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Students are required to read the textbook for the instructor to be able to teach the concepts the course encompasses. (From FOUNDATION SKILLS in the Biology Department Master Syllabus.)
Students, do not just read these notes, to succeed in this class -- at least if you wish to have a high grade, you MUST use your textbook as a LEARNING TOOL!
MAIN TOPICS OUTLINE
9.1 CELL REPRODUCTION
9.2 SEX CELL PRODUCTION
9.3 STAGES IN MEIOSIS
9.4 PRODUCTION OF SEX CELLS (gametes)
9.5 NONDISJUNCTION
LECTURE OBJECTIVES
1. Describe binary fission in bacteria, and distinguish this process from cell division in eukaryotes.
2. Explain why sexually reproducing organisms must form cells with haploid number of chromosomes.
3. Describe the stages in meiosis I.
4. Describe the stages in meiosis II.
5. Compare mitosis and meiosis; cite similarities and differences.
6. Recognize the effect that meiosis has on chromosome number.
7. Compare sex cell production in males and sex cell production in females; cite similarities and differences.
8. Explain how nondisjunction (e.g., trisomy) results from errors in the meiotic process.
9. Explain how genetic variation is generated.
10. Define selected key terms.
Key Termsdef. Asexual reproduction: Type of reproduction in which only one parent gives rise to offspring (genetically identical).
def. Clone: A linkage of genetically identical individuals.
def. Diploid cell: A cell that has two sets of chromosomes -- one from the maternal parent, and one set from the paternal parent.
def. Gene: Unit of heredity, the portion of DNA that determines characteristics (traits)
def. Haploid cell: A cell that has a single set of chromosomes -- in eukaryotes with sexual reproduction resulting from meiosis.
def. Meiosis: A process of nuclear division in eukaryotic sex cells that reduces the chromosome number of sex cells to the haploid number.
def. Mutation: A rare change in the DNA that ultimately creates genetic diversity
def. Sex cells: Sperms or eggs (including the germ cells, which give rise to the sex cells).
def. Sexual reproduction: Type of reproduction in which two parents gives rise to offspring (genetically unique combination of genes inherited from gametes of the two parents).
def. Somatic cells: Cells that are not sex cells (see sex cells).
9.1 CELL REPRODUCTION
Eukaryotic vs. prokaryotic cell division
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• ASEXUAL vs. SEXUAL REPRODUCTION
def. Clone: A linkage of genetically identical individuals.
def. Asexual reproduction: Type of reproduction in which only one parent gives rise to offspring (genetically identical).
def. Sexual reproduction: Type of reproduction in which two parents gives rise to offspring (genetically unique combination of genes inherited from gametes of the two parents).
def. Mutation: A rare change in the DNA that ultimately creates genetic diversity
• Asexual Reproduction
-- single individual gives rise to the offspring (is the sole parent)
‡ very little variation > only from mutations (LATER)
MITOSIS
--"CLONE"
• Sexual Reproduction
-- two individuals give rise to the offspring
‡ greater variation > not only from mutations -- also nature of sexual reproduction
-- variations from both parents
MEIOSIS
• HUMAN CHROMOSOME NUMBERS
• Human somatic cells (all body cells except sperms and eggs) have 46 chromosomes
-- DIPLOID number
• Human sex cells (sperms and eggs -- gametes) only have 23 chromosomes
-- HAPLOID number
• The haploid number ( chromosome reduction ) is obtained by a specialized cell division
-- MEIOSIS
Before fertilization the number of chromosomes -- reduced to half
At fertilization -- resulting in a 46 chromosome zygote: --- 23 + 23 = 46
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9.2 SEX CELL PRODUCTION
• Primitive Sex Cell diploid (2n, 46 chromosomes)
-- in the SEX ORGANS(testes, ovaries)
MITOSIS
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
• Primary Sex Cell diploid (2n, 46 chromosomes)
MEIOSIS I
• Secondary Sex Cell haploid (n, 23 duplicated chromosomes)
MEIOSIS II
• Sperm or Egg haploid (n, 23 unduplicated chromosomes)
At fertilization:
-- 23 (unduplicated from the sperm) + 23 (unduplicated from the egg)
result: a 46 diploid chromosome Zygote (fertilized egg).
9. 3 STAGES IN MEIOSIS
MEIOSIS I Division of 1 Primary sex cell (w. 46 duplicated chromosomes).
– Prophase I “Crossing Over”
– Metaphase I Homologous chromosome pair on the metaphase plate.
– Anaphase I The paired chromosomes separate but do not split.
– Telophase I
– "Interphase" -- 2 haploid Secondary sex cells (w. 23 duplicated chromosomes).
MITOSIS II Division of Secondary sex cells (with 23 duplicated chromosomes).
– Prophase II
– Metaphase II 23 (unpaired) chromosomes line up on the metaphase plate
– Anaphase II The chromosomes split by the centromere.
– Telophase II
– End of Meiosis: -- 4 haploid Gametes (sperms / eggs, w. 23 unduplicated chromosomes)
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9.4 PRODUCTION OF SEX CELLS (gametes)
• Females: Oogenesis
Results in 4 cells, with 23 chromosomes
-- only one is a full size egg. (the other 3 are called polar bodies)
-- not a continuous process, separated in time
• Males: Spermatogenesis
Results in 4 cells (sperms), with 23 chromosomes
-- all full size
-- continuous process
9.5 NONDISJUNCTION
def. Nondisjunction: Abnormal meiotic division resulting in sex cells with too many or too few chromosomes.
def. Down Syndrome: A genetic disorder resulting from the presence of an extra chromosome number 21.
-- If the chromosomes do not separate correctly during meiosis.
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-- Disclaimer: "Dr. Nilsson's CyberOffice", at the time of writing located as a file under the South Texas Community College's (STCC) web server with the general URL http://stcc.cc.tx.us/, is the intellectual property of Dr. Jan A. Nilsson, member of STCC biology faculty. The content of Dr. Nilsson's CyberOffice does not necessarily reflect the opinions and beliefs of the STCC faculty, staff, administration, and Board of Trustees.