Learning Resource 09. The Learning Resource goal is to use the Internet and become independent of the textbook. It is a project in progress...   Ch. x
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(Source: Dr. Nilsson's old lecture notes, and Mader, S.S. 2010. Biology. Ed. 10. Used for educational purpose.Permission given in 2001 by McGraw-Hills then sales representative, Don Grainger, to use the pictures online on lecture notes.)   Genetic variation From S. S. Mader. 2010. Biology. McGraw-Hill.  In order for the chromosome number in one species to stay the same, generation after generation, a special cell division process called meiosis is required for the production of the gametes (the sperms and the eggs). To maintain the human chromosome number 46, the number of chromosomes in the gametes must be reduced to 23, so that at fertilization the resulting fertilized egg (the zygote) will still have have 46 chromosomes (23 + 23 = 46). Meiosis is the cell division that reduces the chromosome number in the gametes. In addition, because of "crossing over" and "independent assortment" meiosis is important for genetic variation. Genetic variation is illustrated in the figure above, where the puppies have a different appearance even though they have the same two parents. This is because of "crossing over" and "independent assortment" in meiosis brought different gametes together.  The Human Life Cycle --- The term life cycle is used for the reproductive events that occur from one generation to the next. In humans, the individual is (all the body cells) always diploid. The only haploid phase of the life cycle is the gametes produced by meiosis.
 Meiosis in males is part of sperm production, and meiosis in females is part of egg production. When a haploid sperm fertilizes a haploid egg, the zygote (fertilized egg) is diploid. The zygote then undergoes mitosis as it develops into a newborn child. Mitosis continues throughout life during growth and repair.
 Halving the Chromosome Number (1st result of meiosis) --- Meiosis provides a way to keep the chromosome number constant generation after generation. Without meiosis -- the "reduction division", the chromosome number of the next generation would continually increase. As discussed in the previous chapter, mitosis is the dividing process that MAINTAINS the normal chromosome number of a species -- the so-called "diploid number". Another process discussed in this chapter, meiosis is the dividing process that REDUCES the normal chromosome number to the number found in gametes (sperms or eggs) -- the "haploid number". To understand this difference you need to know the difference between the two terms diploid vs. haploid, which are two terms used extensively when discussion genetics.  Video: 0.00 min - 2.45 min (below)
Diploid number. All sexually reproducing species have a diploid number of chromosomes -- one set from the father and one set from the mother. For humans the number 46 (23 from dad and 23 from mom). In humans, the somatic (body) cells that make up the body are diploid -- containing two complete sets of homologous chromosomes, one set derived from each parent.Homologous chromosomes are the pair of chromosomes containing the same linear gene sequences (the genes controlling the same trait -- e.g., hair color) that are paired up during meiosis (one from mom and one from dad). Haploid number. For humans the number 23. The number of chromosomes in a sex cell (sperm or egg).   Chromosomes (Cassiopeiaproject) Video: 0.00 min - 3.30 min (below) Source: https://www.youtube.com/watch?v=qIe4-fVjmPE (To navigate to the "chromosome viewer" web site mentioned in the video, click on the image below.) 
• ASEXUAL vs. SEXUAL REPRODUCTION • Asexual Reproduction -- single individual gives rise to the offspring (is the sole parent) -- very little variation • Sexual Reproduction -- two individuals give rise to the offspring -- greater variation -- MEIOSIS responsible for reducing to the haploid number Before fertilization the number of chromosomes -- reduced to half At fertilization -- resulting in a 46 chromosome zygote: --- 23 + 23 = 46
 Genetic Variation (2nd result of meiosis) --- As described above, meiosis provides a way to keep the chromosome number constant generation after generation. Without meiosis -- the "reduction division", the chromosome number of the next generation would continually increase. Another important function of meiosis is to provide genetic variation. Although mutations is a source of variation in providing completely new variations of a trait, "reshuffling" of the chromosomes during sexual reproduction ensures that offspring will have a different combination of genes from their parents. Meiosis brings about genetic variation with two methods: (1) Crossing-over and (2) Independent assortment. (1) Crossing-over, is an exchange of genetic material between the homologous chromosomes (the non-sister chromatids) during meiosis I. In the figure above, part of the blue chromatid switches place with part of the red chromatid on the homologous chromosome. The result is genetic recombination where some of the gametes receive chromosomes with recombined alleles ("traits"). Chromosomes with genes that has "crossed over". (2) Independent assortment, the homologous chromosome that has paired up (i.e., one from mom, one from dad) separate independently in a random manner. In humans, who have 23 pairs of chromosomes, the possible combination is very high. In the figure above, with only three chromosome pairs the possible combinations of male sex cells (sperms) are 8, and the possible combinations of female sex cells (eggs) are 8 (or 2x2x2).  Possible chromosome combinations with 3 pairs of chromosomes.
[Crossing-over will be mentioned again when discussing the stages in meiosis.]  Meiosis --- Meiosis consists of two (2) unique cell divisions, meiosis I and meiosis II. The phases of both these divisions have the same names as the phases in mitosis. See your textbook for details (in edition 10, on page 173-176), or view the video above. Meiosis I. This is the true "reduction division". The homologous pairs of chromosomes separate during metaphase I in meiosis I (chromosomes are NOT split at the centromere -- they still look like the letter X). Video: 13.55 min - 16.45 min (below)
Meiosis II. This is a division similar to mitosis, but with only the haploid number of chromosomes. The chromosomes, as in mitosis, do not pair up in metaphase II in meiosis II (chromosomes are therefore split at the centromere when separated -- separation of sister chromatids -- they no longer look like the letter X). The end result of meiosis is 4 cells (in a male 4 sperms; in a female 1 egg and 3 polar bodies). The cell before the division had a diploid (2n) number of chromosomes; while after the division the numbers in the fours cells are haploid (n). In humans, the cell before the division had a diploid (2n or 2x23=46) number of chromosomes; while after the division the number is haploid (n or 23).
STAGES IN MEIOSIS  MEIOSIS I Division of 1 diploid 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)  1 Prim. Sex Cell (46 duplicated) ---> 2 Sec. Sex Cell (23 duplicated) ---> 4 Gametes (23 unduplicated)
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, and the first polar body normally disintegrates before meiosis II, so only two polar bodies are present at the end.) -- not a continuous process, separated in time (cyclic and only to a certain age) • Males: Spermatogenesis Results in 4 cells (sperms), with 23 chromosomes -- all full size -- continuous process   Changes in Chromosome Number and Structure ---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. The normal chromosome number in humans -- 46 -- resulting from the normal distribution of the chromosomes, disjunction (eupoidy), during meiosis.
Things can go wrong and some cells will end up with the incorrect number or with missing or too many parts of a chromosome. The risk of nondisjunction in the offspring increases with maternal age at pregnancy (Nondisjunction can also happen in the sperms, but since sperms are produced continuosly and in high amounts, the risk is small.).
Autosomal nondisjunction. If the nondisjunction happens with the autosomal chromosomes (pair 1-22), they are usually lethal. In humans, only trisomy 13 (Patau Syndrome), trisomy 18 (Edwards Syndrome), and trisomy 21 are known to be viable. Only trisomy 21 (Down syndrome) is viable beyond early childhood.
Sex chromosome nondisjunction. If the nondisjunction happens with the autosomal chromosomes (pair 23), it is usually not lethal. Two chromosomes X and Y (chromosome pair 23) control gender. A person with two X chromosomes, XX, is a girl; a person with an one X and one Y is a boy, XY. Examples are Turner syndrome, with only one sex chromosome -- an X (0X), and Klinefelter syndrome, with more than one X chromosome, but also a Y chromosome (XXY).
[Non-disjunction will be mentioned again when discussing genetics.]
- Phases of Meiosis (Khan Academy You Tube) -COMMENT ON THE YOUTUBE: This video helped me understand how Meiosis works much better than the textbook. I read the textbook repeatedly and could not understand how it worked, but this video really helped me. If you have problems understanding the concept of the "meiosis" here is an excellent YouTube video from Khan Academy, which will help you understand (even though some of the information is beyond what is required for this class). Phases of Meiosis Video (especially recommended): 0.00 min - 2.45 min (below) Video (especially recommended):: 13.35 min - 16.45 min (below)
Meiosis is similar to mitosis, but only occurs in certain types of cells during certain times of an organism's lifespan. Meiosis facilitates sexual reproduction, and while things can go wrong during the process, sexually reproducing organisms have a greater likelihood of survival than asexually reproducing species because of the genetic diversity. Understanding meiosis and how it increases diversity is important in order to understand genetics. In the next chapter the basic genetic laws (Mendel's Laws) established by the father of genetics Gregor Mendel.
(Source: Mader, S.S. 2010. Biology. Ed. 10. Used for educational purpose.) |
