CYTO L4

OVERVIEW: CELLULAR REPRODUCTION

2 primary types of cells -

  1. Prokaryotic cells

    • No true nucleus,
    • DNA is coiled up in a region called the nucleoid
    • DNA is found in the cytoplasm and not enclosed within the nuclear membrane Found in single celled organisms Ribosomes – where proteins are made are the only organelle in the prokaryotic cells
    • Eg. Bacteria
  2. Eukaryotic cells

    • They have a true nucleus found in the membrane and separated from other cellular structure
    • Don’t have define organelles aside from the ribosomes
    • Larger than prokaryotic cells Found in multi cellular organisms Contain other organelles aside from the nucleus
    • Eg. Cells in plants, animal, fungus
  3. ORGANELLE

    • is a structure within the cytoplasm that performs specific a specific job in the cell

      • Mitochondria – provides energy to the cell
      • Vacuoles – store substances in the cells
    • Organelles allow eukaryotic cells to carry out more functions compared to your prokaryotic cells Allowed eukaryotic cells to have greater cell specificity than prokaryotic cells

  4. PROKARYOTIC CELL REPRODUCTION

    • Reproduce by a process called binary fission
    • The single-celled DNA molecule replicates and the original cell is divided into 2 identical daughter cells
    • The DNA in such cells is contained in a single circular chromosome called plasmids within the cytoplasm
    • The reproductive process starts with the replication of the chromosome
      • New chromosome attach itself to the plasma membrane and the 2 chromosomes migrate to the opposite ends of the cell
      • Plasma membrane grows inward until it closes to separate the cell into 2 compartments; each with the full complement of genetic material
    • The cell then fissions at the center, forming two new daughter cells
    • Binary fission is somehow like mitosis
      • same start and end but different process in between
  5. EUKARYOTIC CELL REPRODUCTION

  6. Eukaryotes grow and produce called mitosis Life of eukaryotic cells is characterized by a cell cycle with two major phases:

    • Interphase
    • Cell division
    • Also reproduce asexually through budding, regeneration and parthenogenesis
    • Eukaryotic cell have more DNA thus the process is more complicated compared to the prokaryotic cell reproduction
  7. Interphase

    • Cell takes in nutrients, grows and duplicates its chromosomes
  8. Cell division phase

    • The nucleus divides in a process called mitosis and then the divided nuclei are established in separate cells in a process called cytokinesis
  9. 2 DIFFERENT TYPES OF CELLULAR REPRODUCTION

    • Mitosis A
      • process that creates a nearly exact copy of the original cell.
      • Somatic cells which include nearly all human cells are created by this process
      • Division of somatic cells
      • Process by which cells reproduce themselves creating 2 daughter cells, genetically identical to one another Examples: when wound heals, growth of nails, growth of hair
    • Meiosis
      • Different form of reproduction that leads to the production of germ cells or sex cells Special type of division that occur only in gametic cells

    These processes are responsible for creating two different types of cells

    1. MITOSIS
    • A eukaryotic cell nucleus splits in 2, followed by a division of the parent cell into 2 daughter cells
    • Mitosis means thread, refers to the threadlike appearance of chromosomes as the cell prepares to divide

Mitosis is divided into 5 phases

  1. Interphase – cell grows and makes a copy of its DNA
  2. Prophase – condensed down into chromosomes (does not happen in prokaryotic cells)
  3. Metaphase – chromosomes a line in the middle 
  4. Anaphase – pulled apart 
  5. Telophase – nuclear membrane starts to reform around the 2 nuclei, appearance of the cleavage furrow is seen 
  • Cytokinesis then follows, the furrow pinched together now the 2 new cells are formed 
BINARY FISSION vs MITOSIS
Binary FissionMitosis 
DNA is uncondensed Chromosomes
DNA moves to polesSpindles: generated by centrioles to pull apart the chromosomes 
Not nearly as organized 
Similarities Both DNA copied and cell divides 

INTERPHASE 

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  • DNA in the cell is copied in preparation for cell division

  • Results in 2 identical false sets of chromosomes

  • Outside of the nucleus are 2 centrosomes, these structures are critical for the process of cell division 

  • During interphase, microtubules extend from these centrosomes

G1 Phase

  • Also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in the later steps

  • Longest and lasts up to 9 hours

S phase 

  • Cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during mitosis phase

  • Lasts for 5 hours in mammalian cells 

  • Some DNA replicate early others later 

G2 phase

  • Cell grows more, makes proteins and organelles and begins to reorganize its contents in preparation for mitosis. G2 phase ends when mitosis begins.

  • Lasts for 3 hours

G1, S, and G2 phases are known as interphase 

  • inter means between, which means interphase begins between one mitotic phase and the next

  • Average mammalian cell cycle lasts about 17-18 hours and is the transition of a cell from one interphase through cell division and back to interphase 

  • Cell cycle is divided into 4 major stages

  • First 3 stages: G1, synthesis and G2 phase

  • 4th stage: mitotic phase – very important phase  

  • Cells are metabolically active during G1 phase and when protein synthesis takes place

  • A cell might be permanently arrested in this stage known as gap zero phase, if it does not undergo further division 

  • Final step in the cell cycle is mitosis which lasts for 1-2 hours in mammalian cells 

PROPHASE

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  • Chromosomes condense into x-shaped structures that can be easily seen under a microscope

  • Each chromosome is composed of 2 sister chromatids containing identical genetic information

  • The chromosomes pair up so that both copies of chromosome 1 are together so are chromosome 2 

  • At the end of prophase the membrane around the nucleus in the cell dissolves away releasing the chromosomes

  • The mitotic spindle(contains microtubules and other proteins), extends across the cell between the centrioles as they move to opposite poles of the cell

  • Chromosomes are at their greatest elongation and are not visible as discreet structures under the light microscope during interphase

  • But during prophase chromosomes begin to coil thus becomes more condensed and becomes visible as discreet structures

  • Nuclei are visible early during prophase but disappear as the stage progresses 

PROMETAPHASE

  • A short period between prophase and metaphase during which the nuclear membrane disappears and the spindle fibers begin to appear
  • Chromosomes attach to the spindle fibers at their kinetochores

METAPHASE

  • The mitotic spindle is completed

  • Chromosomes line up neatly end to end along the centre(equator) of the cell

  • The centrioles are now at opposite poles of the cell with the mitotic spindle fibers extending from them

  • Chromosomes reach their maximum state of contraction during this phase 

  • It is metaphase chromosomes that are traditionally studied in cytogenetics

    ANAPHASE

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    • Centromere is divided longitudinally 
    • Sister chromatids are then pulled apart by the mitotic spindle which pulls one chromatic to one pole and the other chromatids to the opposite pole

    TELOPHASE

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    • Chromosomes uncoil and become indistinguishable again

    • Nuclei reform and the nuclear membrane is reconstructed

    • At each pole of the cell a full set of chromosomes gather together

    • A membrane forms around each set of chromosomes to create 2 new nuclei 

    • The single cell the pinches in the middle to form 2 separate daughter cells each containing a full set of chromosomes within the nucleus 

    • This process is known as cytokinesis 

    • Usually followed by cytokinesis or cytoplasmic division 

    The products of mitosis are 2 genetically identical daughter cells, each of which contains the complete set of genetic material that was present in the parent cell

     CYTOKINESIS

    • The cytoplasm of the cell is split in 2 making 2 new cells
    • Begins just as mitosis is ending with a little overlap
    • Takes place differently in animal and plant cell

    MEIOSIS

    • Production of gametes- sex cells/sperm and eggs

    • Takes place only in the ovaries and testes, a process involving one duplication of the DNA and 2 cell division namely meiosis 1 (reductional division) and 2 (equational division) which reduces the number of chromosomes from the diploid number: 46 (one with 2 sets of chromosomes) to haploid: 23 (one with a single chromosome)

    • Its goal is to make daughter cells with exactly half as many chromosomes as the starting cell

    • Each gamete produce contains only 1 copy of each chromosome

    • In humans the haploid cells made in meiosis are sperm and egg, when they join in fertilization it restores the diploid number in the zygote

    • It takes place in the gonads of animal cellsMeiosis in humans is a division process that takes a diploid cell- to haploid cells

    • in fertilization the 2-haploid seta of chromosomes form a complete diploid set; a new genome

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    PHASE OF MEIOSIS

    • Meiosis is a lot like mitosis

    • It still needs to separate sister chromatids (the 2 halves of a duplicated chromosomes) as in mitosis

    • But it must also separate homologous chromosomes, the similar but non identical chromosomes pair an organism receives from its 2 parents

    • In each round of division cells undergo 4 stages prophase, metaphase, anaphase and telophase 

    • Homologues pairs separate during a first round of cell division called meiosis I

    • Sister chromatids separate during a second round, called meiosis II

    • Since cell division occurs twice during meiosis, one starting cell can produce 4 gametes

    PROPHASE I

    • before entering meiosis I a cell must first go through interphase as in mitosis the cell grows in G1 phase copies all its chromosomes during s phase and prepares for division during the G2 phase

    • During prophase I in meiosis I they also pair up. Each chromosome carefully aligns with its homologue partner (also called bivalent chromosomes) so that the 2 match up at corresponding positions along their full length 

    • During synapsis there is an exchange of genetic material between homologous chromosomes

    Crossing over

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    • Letters A,B and C represent genes found at particular spots on the chromosomes

    • DNA is broken at the same spot on each homologue between genes B and C, and reconnected in a crisscross pattern so that the homologue exchange part of the DNA 

    • At the end of prophase I instead of having 2 capital letters AA and 2 small letters aa they will now carry a capital A and a small a allele in both chromosomes

    • Its helped along by a protein structure called the synaptonemal complex that holds the homologues together

    • The point where cross-overs occurs is called the chiasma(plural: chiasmata)

    • Bivalents have 2 chromosomes and 4 chromatids 

    Prophase I is further specified by subdividing into 5 recognizable stages

    • Leptonema: Thin thread stage. The chromosomes are long and slender with many bead like structure called chromomere 

    • in leptotene there are 46 chromosomes each comprised of 2 chromatids. The chromosomes begin to condense but are not yet visible by light microscopy

    • once leptotene takes place the cell is commited to meiosis  

    • Zygonema: stage where homologous chromosomes pair to form bivalent or tetrad. The pairing is called synapsis. Synaptonemal complex id formed

    • Appear as long threadlike structure pair locust for locust 

    • Can be seen with electron microscopy

    • The zygonema complex is necessary for the phenomenon of crossing over that will take place in prophase I

    • Pachynema: stage where most chromosomes appear as thick threads. The bivalents due to coiling and chromatid break occur

    • Synapses is complete, chromosomes continue to condense now appear as thicker threads

    • The paired homologues from structures called bivalents sometimes tetrads because they are composed of 4 chromatids

    • Crossing over takes place during pachytene segments of DNA  are exchanged between non sister chromatids of the bivalence 

    • The result of a crossover is a reshuffling of genetic material between homologues creating new combination of genes in the daughter cells 

    • Diplonema: longitudinal separation of bivalents

    • In diplotene chromosomes continue to shorten and thicken and the homologues chromosomes begin to repel each other, this repulsion continues until the homologous chromosomes are held together only in points of crossing over took place

    • These point are referred to as chiasmata, in males the sex vesicle disappears and the xy chromosomes associate end to end

    • Diakinesis: there is maximal contraction of bivalents showing a unique configuration due to repulsion of bivalents  

    • Bivalents are distributed throughout the nucleus 

    METAPHASE I

    • After crossing over the spindle begins to capture chromosomes and move them in the center of the cell/metaphase plate 

    • Homologue pairs – not individual chromosomes, line up at the metaphase plate for separation

    • Each chromosome attaches to microtubules from just one pole of the spindle, and the two homologues of a pair bind to microtubules from opposite poles

    • In an organism with 2 sets of chromosomes, there are 4 ways in which the chromosomes can be arranged resulting in differences in chromosomal distribution in daughter cells after meiosis I 

    • 50/50 chance that the daughter cells get either the mother’s or the father’s homologue

    • Characterized by the disappearance of the nuclear membrane and the formation of the meiotic spindle 

    ANAPHASE I

    • Homologues chromosomes separate
    • The centromeres of each bivalent separate and migrate to opposite poles unlike mitosis, the centromeres do not split and the sister chromatids remain paired in anaphase I

    TELOPHASE I AND CYTOKINES

    • In telophase the 2 haploid sets of chromosomes reach opposite poles and the cytoplasm divides 

    • A nuclear membrane forms around each set of new chromosomes than results in 2 cells containing 23 chromosomes each comprised of 2 chromatids

    • The homologues of bivalent arrive at opposite poles of the cell

    • A new nuclear membrane forms around each set of chromosomes

    • Cytokines then divides the cell into 2 daughter cells

    • Each of the 2 daughter cells is now haploid

    RECAP 

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    MEIOSIS II

    • Cells move from meiosis I to meiosis II without copying their DNA 

    • Meiosis II is a shorter and simpler process compared to Meiosis I 

    • Resembles mitosis more than meiosis I

    • Meiosis 2 begins with 2 haploid (n=2) cells and end with 4 haploid (n=2) cells 

    • The four meiocytes are genetically different than one another

    PROPHASE II

    • Spindle fibers reform and attach to centromeres

    METAPHASE II

    • Chromosomes align on the metaphase plate in preparation for centromeres to divide in the next phase

    ANAPHASE II

  • Chromosomes divide at the centromeres and the resulting chromosomes each with one chromatid move towards opposite poles of the cell 

TELOPHASE II AND CYTOKINES 

  • Nuclear membrane form around each set of chromosomes. Four haploid nuclei are formed in telophase II. Division of the cytoplasm during cytokines results in four haploid cells 

  • These 4 cells are not identical as random arrangement of bivalence and crossing over in meiosis I leads to different genetic composition of these cells 

  • In humans’ meiosis produces genetically different haploid daughter cells each with 23 chromosomes that consists of one chromatid 

  • These haploid cells become unfertilized eggs in females and sperms in males

  • The genetic differences ensure that siblings of the same parents are never entirely genetically identical 

RECAP

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HOW MEIOSIS MIXES AND MATCHES GENES

Crossing over: the points where homologues cross over and exchange of genetic material are chosen more or less at random, and they will be different in each cell that goes through meiosis. If meiosis happens many time as in humans, crossovers will happen at different points.

Random orientation of homologue pairs: the random orientation of homologues pairs in metaphase I allows for the production of gametes with many different assortments of homologous chromosomes

COMPARISON:

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Mitosis Body cellsDiploid – ends with 46 chromosomes with 2 identical diploid cells
Meiosis Sex cellsDiploid cell- ends with 4 non identified haploid cell

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SPERMATOGENESIS VS OOGENESIS 

  • are the processes of formation of male and female gametes

spermatogenesis: leads to the formation of sperms 

  • is the process of formation of haploid sperms from a diploid stem cell known as spermatogonium. The process occurs inside the seminiferous tubules in the testis. The entire process takes about 70 days
  • spermatogenesis converts the spermatocyte into 4 spermatids 

oogenesis: helps in the formation of ova 

  • the fertilization of the sperm and ova lead to the formation of a zygote which further develops into an embryo

  • process of formation of ovum. The process occurs in the ovaries of the female. One oogonium produces a single ovum

  • during oogenesis asymmetric cell division reduces one large cell and three small ones that degenerate into 3 polar bodies 

GAMETOGENESIS

  • Spermatogenesis takes place in the seminiferous tubules of the male testis; the process is continuous and each meiotic cycle of a primary spermatocyte results in the formation of 4 non identical spermatozoa 
  • Begins with sexual maturity and occurs throughout the post pubertal life of a man 
  • The spermatogonia contains 46 chromosomes- through mitotic cell division they give rise to primary spermatocyte; which enter meiosis I and give rise to the secondary spermatocyte which now contain 23 chromosomes, each consisting of 2 chromatids 
  • The secondary spermatocyte undergoes meiosis II and give rise to the spermatids, which contain 23 chromosomes each consisting of a single chromatid  
  • The spermatids differentiate to become spermatozoa or the mature sperm

OOGENESIS 

  • Begin in prenatal life, the ova develop from oogonia within the follicles in the ovarian cortex
  • About the 3rd month of fetal development the oogonia through mitotic cell division begin to develop into diploid primary oocyte 
  • Meiosis I continues to diplotene where it is arrested until some time of the post pubertal reproductive life of a woman  
  • This suspended diplotene is referred to as dictyotene 
  • Subsequent to puberty several follicles begin to mature with each menstrual cycle 
  • Meiosis I rapidly proceeds with an uneven distribution of cytoplasm and cytokinesis of meiosis I
  • Resulting in the secondary oocyte containing most of the cytoplasm and a first polar body
  • The secondary oocyte has been ovulated which begins in meiosis II
  • Meiosis II continues only if fertilization takes place 
  • The completion of meiosis II results in a haploid ovum and a second polar body 
  • The first polar body might undergo myosis II or it might be generate so only one of the potential for gametes produced each menstrual cycle is theoretically viable 

FERTILIZATION

  • The chromosomes of the egg and sperm produced in meiosis II are each surrounded by a nuclear membrane within the cytoplasm of the ovum and are referred to as pronuclei

  • The male and female pronuclei fuse to form the diploid nucleus of the zygote, and the first mitotic division begins

  • Spermatogenesis is a continuous process

  • Puberty to old age 

  • Oogenesis is a discontinuous process 

  • Because the early stages take place in the fetus and the rest is in the later stages of life 

  • Prior to birth stops and reactivated in the puberty of females then stops again and is reactivated during fertilization