Eukaryotic Cell Biology

• All organisms consist of one or more cells.
• Cells are categorized as either prokaryotic or eukaryotic.
• Prokaryotic cells are found in bacteria and cyanobacteria.
• Eukaryotic cells are found in plants, animals, fungi, and protists.
• In prokaryotic cells chromosomes are not contained within a membrane-bound nucleus.
• The chromosomes reside in the nucleoid region of the cytoplasm.
• In eukaryotic cells the nucleus is enclosed by a nuclear membrane.
• The nuclear membrane separates nuclear from cytoplasmic material.
• Membrane-bound organelles like the endoplasmic reticulum, mitochondria, Golgi apparatus, chloroplasts, and lysosomes are present in eukaryotic cells.
• All cells are composed of a plasma membrane, cytoplasm, DNA, and ribosomes.

Parts of Eukaryotic Cell

• The plasma membrane is a semi-permeable phospholipid bilayer that regulates molecular traffic into and out of a cell

Nucleus

• The nucleus is the largest, most conspicuous organelle in eukaryotic cells.
• It contains most of the cell's genetic material.
• It controls cellular activities.

Nuclear Membrane

• The nuclear membrane is a double membrane enclosing the nucleus.
• It separates the cytoplasm from the nucleoplasm (the contents of the nucleus).

Nuclear Pores

• Nuclear pores are small openings in the nuclear membrane.
• They facilitate the exchange of materials between the nucleus and cytoplasm.

Nucleolus

• The nucleolus consists of spherical bodies that make ribosomes.

Chromatin

• Composed of DNA and histone proteins, chromatin makes up the chromosomes.

Chromosome

• The chromosome is a threadlike structure containing the genetic information of an organism.

Cytoplasm

• The cytoplasm is the region surrounding the nucleus.
• Various organelles are present in the cytoplasm, and their presence/size vary between different cells/tissues.

Mitochondrion

• A sausage or rod-shaped organelle enclosed by a double membrane.
• The inner membrane consists of infoldings known as cristae.
• It generates energy through aerobic respiration, and is often called the "powerhouse" of the cell.

Golgi apparatus

• Consists of a stack of flat, membranous sacs.
• It is involved in packaging, modification, and transport of molecules.

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is a network of tubules and flattened sacs, and is classified as smooth (SER) or rough (RER).
• SER lacks ribosomes and is involved in lipid/carbohydrate metabolism, and detoxification.
• RER has ribosomes attached and is active in protein synthesis.

Lysosome

• The lysosome is a membrane-bound organelle containing digestive enzymes.
• It digests macromolecules.

Ribosomes

• Ribosomes are small particles that are the main site of protein synthesis
• They are either free-floating or attached to the ER.
• Ribosomes are made of two subunits that differ in sedimentation coefficient.
• In bacteria, the subunits are 30S and 50S.
• Eukaryotic cells have subunits of 60S and 40S.

Centriole

• The centriole is a cytoplasmic organelle involved in forming spindle fibers during cell division.

Chloroplast

• The chloroplast is a two-layered membrane-enclosed organelle.
• It contains chlorophyll.
• It is the site of photosynthesis.

Cytoskeleton

• The cytoskeleton is a network of tubules and filaments that gives shape, strength, and anchors organelles.
• It provides mechanical support allowing for cell movement and division.
• It includes microtubules, intermediate filaments, and microfilaments.

Chromosome Structure

• Chromosomes contain the genetic material of organisms.
• Chromosome number varies by species.
• Humans have 46 chromosomes.
• Dogs have 48 chromosomes.
• Houseflies have 12 chromosomes.
• Mice have 40 chromosomes.
• Fruit flies have 8 chromosomes.
• Rice has 24 chromosomes.
• Corn has 20 chromosomes.
• Cats have 38 chromosomes.

Parts of Chromosome

• Mitotic chromosomes are made of structures like centromeres, kinetochores, secondary constrictions, satellites, and telomeres.
• The Centromere (primary constriction) is a defined region of a chromosome.
• the Kinetochore is a protein associated with the centromere that serves as spindle attachment. Secondary constriction is a constricted region found at any point other than the centromere.
• The Satellite is a bulge on the telomeric end containing repetitive, heterochromatic DNA sequences.
• Telomeres are repetitive nucleotide sequences at each chromatid end.
• Telomeres protect the chromosome from deterioration/fusion.

Classification of Chromosomes

• Chromosomes can be classified based on size and centromere location.
• Centromere location can be at the terminal end (telocentric), near the end (acrocentric), near the center (submetacentric), or at the center (metacentric).
• Chromosomes are classified as somatic chromosomes (autosomes) or sex chromosomes.
• Autosomes control somatic characteristics.
• Sex chromosomes determine sexuality.
• Humans have 22 pairs of autosomes and 2 sex chromosomes.
• XX chromosomes = female.
• XY chromosomes = male.

Cell Cycle

• The cell cycle is the series of events for cell growth and division.
• It includes interphase (G1, S, G2 phases) and M phase.
• Interphase is the time between cell divisions when the cell grows, develops, and functions.
• M phase includes karyokinesis (nuclear division/mitosis) and cytokinesis (cytoplasmic division).
• The cell cycle is important for passing genetic material.

Mitosis

• Mitosis produces two genetically identical cells.
• Mitosis includes interphase and M phase, and is a type of cell reproduction
• During interphase, the cell is metabolically active and prepares for division.
• Chromosomes are not clearly visible in the nucleus during interphase.
  • During the G1 phase (first gap), the nucleus and cytoplasm enlarge.
  • Cytoplasmic organelles such as ER, Golgi apparatus, and mitochondria are formed.
  • During the S phase (synthesis), DNA and histone are synthesized and chromosomes are duplicated.
  • The amount of DNA doubles
  • During the G2 phase (second gap), biochemical events necessary for cell division occur, and RNA and protein are synthesized.
• The M phase is active cell division: mitosis (nuclear division) and cytokinesis (cytoplasmic division).
  • During prophase, chromosomes condense and become visible, centrosomes move apart, microtubules form fibers, and the nucleoli and nuclear envelope disintegrate.
  • During metaphase, chromosomes align at the equatorial plane and centrioles are at opposite poles of the cell.
  • During anaphase, cohesin protein breaks down, the centromere divides, and sister chromatids separate and move to opposite poles.
  • During telophase, chromosomes are at opposite poles, the nuclear envelope is formed, nucleoli reappear, and spindle microtubules disintegrate.
  • Cytokinesis follows immediately.
• Cytokinesis is the division/partitioning of the cytoplasm.
• In animal cells, a cleavage furrow forms, and partitioning starts from the side to the center.
• In plant cells, a cell plate forms, and division starts from the center to the sides.
• Genetic consequences of mitosis and cytokinesis include producing two identical daughter cells with the same number of chromosomes as the parent cell.

Meiosis

• During maturity, sexually reproducing organisms form gametes in gonads (testes/ovaries) in animals and stamens/pistils in plants.
• Chromosome transmission from parents to offspring is accomplished by fertilization of haploid (n) gametes to form a diploid (2n) zygote.
• Meiosis reduces the chromosome number prior to fertilization.
• Meiosis occurs during gametogenesis (spermatogenesis in males, oogenesis in females) and sporogenesis in higher plants.
• Microsporogenesis is the formation of male gametophytes, while megasporogenesis is the formation of female gametophytes.
• Meiosis is preceded by G1, S, and G2 phases of the cell cycle.
• It includes two successive nuclear division cycles where the first division separates homologous chromosomes resulting in haploid nuclei, and the 2nd division results in four haploid nuclei.
• Meiosis I reduces the chromosome number from diploid (2n) to haploid (n).
  • It includes Prophase I, Metaphase I, Anaphase I, and Telophase I.
    • During prophase I, cells increase in size, contain two duplicates coiled together, and is the longest/most complex phase.
      • In leptotene, chromosomes begin to condense and appear as long/thin threads, and beadlike chromomeres are observed along the chromosomes.
      • In zygotene, homologous chromosomes pair (synapsis), synapsed chromosomes form a bivalent of four chromatids, and the synaptonemal complex is formed.
      • During pachytene, crossing over/exchange chromosome segment occurs between non-sister chromatids of homologous chromosomes, and the point of contact is called chiasma (source of genetic variation).
      • During diplotene, chromatids shorten/thicken, the synaptonemal complex disintegrates, and homologous chromosomes separate (except at the chiasma).
      • During diakinesis, chromosomes become shorter/thicker (condensation), the nuclear envelope/nucleolus disappears, and centrioles move to opposite poles and spindle fibers form.
    • During metaphase I, bivalents align at the middle of the cell (equatorial plane).
    • During anaphase I, each univalent separates and moves toward opposite sides of the cell.
    • During telophase I, two haploid nuclei are formed, the nucleolus/nuclear membrane reappear, chromosomes uncoil/lengthen, and cytokinesis may follow.
• Meiosis II is similar to mitosis except that it involves a haploid chromosome number.
  • Prophase II similar to mitotic prophase; chromosomes contract, the nucleus/nuclear membrane disappears.
  • Metaphase II similar to mitotic metaphase; chromosomes are at the middle of the cell.
  • Anaphase II similar to mitotic anaphase; sister chromatids move toward opposite sides.
  • Telophase II similar to mitotic telophase; chromosomes uncoil/lengthen, nucleolus/nuclear membrane reappear, four haploid are formed, and cytokinesis follows.
• Meiosis reduces chromosome number from diploid to haploid.
• This ensures the chromosome number of the species is maintained.
• Crossing over generates genetic differences.

Life Cycle of Flowering Plants

• This life cycle consists of sporophyte and gametophyte stages.
  • The diploid sporophyte produces haploid spores by meiosis in sporangia.
  • These grow into haploid gametophytes.
  • The haploid gametophyte produces haploid gametes (sperm/egg) through mitosis.
  • The gametes fuse to form a zygote which develops into a multicellular sporophyte.
    • Microsporogenesis develops male gametophytes (pollen grains).
      • The Pollen grain formation occurs in microsporangia (pollen sacs) of anthers at the stamen tips.
      • Microsporangia contain diploid microsporocytes (microspore mother cells). -Each microsporocyte undergoes meiosis producing four haploid microspores.
      • Haploid microspores divide by mitosis to form a generative cell and a tube cell.
      • Tube cell gives rise to a pollen tube that burrows through the style after pollination.
      • The generative cell divides mitotically to form two sperm nuclei as the pollen tube grows.
      • Megasporogenesis forms the female gametophyte or embryo sac inside the ovule.
      • Megasporocytes (megaspore mother cell) are located inside the ovule.
      • The megasporocyte divides by meiosis into four haploid megaspores.
    • In most species only one survives, while three disintegrate, therefore the Nucleus of the surviving megaspore divides mitotically three times.
    • This forms one large cell with 8 haploid nuclei: 3 antipodal nuclei, 2 polar nuclei, 1 egg nucleus, and 2 synergids.
  • During pollination, generative cells of pollen grain divide mitotically to form 2 sperm (male mature gametophyte).
  • The pollen tube enters embryo sac through micropyle discharging its two sperm nuclei.
  • One sperm nucleus unites with egg nucleus to form zygote.
  • The second sperm nucleus fuses with polar nuclei to form endosperm.

Animal Life Cycle

• Gametogenesis creates mature reproductive cells (gametes).
• Spermatogenesis is the formation of male gametes (spermatozoa).
  • Oogenesis is the production of female gametes (ova).
  • Spermatogenesis occurs in the seminiferous tubules of the testis.
  • Primordial germ cells differentiate into spermatogonia.
  • Diploid spermatogonium grows and develops to primary spermatocytes.
  • Undergoes the first meiotic division to produce two haploid secondary spermatocytes.
  • Undergoes the second meiotic division to form two haploid spermatids, thus differentiating into functional spermatozoon.
  • Oogenesis takes place in the ovary; and the oogonia are derived from the primordial germ cell.
  • Diploid oogonia develop into primary oocytes.
  • Enter the first meiotic division.
• Unlike spermatogenesis, each primary oocyte divides unequally; producing a larger secondary oocyte and smaller first polar body. The secondary oocyte completes the second meiotic division; produces an ootid and secondary polar body.
• The secondary polar body disintegrates.
• the ootid differentiates into an ovum. Only one functional ovum forms. During fertilization the ovum can be fertilized by spermatozoa carrying either X chromosome or Y chromosome, thus producing a female or male individual.