importance of Mitosis

• Initiates with a single cell, enabling all body cells to have copies of the original 46 chromosomes.
• Trillions of cells in the body trace back through mitotic divisions to the first cell following fertilization.

Terms to Know

• Cell cycle is the ordered sequence of events from cell formation to division.
• Interphase is the period when the cell isn't actively dividing, constituting most of the cycle.
• Mitosis is nuclear division producing two identical nuclei; together with cytokinesis, it constitutes the mitotic (M) phase.

Mitotic Phase Stages

• Prophase: Chromatin condenses into chromosomes, mitotic spindle starts forming as centrosomes move apart.
• Prometaphase: Nuclear envelope fragments, microtubules attach to kinetochores, and chromosomes move toward the cell's center.
• Metaphase: Spindle fully formed, chromosomes align on the metaphase plate, and kinetochores attach to microtubules from opposite poles.
• Anaphase: Centromeres separate, motor proteins move daughter chromosomes to opposite poles, shortening kinetochore microtubules, elongating cell.
• Telophase: Cell elongation continues, daughter nuclei form, mitotic spindle disappears, mitosis completes.
• Cytokinesis: Cytoplasm divides, two daughter cells separate.

Mitotic Spindle and Centrosomes

• The mitotic spindle—a structure of microtubules—separates chromosomes.
• Spindle microtubules originate from centrosomes—microtubule-organizing regions.

Animal Cell Cytokinesis

• Cytokinesis is achieved through cleavage.
• A cleavage furrow forms, and a ring of actin microfilaments contracts, pinching the cell in two.

Plant Cell Cytokinesis

• Cell walls of plant cells prevent contraction.
• Vesicles containing cell wall materials form a cell plate in the center of the cell.
• The membrane fuses with the plasma membrane, and its contents integrate with the parental cell wall, forming two daughter cells.

Homologous Chromosomes

• Human somatic cells have 46 chromosomes, arranged in 23 matching pairs during mitosis.
• Homologous chromosomes are matching pairs with genes controlling the same inherited traits.
• May carry different versions of the same gene.

Sex Chromosomes

• Females have two X chromosomes (XX), while males have one X and one Y (XY).
• With small homologous parts, X-linked genes on the Y chromosome are mostly absent.
• Sex chromosomes determine sex.

Autosomes

• The 44 non-sex-determining chromosomes in humans.

Terms to Know

• Homologous chromosomes are paired chromosomes with the same length, centromere position, and staining pattern, carrying genes for the same traits.
• Locus is the site where a gene is found on the chromosome.
• Fertilization is the union of sperm and egg nuclei to produce a zygote.
• Diploid cells contain two homologous sets of chromosomes (2n), one from each parent.
• The total number of chromosomes in diploid organisms is called the diploid number (2n). For humans, 2n = 46.
• A zygote is a diploid fertilized egg cell.
• Sex chromosomes determine the sex of an individual.
• Haploid cells contain a single set of chromosomes (n).
• Autosomes are chromosomes not involved in determining sex.
• Gametes are sex cells, either a haploid egg or sperm; fertilization of two gametes forms a zygote. Gametes have a single set of chromosomes: 22 autosomes plus a sex chromosome (X or Y). For humans, n = 23.

Meiosis I & II

• Meiosis is a cell division process that creates haploid gametes in diploid organisms, which can combine through fertilization to form a diploid zygote.
• The alternation of fertilization and meiosis helps maintain a consistent chromosome number across generations.
• Meiosis involves two consecutive cell divisions (meiosis I and meiosis II) after chromosome duplication, resulting in four daughter cells, each with half the chromosome number of the parent cell.
• Meiosis I segregates homologous chromosomes into separate haploid daughter cells.
• Meiosis II separates sister chromatids, resulting in four haploid daughter cells, each containing a single chromosome from the homologous pair.
• Meiosis II is essentially the same as mitosis, but starts with a haploid cell containing a single set of still-duplicated chromosomes.

Meiosis Stages:

• Interphase: Chromosomes duplicate, and each chromosome consists of two genetically identical sister chromatids as the cell's centrosome also duplicates.
• Prophase I: Homologous chromosomes form pairs called tetrads, and nonsister chromatids exchange segments via crossing over; chromosomes coil tightly, nuclear envelope breaks, and tetrads move toward the center of the cell.
• Metaphase I: Chromosome tetrads align on the metaphase plate as spindle microtubules attach to the kinetochores with homologous chromosomes poised to move toward opposite poles.
• Anaphase I: Homologous chromosomes move toward the poles, but sister chromatids remain attached.
• Telophase I & Cytokinesis: Chromosomes arrive at the poles of the cell with each pole having a haploid chromosome set with chromosomes still in duplicate form; cytokinesis occurs, forming two haploid daughter cells.
• Prophase II: A spindle forms and moves chromosomes toward the middle of the cell.
• Metaphase II: Chromosomes align on the metaphase plate, with kinetochores of sister chromatids pointing toward opposite poles.
• Anaphase II: Centromeres of sister chromatids separate as sister chromatids move toward opposite poles.
• Telophase II & Cytokinesis: Nuclei form at the cell poles and cytokinesis occurs, resulting in four daughter cells, each with a haploid number of single chromosomes.

Crossing Over

• During prophase I of meiosis, non-sister chromatids can exchange segments through crossing over, contributing to genetic variability.
• In prophase I, the four chromatids of each homologous pair align and touch each other for crossing over to occur.

Cell division ensures genetic information is passed to offspring. Asexual reproduction involves a single parent producing identical offspring. Sexual reproduction requires gamete fusion, resulting in genetically diverse offspring.