Mitosis, meiosis and embryo development

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Mitosis, meiosis and embryo development
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Mitosis

• Prokaryotic organisms and most eukaryotic cells
  in the body undergo mitosis
• Mitosis is a means to replace damaged cells,
  cells that have been sloughed off, or to
  facilitate overall organismal (animal and plant)
  growth
• Different types of cells or tissues have specific
  cell cycle rates
• Diploid (or 2n) cells start with the normal
  number of chromosomes and end with same number
• For example Diploid human cells have a total of
  46 chromosomes (23 pairs) and end with 46
  chromosomes

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Overview of cell cycle

• All cells start off with a period of growth
• As cells grow in size, they produce organelles,
  and perform various functions
• Before cell division, chromatin (DNA) must be
  replicated
• Chromatin must be coiled up into chromosomes
• Chromosomes consist of a pair of chromatids
  called sister chromatids
• Each new daughter cell has one chromatid from
  each pair of chromosomes

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What was that about chromosomes?

• Chromosomes are a way of organizing and
  condensing very long strands of DNA
• DNA is wrapped around histone proteins and many
  histone coils are found in a chromosome

Histones
Nucleosomes
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Stages of mitosis

• The longest phase of the cell lifecycle is
  interphase
• Interphase consists of three phases
• Cell division comprises a much smaller portion of
  the cell lifecycle

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Mitosis Interphase, and prophase
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Fig. 5.25 (TEArt)
Centrioles are assembly points for microtubules
Microtubule triplets
Spindle fibers are made of microtubules which act
as guiding wires to push chromosomes through the
cell
Microtubules are made of proteins
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Mitosis Metaphase, anaphase, telophase
Cytokinesis (cell division) occurs during
telephase
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Cytokinesis in animal cells

• Note the presence of a cleavage
• furrow
• Cells are pulled apart by
• microfilaments (protein called actin)

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Cytokinesis in plant cells

• No cleavage
• Cell gradually expands horizontally
• Cell reconstructs cell wall from materials
• found in vesicles
• Cell plate becomes new cell wall

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Recognizing stages of mitosis
Allium root tip
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Recognizing stages of mitosis
Gastrulation from zygote to blastula stage
Whitefish blastula
Zygote
Blastula
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Mitotic diseases Leukemia
Erythrocytes (red blood cells)
Abnormal leukocytes
Leukocytes (white blood
cells)
Acute lymphocytic leukemia
Normal human blood smear
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More about cancer

• Cancer cells do not respond to cell cycle control
  system and excessively divide
• A normal cell may transform and become a type of
  cancer cells (such as a tumor) if it escapes
  detection by the immune system after
  transformation
• Benign tumors are localized and malignant tumors
  spread to other parts of the body via the blood
  or lymphatic system
• Chemotherapy drugs interrupt cancer cells during
  certain phases of mitosis
• Radiation treatment attempts to thwart cancer
  cells by destroying cancer cell DNA cancer cells
  are typically unable to repair damaged DNA

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More about human chromosomes

• A typical human cell is called a somatic cell
• In a somatic cell there are 46 chromosomes (23
  homologous pairs)
• 22 of these pairs are autosomes
• 1 pair of sex chromosomes
• XX Female, XY Male
• Diploid cells (2n) have 46 chromosomes
• Haploid cells (n) have 23 chromosomes (22
  autosomes and 1 sex chromosome)

Normal human karyotype
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Meiosis

• Meiosis is a means to transfer genetic
  information onto the next generation without
  doubling the number of chromosomes in each
  generation
• Meiosis consists of cell division phases
  (prophase, metaphase, anaphase, telophase)
  similar to mitosis except there are two meiotic
  cycles (meiosis 1 and meiosis 2)
• Gametes start as diploid cells (2n) end as
  haploid cells (n), one chromosome from each pair
• Meiosis in humans occurs in gametes (egg and
  sperm)
• When the egg and sperm unite during sexual
  reproduction, a diploid (2n) zygote is produced

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Meiosis 1
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Meiosis 2
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Gamete production in men

• Sperm cells develop from primary spermatocytes
  in seminiferous tubules
• Primary spermatocytes differentiate into
  secondary spermatocytes after meiosis 1
• Secondary spermatocytes differentiate into sperm
  cells after meiosis 2

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Sperm cell development
Testes consists of chambers in which sperm are
produced
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Gamete production in women

• An ovum develops from an oocyte (egg
• cell)
• Primary oocyte and first polar body develops
  after meiosis 1 first polar body may also
  undergo meiosis and produce 2 polar bodies
• Secondary oocyte and second polar body develops
  after meiosis 2
• Presence of sperm completes meiosis 2 causing
  secondary oocyte to develop into ovum and one
  polar body
• Thus, 3 polar bodies and 1 ovum may be produced
  after successful completion of meiosis

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Female gamete production and embryo implantation
Ovary
Ovary, oviduct, uterus
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Oocyte development
Immature oocytes

• Females are born with thousands of oocytes
• Oocytes are arrested until puberty
• After puberty, oocytes begins to mature each
  month

Developing oocyte
Mature oocyte
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Embryo development

• After zygote formation, cleavage occurs
• Cleavage consists of a series of rapid cell
  divisions until blastula formation overall
  embryo size does not change
• A blastula (background) transforms into a
  gastrula through a process known as gastrulation
• Gastrulation is complete when three embryonic
  germ layers are formed

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Embryo development continued
These cells undergo mitosis
Embryo develops tissue layers which become organs
and other structures
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YouTube - Amphioxus embryonic development