theme: reproduction on a cellular level lecturer: prof. fedonyuk l.ya
TRANSCRIPT
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Theme: Reproduction on a cellular level
Lecturer: prof. Fedonyuk L.Ya.
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Plan of lecture:
• Structure of nucleus• The levels of organization of eukariotic
chromosomes.• Сhromosomes types.• Normal human karyotype
characteristics.• Cell (mitotic) cycle, its stages. Mitosis,
its stages. • Cytological and cytogenetically
characteristics of meiosis.
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Nucleus
consists of nuclear envelope nucleolus nucleoplasm chromatin
(chromosomes)
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Nuclear envelope• surrounds the nuclear
material• consists of two parallel
membranes separated from each other by a narrow perinuclear cisternae
• is perforated at intervals by openings called nuclear pores
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Nuclear Membrane or Envelope - two membranes which formthe nucleus, is porous. Allows RNA to leave nucleus.
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Assembly and Disassembly of Nuclear Envelope
•Nuclear envelope is a cell cycle dependent structure that disperses at the onset of mitosis (late prophase) and reassembles around the reforming nucleus in the late telophase.
•The correlations of breakdown of the nuclear envelope, formation of chromosomes and mitosis are essential for cell division and the ability of cells to divide in an orderly manner.
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Nucleoplasm
• is the portion of the protoplasm that is surrounded by the nuclear envelope
• is consists of a matrix and various types of particles.
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Whole Mount Electron Microscopy Demonstrating Fibrogranular Structure of the Internal Nuclear
Matrix
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Nucleolus
• is a well-defined nuclear inclusion (sometime more than one)
• is present in the cells that are actively synthesizing proteins
• become detectably only when the cell is in interphase
• is involved in the synthesis of rRNA and its assembly into precursors of ribosomes
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Chromatin• is double-stranded DNA
complexed with histones and acidic proteins
• is responsible for RNA – synthesis, resides within the nucleus in two forms: heterochromatin and euchromatin
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Heterochromatin• is a highly condensed portion
of chromatin• is visible in light microscope• appears in the light
microscope as basophilic clumps of nucleoprotein
• is not transcribed into RNA
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Euchromatin• isn’t condensed portion of
chromatin during interphase• from which RNA is
transcribed, its genes can be activated, is transcriptionally active, mostly encodes proteins
• does not visible in light microscope.
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Levels of organization of eukaryotic chromosomes:
1. The DNA is associated with basic proteins called histones to form nucleosomes, each of which consists of 8 histones bead with DNA wrapped around it.
2. The nucleosomes are organized into large coiled loops held together by nonhistone scaffolding proteins.
3. The chromonema is a single double-stranded DNA molecule with a protein coat
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Chromosome = DNA (deoxyribonucleic acid) + associatedproteins (mainly histones) = “packaged” DNA
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Organization of eukaryotic chromosomes
• The chromosomes have already doubled, and each now consists of two identical sister chromatid
• The chromatid is composed of a very fine filament, called as chromonema
• The two chromatids remain attached to each other at a point of primary constriction, the centromere.
• The centromere is a specific DNA sequence of about 220 nucleotides, to which is bound a disk of protein called a kinetochore.
• It is a place, where the spindle fibers attach during cell division.
• Regions on either sides of centromere are called arms.
• The long arm of a chromosome is designated “q” and the short arm – “p”.
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Metaphase chromosome structure
1 - long arm 2 - short arm 3 - centromere 4 - secondary constriction 5 - satellite 6 - chromatids
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Types of chromosomes
1 – Metacentric (the centromere divides it into two equal arms) 2 – Submetacentric (the centromere is slightly displace from the
center of chromosome)3, 4 – Acrocentric (the centromere establishes one long arm and
one short arm) а – centromereб - secondary constriction
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Karyotype • is a diploid number of chromosomes • is represented in humans by the 22
pairs of autosomes and the 1 pair of sex chromosomes (either XX or XY) totaling 46 chromosomes
• Pair of chromosomes, with the same gene loci in the same order, are known as the homologous chromosomes.
• The chromosomes of each pair have characteristic size and shape.
• An ideogram is a karyotype, which displays chromosomes arranged in pairs in descending size order.
2n=44a+XX2n=44a+XX ((femalefemale karyotypekaryotype))2n=44a+XY2n=44a+XY ((malemale karyotype)karyotype)
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• Two important characters of living organisms, Growth and Reproduction are due to Cell division.
• Cell division involves two phases – 1. Division of nucleus.– 2. Division of cytoplasm.
• Two major types of cell division– 1. Mitosis - similar daughter cells (2n)– 2. Meiosis - Haploid/gametes (n)
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The cell cycle
•Covers a time from one division of cells till other division or destruction (perish) of cell
•It has two major stages: 1) mitosis 2) interphase
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Cell Cycle: Interphase• Before mitosis• Time of high metabolic activity• DNA replicated and synthesized• Three phases: G1, S, and G2• G1(gap 1)- longest stage of cell
cycle, RNA, protein sysnthesis• S (synthesis)- DNA replicated , 2
chromatids per chromosome, chromatids genetically identical
• G2(gap 2)- RNA synthesis, not well understood
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The human cell cycle
G1
S
G2M
DNA synthesis synthesis
Growth and preparation forcell division
Rapid growth and preparation for DNA synthesis
phase
phase
phase
phase
Mitosis
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Cell Cycle: Mitosis
• Process of cell division(nuclear division) which produces daughter cells genetically identical to the parent cell
• Four Phases (P-M-A-T): prophase, metaphase, anaphase, and telophase.
• Upon completion of the phases of mitosis (nuclear division) the cell “officially” divides into two by a process called cytokinesis - division of cytoplasm
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Cell Cycle
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Interphase
Not part of mitosisDNA is replicatedchromosomes start to condense
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Chromosome duplication
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Prophase
*Chromosomes coil and condense further.*Nucler membrane breaks down/ disappears.*Microtubules increase in number, spindle apparatusforms.
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G2, Prophase
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Metaphase*Nuclear membrane completely disappeared*Chromosomes move to equator of cell - begin to line up*Chromosomes attach to spindle via kinetochore
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Centromeric Region
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Anaphase*Movement of chromosomes via microtubulesto opposite sides of the cell. One chromatid to one end the otherChromatid to the opposite end
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Chromatid Separation
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• In anaphase, the sister chromatids separate.– Two daughter cells– Each has a complete and
identical set of chromosomes
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Telophase
*Genetically identical info at each pole*Spindle fibers disappear*Chromosomes uncoil*Nuclear envelope reforms around
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METAPHASE
Metaphaseplate
Spindle Daughterchromosomes
Nuclearenvelopeforming
ANAPHASE
Metaphase, Anaphase, Telophase
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Mitosis in a plant cell
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CytokinesisCytokinesis - is separate from mitosis, = pinching of cell/divison of cytoplasm.
Mitosis + Cytokinesis result in twoidentical daughter cells.
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Mitosis
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Histologic picture of Mitosis:
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Mitosis:• Interphase: No morphological changes,
Replication of DNA and organelles.
• Prophase: Visible chromosomes
• Metaphase: equatorial plate formation
• Anaphase: Separation of chromosome pairs
• Telophase: Two separate nuclei formation.
• Cytokinesis: Separation of daughter cells.
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Chromosomes and Chromatids During Mitosis
Begin
Interphase
After
Interphase
After
Prophase
After
Metaphase
After
Anaphase
After
telophase
№ of
Chromosomes46 46 46 46 92 46
№ of
Chromatids46 92 92 92 92 46
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Cell division
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Molecular Basis of Carcinogenesis
• Genes control cell division by cytokines.
• Four classes of regulatory genes.1.Promotors – Proto-
oncogenes2.Inhibitors – Cancer-
suppressor genes – p533.Genes regulating Apoptosis.4.DNA repair genes.
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Meiosis
• Cell division which results in halpoid “sex” cells (i.e., egg and sperm)
• One replication of the genetic material (DNA) during interphase, but two nuclear divisions (meiosis I and meiosis II).
• Results in haploid (N) cells (= gametes in animals) from an initial diploid (2N) cell
• Very similar to mitosis except that the cells produced are not genetically identical.
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Overview of meiosis: how meiosis reduces chromosome number
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Meiosis Meiosis I (reduction) Prophase I is divided into the
following five stages:• Leptotene -the chromatin
condenses into the visible chromosomes, each of which contain two chromatids joined at the centromere
• Zygotene - homologous maternal and paternal chromosomes pair and make physical contact (synapsis), forming a tetrad
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Crossing-Over
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. Prophase I • Pachytene - the chiasmata are formed and crossing over (random exchanging of genes between segments of homologous chromosomes) occurs – an event that is crucial for increasing generic diversity
• Diplotene - the chromosomes continue to condense and chiasmata can be observed, indicating where crossing over has taken place
• Diakinesis - the nucleolus disappears, chromosomes are condensed maximally, and the nuclear envelope disappears
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Metaphase I• Spindle formation is
complete• Bivalents are aligned
at equator• Kinetochore
microtubules of the homologues point to opposite poles
Anaphase I• Homologues
separate and move toward opposite poles
• Cytokinesis begins
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The stages of meiotic cell division: Meiosis I
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Telophase I
• Spindle dissolvesSpindle dissolves• Nuclear envelopes reformNuclear envelopes reform• Daughter nuclei are haploidDaughter nuclei are haploid• Each chromosome is still Each chromosome is still duplicatedduplicated• Cytokinesis results in 2 Cytokinesis results in 2 daughter cellsdaughter cells
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Meiosis II Equatorial division -
begins soon after the completion of meiosis I, following a very brief interphase without DNA replication.
Prophase II • chromosomes are still
duplicated• spindle formation begins• nuclear envelope
fragments
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Metaphase II• Spindle formation is
complete• Duplicated chromosomes
are aligned at equator• Kinetochore microtubules
of sister chromatids point to opposite poles
Anaphase II• Centromeres divide• Haploid sets of chromosomes
move toward poles• Cytokinesis begins
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Telophase II
• Spindle dissolves• Nuclear envelopes
reform• Daughter nuclei are
haploid and genetically dissimilar from parent cell and each other
• Cytokinesis results in 4 daughter cells
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The stages of meiotic cell division: Meiosis II
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Independent Assortment
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The biological significance of meiosis
• meiosis enables a species’ chromosome number to remain constant over generation.
• Meiosis produces novel combination of genes.
• Meiosis produces novel combination of non homologies chromosomes
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“To a brave man, good and bad luck are like
his right and left hand, He uses both“
St Catherine of Siena
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Thank you for attention !