SCIENCE Q4

• Meiosis may not always proceed normally. Accidents sometimes happen.
• Chromosomes sometimes fail to separate and segregate. The gametes produced may contain an additional or a missing chromosome.
• Down's syndrome


  Caused by the presence of a third copy of chromosome number 21. People with this condition suffer from variable degrees of mental retardation, sterility, and increased risk of Alzheimer's disease beyond the age of 40.
• Edward's syndrome


  Also known as trisomy 18. Babies are small, have heart defects, and usually do not survive for much longer than a week.
• Patau syndrome


  Also known as trisomy 13. There is severe intellectual disability and physical abnormalities in many parts of the body. There are often heart defects, brain or spinal cord abnormalities. The body is very small or poorly developed and there may be extra fingers or toes, cleft palate, and weak muscle tone.
• Turner syndrome


  The absence of the other X chromosome in females. This syndrome can cause a variety of medical and developmental problems including short height, failure of the ovaries to develop, and heart defects.
• Cri du chat syndrome


  Also known as 5p- (5p minus) syndrome. It is a rare chromosomal disorder caused by missing or deleted portions of chromosome number 5. Babies who are born with this syndrome often have a high-pitched cry that sounds like a cat.
• Meiosis ensures that the chromosome number of organisms stays constant generation after generation.
• Meiosis ensures that each daughter cell receives a copy of each kind of chromosomes and that the chromosomes are distributed to the daughter cells in various combinations bringing about variations in the characteristics of organisms that would allow them to better adapt to the changing environment.
• Prophase (Early)


  1. Each chromosome consists of two sister chromatids which appear as long thin threads

  2. Asters start to form around centrioles

  3. Two centrioles move apart to opposite poles
• Prophase (Late)


  1. Nucleolus and nuclear membrane disappear

  2. Spindle fibers appear

  3. Centrioles are nearly at the opposite sides of the nucleus
• Metaphase


  1. Centrioles have moved toward the poles

  2. Chromosomes line up along the equatorial plate

  3. Each chromosome is attached to a spindle fiber at the centromere

  4. Nuclear membrane has completely disappeared
• Anaphase (Early)


  1. Each chromosome splits

  2. Spindle fibers start to pull the chromatids apart to opposite poles of the cell
• Anaphase (Late)


  1. Each chromatid is now considered a full-fledged chromosome. Chromosomes are at the opposite poles

  2. Cytokinesis begins

  3. Slight cleavage furrow on cell membrane begins to form in the region of the equator
• Telophase


  1. Chromosomes uncoil or unfold and appear thread-like

  2. Spindle fibers disappear or disintegrate

  3. Nuclear membrane or envelope appears

  4. Nucleolus becomes apparent again
• Cytokinesis


  The final phase of cell division marked by the division of the cytoplasm. Cytoplasm divides forming two identical cells.
• Cellular reproduction, commonly known as cell division, is a process by which cells duplicate their contents and then divide to yield multiple cells with similar or duplicate contents.
• Chromosome


  The genetic material that serves as the set of instructions that direct the activities and functions of the cell. These genetic materials, also known as the deoxyribonucleic acid (DNA), carry the hereditary information that are passed on from one generation to the next to ensure the continuity of life.
• Centromere


  The constriction point that divides the chromosome into two sections or "arms".
• p arm


  The short arm of the chromosome.
• q arm


  The long arm of the chromosome.
• Sister chromatids


  The two identical copies of the same chromosome formed by DNA replication during the S phase of the cell cycle.
• The number of chromosomes per nucleus normally remains constant for all the individuals of a species.
• Stages of the cell cycle


  • G1 phase
  • S phase
  • G2 phase
  • M phase
• Interphase


  The period between two successive mitotic cell divisions. It is the longest period in the cell cycle during which the cell is not dividing.
• M phase


  The cell division phase (mitosis/meiosis) where nuclear and cytoplasmic division take place.
• G0 phase


  The time when the cell is neither dividing nor preparing to divide. It is described as a cellular state outside of the cell cycle.
• Cell division is important for two reasons: (1) to produce offspring; and (2) to generate new cells that will replace worn out or damaged cells.
• Mitosis


  The division of the nucleus or nuclear material to form two identical daughter cells. This cell division takes place in our body cells (somatic cells) such as those in the muscles, skin, glands, and bones.
• Meiosis


  A process wherein a diploid parent produces four unique haploid cells. It reduces the number of chromosomes each daughter cell will receive. It is a special type of cell division occurring only in the reproductive organs (ovaries and testes) producing sex cells or germ cells (egg cells and sperm cells).
• Meiosis I (Prophase I)


  1. Each chromosome duplicates and remains closely associated. These are called sister chromatids

  2. Chromosomes shorten, thicken, and become visible

  3. Homologous chromosomes pair up and crossing over occurs
• Meiosis I (Metaphase I)


  1. The nuclear membrane and nucleolus disappear

  2. Centrioles produce spindle fibers

  3. Homologous chromosomes align at the equatorial plate
• Meiosis I (Anaphase I)


  Homologous chromosomes separate and move to opposite poles
• Meiosis I (Telophase I)


  1. The chromosomes reach the poles

  2. A nuclear membrane forms around the chromosomes in each pole

  3. Two cells eventually result, each new cell having one pair of chromosomes
• Meiosis II (Prophase II)


  1. Chromatids condense and become visible

  2. Centrioles are doubled

  3. Nucleoli are still visible. DNA does not replicate.
• Meiosis II (Metaphase II)


  1. Nuclear envelopes and nucleoli disappear

  2. Chromosomes align at the equator of the equatorial plate
• Meiosis II (Anaphase II)


  1. Centromeres divide and sister chromatids migrate separately toward opposite poles

  2. A cleavage furrow is produced due to the opposite pull of the centrioles
• Meiosis II (Telophase II)


  1. Chromosomes reach the poles and a new nuclear membrane forms around them in each pole

  2. Four haploid daughter cells are formed

  3. Each new nucleus has one chromosome
• Meiosis I (First Division)


  1. Chromosomes reach the poles

  2. A nuclear membrane forms around the chromosomes

  3. Two cells eventually result, each new cell having one pair of chromosomes

  4. The chromosome number is only half that of the original cell

  5. The chromosomes are double-stranded
• Meiosis II (Second Division - Gamete Formation)


  1. Chromatids condense and become visible

  2. Centrioles are doubled

  3. Nucleoli are still visible

  4. DNA does not replicate

  5. Nuclear envelopes and nucleoli disappear

  6. Chromosomes align at the equator of the equatorial plate

  7. Centromeres divide and sister chromatids migrate separately toward opposite poles

  8. A cleavage furrow is produced due to the opposite pull of the centrioles

  9. Chromosomes reach the poles and a new nuclear membrane forms around them in each pole

  10. Four haploid daughter cells are formed

  11. Each new nucleus has one of each type of chromosome found in Telophase 1