Lecture Notes 1 (Cell Reproduction and Differentiation) PDF

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GENERAL BIOLOGY 2

Stages of Cell Cycle

QUIZ NO. 1 Coverage Lecture (1/24/20) Cell Reproduction and Differentiation Cell Division is necessary for: ❖ ❖ ❖ ❖ ❖

Growth Healing Cell Repair Cell Replacement Reproduction

Cell Reproduce by Dividing into Two ❖ Unicellular organisms o Cell division = reproduction ❖ Multicellular organism o Cell division = development ▪ Growth from a fertilized egg to a multicellular individual ▪ Renewal and repair in fully grown multicellular organism Types of Cells ❖ Somatic Cells (n  2n): Mitosis ❖ Meiotic Cells (2n  n): Meiosis

The Cell Cycle Creates New Cells Cell cycle includes two major phases: ❖ Interphase “between cell division” ❖ Mitotic Phase (nuclear division and cytokinesis) The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed during division of a parent cell until its own division into two daughter cells. (Our use of the words daughter or sister in relation to cells is the biological convention and is not meant to imply gender.) Passing identical genetic material to cellular offspring is a crucial function of cell division. The cell cycle consist of two distinct periods — the INTERPHASE (or the stagnant phase) and the MITOTIC PHASE (which is the cell division proper)

Go – cells stop growing Interphase ❖ Long growth period between cell divisions ✔ G1 (first gap) ▪ Primary growth phase, very active growth ✔ S (synthesis) ▪ Synthesis of DNA for next cell division ✔ G2 (second gap) ▪ Final growth phase before cell division Mitotic Phase (cell division phase) A. Mitosis ❖ Nuclear division ▪ Duplicated DNA is distributed between two daughter nuclei, nucleus divides. B. Cytokinesis ❖ Cytoplasm divides ❖ Two new daughter cells are formed. Cell Cycle…. ❖ Complete cell cycle takes 18-24 hours ❖ Mitosis and cytokinesis take less than one hour of the complete cell cycle ❖ Many cells enter a non-dividing state, Go, either temporarily or permanently ▪ Osteocytes enter Go after adolescence.

DNA STRUCTURE AND FUNCTIONS ❖ Human DNA: organized into 46 separate chromosomes containing 3 billion base pairs of DNA ❖ Chromosomes consist of DNA and histones (proteins) ❖ Throughout most of the cell cycle, DNA is loose and diffuse, referred to as chromatin. ❖ During mitosis, DNA is compacted into chromosomes that are visible under the microscope, each consisting of two sister chromatids joined at the centromere ❖ Gene: short segment of DNA that contains the code, or recipe for a protein ❖ Twenty thousand genes on the 46 chromosomes.

CELL REPRODUCTION: ONE CELL BECOMES TWO MITOSIS ❖ Maintains the chromosome ❖ Used in growth, development, replacement of damaged/dead cells, or as part of asexual reproduction ❖ Generates new diploid cells ❖ Produces Cells identical to the parent cell ❖ All body cells other than sperm and eggs divide by mitosis and have 46 chromosomes (23 pairs of homologous chromosomes) ➢ 22 pairs are autosomes (chromosomes other than sex chromosomes) ➢ 1 pair of sex chromosomes

➔ Prophase ◆ Mitotic spindle formed ● Centrosomes, microtubules, asters ◆ Centrosomes migrate to cell poles ● Only found in animal cells ◆ Chromatin condenses into visible chromosomes (sister chromatids) ◆ Nucleoli disappear ◆ Metabolic activity decreases ➔ Prometaphase ◆ Nuclear envelope fragments ◆ More condensed chromosomes ◆ Each chromatid now has kinetochores ◆ Kind of Microtubules ● Kinetochore Microtubules ● Non-Kinetochore ● Some microtubules attach to the kinetochores - these are kinetochore microtubules ● Non-kinetechore microtubules interact with those at the opposite pole of the spindle

➔ Metaphase ◆ Duplicate chromosomes form single line at the equator between centriole poles “metaphase plate: ◆ Meta - “after” ◆ Centrosomes are at opposite poles of the cell

➔ Anaphase ◆ Sister chromatids separate ● Daughter chromosomes are pulled toward poles by microtubules ● Cell elongated as nonkinetochore microtubules lengthen. ◆ Shortest stage

➔ Telophase ◆ Reverse of prophasev ◆ Mitotic spindle disintegrates ◆ Nuclei forms ● Nuclear membrane reforms ● Nucleoli reappear ◆ Chromosomes uncoil and revert to chromatin

➔ Cytokinesis ◆ Animal cells ● Contractile ring of filaments forms are midsection of cell and

tightens, forming a cleavage furrow ◆ Result: two identical daughter cells (diploid) ◆ Plant cells: ● Vesicles derived from Golgi apparatus move along microtubules to the middle of the cell forming a cell plate ● Cell wall materials carried in the vesicles collect inside the cell as it grows ● The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell ◆ Result: two identical daughter cells (diploid)

❖ Halves the chromosome number ❖ Used in sexual reproduction, in which two parents contribute genes to offspring ❖ Generates haploid gametes ❖ Gametes are haploid ❖ Reduction in chromosome number from diploid to haploid is accomplished by meiosis, a special cell division process that occurs in ovaries and testes ❖ Meiosis includes two successive cell division processes: Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokinesis) and Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II and Cytokinesis)

Mitosis Produces Cells Identical to the Parent Cell ▪All cells in human body divide by mitosis, with the exception of the cells that form sperm and eggs ▪All body cells other than sperm and eggs have 46 chromosomes (are diploid) –These represent 23 pairs of chromosomes –The chromosomes in each pair are called homologous chromosomes ▪23 pairs of chromosomes –22 pairs are autosomes (chromosomes other than sex chromosomes) –1 pair of sex chromosomes –XX in females, XY in males MEIOSIS Meiosis: Prepares Cells for Sexual Reproduction ▪Gametes (sperm, eggs) are haploid ▪Have just one set of 23 chromosomes ▪Reduction in chromosome number from diploid to haploid is accomplished by meiosis, a special cell division process that occurs in ovaries and testes Meiosis

➔ Prophase I ◆ Duplicated homologous chromosomes (can be collectively called as tetrads) pair up ◆ Before crossing over occurs, the two HOMOLOGOUS PAIRS should find each other and move side by side. Crossing over is possible through synapsis, wherein two chromatids intertwine together.

● Chiasma - point of crossing over, or exchange of chromosome segments between nonsister chromatids; it happens randomly ◆ Crossing over is important because it leads to genetic variation

➔ Metaphase I ◆ Spindle fibers attach to the kinetochore and enable homologous pairs of chromosomes line up ◆ Double line of chromosome pairs ➔ Anaphase I ◆ Pairs of chromosomes separated, but duplicated chromosomes stay intact. ◆ Disjunction occurs here. Disjunction is the separation of the two recombinant chromosome. Either chromosome may end up on either side→ Law of Segregation. ➔ Telophase I ◆ The nuclear envelope starts to develop again. Cleavage furrow starts to happen and cytoplasm starts to divide. ◆ Two haploid daughter cells, but chromosomes are still in duplicated state.

➔ Meiosis II ◆ Each of the two daughter cells from meiosis I goes through meiosis II ● Similar process to mitosis ● Prophase II ◆ Metaphase II ◆ Anaphase II ● Duplicated chromosomes (chromatids) and the sister chromatids are separated from each other just as they have in mitosis. ◆ Telophase II and cytokinesis ● All the genetically different haploid nucleus are settled at the poles. Nuclei have the haploid chromosomes number. However, because of crossing over during meiosis I, none of the four haploid daughter cells are exactly alike. ◆ End of meiosis II: 4 haploid daughter cells

WHAT WOULD HAPPEN IF MITOSIS AND MEIOSIS DID NOT OCCUR?? SEX DIFFERENCES IN MEIOSIS: FOUR SPERM BERSUS ONE EGG Males -

Four sperm produced from each cell entering meiosis All viable, functional

Female

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unequal cytokinesis during meiosis I and II One egg and three polar bodies produced from each cell entering meiosis Only the egg is viable If fertilized, an egg will need a lot of energy to grow and develop. A large egg with plenty of cytoplasm and lots of organelles has a better chance of surviving the early stage of development. In females, therefore, as much of the cytoplasm as possible is reserved for only one daughter cell at each meiotic division. The smaller cell produced at each division is called a polar body. The polar body produced during meiosis I, may or may not divide again during meiosis II, but in any event the two or three polar bodies eventually degenerate. Some references say that it provide nutrients for the egg.

HOW CELL REPRODUCTION IS REGULATION -

Not all cells divide at the same rate

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Cells also have internal surveillance and control mechanism Several key checkpoints with “go ahead” signals - G1, G2, M checkpoints Outside influences - Can modify cell cycle - Hormones, growth factors, presence of other cells

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ENVIRONMENTAL FACTORS INFLUENCE CELL DIFFERENTIATION

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● All body cells have the same DNA, yet there are great differences between the shape and function of different cell types ● Differentiation ○ Process by which a cell becomes different from its parent or sister cell ○ Differentiation is based on different gene expression

EARLY EMBRYONIC STAGE

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It contains an Inner cell mast - small clump of cell where embryo develops - Human Chorionic Gonadotropin (HCG) - hormone produced by placenta and indicator of pregnancy in the pregnancy test 4. Gastrulation - process of early development that produces the three germ layers ○ Germ Layers - three primary tissues that form as an early embryo develops

1. Gamete formation 2. Fertilization 3. Cleavage - cell division converts the zygote into a ball of cells ○ Morula - sixteen-celled zygote; mulberry; uterus ○ Blastula - embryo having the form of a hollow fluid-filled; blastula in mammals is specifically called blastocyst ○ Blastomere - each new cell that forms during cleavage Implantation Blastocyst -

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Embryonic stage that develops from morula and will eventually implant in the uterine wall; once implanted, placenta secretes HCG Happens when the Morula enters the uterus - Trophoblast - outer layer or surface epithelium of the blastocyst consisting of layers of cells

○ Ectoderm ■ Nervous system and sense organs ■ Pituitary gland ■ Epidermis and associated structures ○ Mesoderm ■ Cartilage, bones, muscles ■ Cardiovascular, lymphatic, urinary, reproductive ■ Outer layers of digestive system ○ Endoderm ■ Lining of digestive tube ■ Lining of respiratory airways 5. Organ Formation 6. Growth tissue specialization ■ Cell Differentiation Newly formed cells become specialized for a certain function ■ Morphogenesis - The beginning of form; Specific organs and tissue form

EARLY HUMAN DEVELOPMENT

○ Egg is fertilized in vitro, and allowed to divide to the eight cell stage ○ Cells of eight-cell stage are carefully separated and each is implanted into a different surrogate mother in which it develops ● Results, clones are genetically identical to each other but not to either parent ● process has not been attempted with humans DIFFERENTIATION LATER IN DEVELOPMENT ● External substances harmful to fetuses: ○ Cigarette smoke: retards growth ○ Alcohol: fetal alcohol syndrome ○ Medications: pass through placenta ○ Illegal drugs: child born addicted ○ Environmental chemicals: in air, water, soil ○ Radiation: radon, x-rays ○ Intrauterine infections: HIV, syphilis, rubella REPRODUCTIVE CLONING REQUIRES AN UNDIFFERENTIATED CELL ● Reproductive cloning ○ Producing a copy of an entire organism ○ Requires a completely undifferentiated cells as the starting point ○ Two methods ■ Embryo splitting ■ Somatic cell nuclear transfer EMBRYO SPLITTING: PRODUCING IDENTICAL OFF-SPRING

● Procedure

SOMATIC CELL NUCLEAR TRANSFER PRODUCES CLONE OF AN ADULT

THERAPEUTIC CLONING: Creating Tissues and Organs ● Therapeutic cloning ○ The cloning of human cells specifically for treating patients ○ Ideally, remove a single cell from a patient and nurture it to develop and differentiate into the cell type needed to treat the disease ○ Potential for creating new cells, tissues, or organ as yet unrealized --Edited and updated by Ms. Lorraine Del Rosario

● Somatic cell: any cell other than a gamete. Racb somatic cells has a full diploid set of chromosomes ● Procedure ○ Somatic cell from the adult organism to be cloned is inserted into an enucleated fertilized egg ○ An electrical current is used to fuse the cells ○ Fused cell is implanted into uterus of surrogate mother and allowed to develop ● Result ○ Offspring is a clone of the adult organism that provided the somatic cell....