Cell Reproduction

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Cell Reproduction
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Reproduction of Prokaryotes

• Prokaryotes reproduce through a process called
  binary fission
• The single, circular chromosome (which is
  attached to the plasma membrane) replicates
• The cell grows elongates, separating the two
  chromosomes
• The plasma membrane and cell wall pinch inward,
  eventually forming two daughter cells

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Eukaryotic Chromosomes and Cell Division

• Almost all of a eukaryotic cells genes are found
  in the cells nucleus, and are grouped into
  multiple chromosomes
• Chromosomes are only visible when a cell is in
  the process of dividing
• The rest of the time, the DNA protein threads
  exist as a diffuse mass called chromatin

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• Before dividing, the eukaryotic cell replicates
  its chromosomes
• The two copies of the chromosome are called
  chromatids, and they are connected to each other
  at a region called the centromere
• The pair of chromatids is still only one
  chromosome, and the chromatids are not considered
  chromosomes until they separate from one another

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The Eukaryotic Cell Cycle

• A eukaryotic cell spends most of its time in
  interphase, the period of time between cell
  divisions
• During interphase, metabolic activity is high
  the cell grows, produces cell parts, and in the
  middle of interphase the cells chromosomes
  duplicate
• Interphase accounts for 90 of the total time
  required for the cell cycle

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Interphase

• Interphase may be divided into three subphases
• The first subphase is called G1 (the G is for
  gap) the cell is producing proteins and
  organelles
• The second subphase is the S phase, which is when
  DNA synthesis occurs
• The third subphase is called G2, which is a time
  of metabolic activity proteins needed for cell
  division are produced

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Mitotic Phase (M Phase)

• Consists of two processes mitosis and
  cytokinesis
• During mitosis, the chromosomes divide and are
  distributed into two daughter nuclei
• During cytokinesis, the cytoplasm is divided into
  two
• These two processes result in the production of
  two genetically identical daughter cells

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Phases of Mitosis - Prophase

• Chromatin fibers condense
• Nucleoli disappear
• Each chromosome has two chromatids joined at the
  centromere
• The spindle begins to form microtubules grow out
  of the centrosomes, which contain centrioles
• The nuclear envelope fragments

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• Microtubules grow until they can reach the
  chromosomes
• Each chromatid has a special region at the
  centromere, called the kinetichore, where the
  spindle will attach
• Spindles that dont attach to kinetichores extend
  toward the opposite side of the cell
• The chromosomes begin moving toward the center of
  the cell

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Phases of Mitosis - Metaphase

• The spindle is fully formed
• All of the centromeres of the chromosomes are
  lined up at the metaphase plate (the middle of
  the cell)
• The kinetichores of the chromatids face opposite
  sides of the cell

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Phases of Mitosis - Anaphase

• The centromeres of the chromosome come apart and
  the sister chromatids separate (they are now
  considered daughter chromosomes)
• Motor proteins on the kinetichores walk the
  chromosomes toward opposite sides of the cell
• Kinetichore microtubules shorten and
  non-kinetichore microtubules lengthen (the cell
  elongates)

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Phases of Mitosis - Telophase

• Basically the reverse of prophase
• Cell elongation that began in anaphase continues
• Daughter nuclei appear-nuclear envelope appears
• Chromatin fibers uncoil
• Nucleoli reappear
• Spindle disappears

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Cytokinesis

• Occurs along with telophase
• In animal cells, cytokinesis involves a cleavage
  furrow, which pinches the cell in two
• In plants, cytokinesis involves a cell plate,
  which grows outward and accumulates cell wall
  material

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The Cell Cycle Cancer

• Cancer is a disease of the cell cycle
• Systems that control the cell cycle do not work
  in cancer cells
• As a result, cancer cells divide excessively and
  can invade other body tissues
• The excessive growth can result in an abnormal
  mass called a tumor

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Tumors

• Benign tumor-abnormal mass of normal cells can
  cause problems if they grow in an organ remain
  in their original site can usually be completely
  removed by surgery
• Malignant tumor-cancerous capable of spreading
  to other body areas displaces normal tissue can
  form new tumors by breaking apart and spreading
  through the circulatory system (called metastasis)

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Types of Cancers

• Carcinomas-originate in the external or internal
  coverings of the body (skin)
• Sarcomas-originate in tissues that support the
  body (bone and muscle)
• Leukemias and Lymphomas-cancers of blood forming
  tissues (bone marrow, spleen, and lymph nodes)

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Treatments for Cancer

• Radiation- parts of the body with cancerous
  tumors are exposed to high energy radiation,
  which disrupts cell division (damage to ovaries
  or testes may cause sterility)
• Chemotherapy-administration of drugs that disrupt
  cell division some interfere with the formation
  or functioning of the spindle

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Meiosis

• Essential for the formation of gametes (ova and
  sperm) that will be used for sexual reproduction
• Occurs in two consecutive cell divisions
• Reduces the chromosome number by ½ (for humans,
  this means creating gametes that have 23
  chromosomes)

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Human Chromosomes

• Occur in pairs (there are 23 pairs) in somatic
  cells (typical body cells) these are called
  diploid cells (di2)
• Occur unpaired (a single set of 23) in the
  gametes these are called haploid cells
• The paired chromosomes in somatic cells are
  called homologous chromosomes, because they have
  genes for the same traits

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Human Chromosomes

• 22 of the chromosome pairs are called autosomes
• The 23rd pair consists of the sex chromosomes
• Human females have a pair of sex chromosomes
  called X chromosomes (XX)
• Human males have one X chromosome and one Y
  chromosome (XY)

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Inheritance of Human Chromosomes

• One chromosome from each pair is inherited from
  our mothers, and the other chromosome in the pair
  is inherited from our fathers
• At the time of fertilization, the two haploid
  gametes (sperm and ovum) unite to form a diploid
  cell called the zygote
• Fertilization results in the formation of a
  diploid cell, thus restoring the normal diploid
  number of human chromosomes

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Meiosis Produces Gametes

• Meiosis occurs in the reproductive organs