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I. Sexual Reproduction Figure 1 : Human Sexual Lifecycle
- Sexual Reproduction involves the production of sex cells or Gametes from 2 parents. The formation of these cells occurs by a form of cell division called Meiosis , which only occurs within tissues called Gonads (ovaries or testis). - During gamete formation, the genomes of both parents are halved & genetically recombined (“shuffled”). As a result, the gametes formed by each parent are genetically unique ; by extension the resulting zygote (should fertilization occur) is also genetically unique. Figure 2 : Diploid vs Haploid Cells
- Cells other than gametes are called Somatic Cells. These cells are considered to be Diploid (2n) in their chromosome count because they possess 2 complete sets of chromosomes , one maternal set (inherited from mom) & one paternal set (inherited from dad). In human somatic cells, the diploid number is 46 (23 maternal + 23 paternal chromosomes). - In contrast to somatic cells, gametes are considered to be Haploid (n) in chromosome count because they possess only 1 chromosome set. In human gametes, the haploid number is 23. As seen in fig. 2 , the union of haploid gametes during fertilization gives rise to a diploid zygote → embryo → multi-celled individual. Figure 2 .1: Diploid Cells: Homologous Chromosomes - Within the nucleus of diploid cells, each chromosome within the paternal set can be “matched” with a particular chromosome within the maternal set. Such chromosomes are said to be Homologous , for they are similar in terms of: 1) 2) 3) - Based on the aforementioned physical characteristics, the 46 chromosomes within human diploid cells can be organized into 23 Homologous Pairs. Figure 2 .2: Homologous Chromosomes: Gene Content - The specific combination of alleles that determines a trait is called the Genotype (may be homo or heterozygous). An individual’s appearance with regards to a particular trait, determined by genotype, is called the Phenotype.
Figure 3 .1: Genetic Recombination: Crossing Over *Because crossing over results in producing chromatids with new & unique gene combinations, it is considered to be a form of Genetic Recombination. Another major form of genetic recombination, Independent Assortment , is described below.
- The nuclear envelope breakdowns by late prophase I. In addition, formation of the meiotic spindle is complete. Tetrads attach to the microtubules of the spindle & start migrating toward its center. Figure 9.2: Meiosis I (Reduction Division): Metaphase I to Telophase I/Cytokinesis - Metaphase I: Due to the interaction between the tetrads & mitotic spindle, the tetrads line up on the Metaphase Plate , an imaginary plane equidistant from both poles. In the process of aligning along the metaphase plate, tetrads exhibit a second form of genetic recombination, Independent Assortment :
Figure 3 .3: Genetic Recombination: Independent Assortment
- When aligning along the metaphase plate, maternal & paternal homologs of each tetrad may line up on the right OR left side. The side they occupy is determined at RANDOM. As a result, each side of the metaphase plate will exhibit a random assortment of maternal & paternal chromosomes … - As a result of the independent assortment of tetrads, each side of the metaphase plate will exhibit a RANDOM SAMPLE of recombined paternal & maternal chromosomes … Figure 3 .4: Genetic Recombination: Effects of Independent Assortment *As a result of independent assortment, each gamete receives a random collection of genetically recombined chromosomes. Combined with crossing over, genetic recombination leads to the production of genetically unique gametes that provide much of the diversity upon which natural selection acts.
III. Specific Forms of Meiosis: Spermatogenesis & Oogenesis Figure 4 : Spermatogenesis
- Spermatogenesis is a form of meiosis that occurs exclusively within the testis. As a result of this process, 4 genetically unique sperm cells are formed for every cell that initially divides. Figure 4.1: Oogenesis - Oogenesis is a form of meiosis that occurs exclusively within the ovaries. As a result of this process, 1 egg cell (ovum) & 3 smaller polar bodies are formed for every cell that initially divides. Because of their small size, the polar bodies disintegrate within the ovary & are never ovulated into the oviduct.
IV. Embryonic Development Figure 5 : Female Reproductive System Figure 5.1: Stages of Early Embryonic Development
Figure 6 : Animal Groups: Protostomes & Deuterostomes Figure 6 .1: Protostome Embryonic Development
- Protostome animals include (1) Mollusks (clams, octopi, etc) (2) Annelids (earthworms, leeches, etc) & (3) Arthropods (insects & crustaceans). - Protostome development involves the following characteristics: a) Spiral Cleavage: as the embryonic cells divide, each cell lies at an acute angle to the one below it to produce a spiral pattern when viewed from the top. b) Determinate Development: the embryonic stem cells differentiate into specialized cell types at a very early stage. Thus, if a single cell were to break free of the main embryo, it would develop into a specific cell type, not another embryo (identical twin formation not likely). c) Blastopore Fate: the blastopore will develop into the mouth (archenteron develops into the digestive tract). d) Body Cavity Formation: the body cavity (coelom) forms from the splitting & expansion of the mesoderm that arises between the archenteron & ectoderm.
Figure 6.2: Deuterostome Embryonic Development
- Deuterostome animals include (1) Echinoderms (starfish, sand dollars, sea urchins) & (2) Chordates (vertebrates). - Deuterostome Development: involves the following characteristics: a) Radial Cleavage: as the embryonic cells divide, each cell lies immediately above one another to produce a radial pattern when viewed from the top. b) Indeterminate Development: the embryonic stem cells differentiate into specialized cell types at a later stage. Thus, if a single cell were to break free of the main embryo, identical twin formation is possible. c) Blastopore Fate: the blastopore will develop into the anus (archenteron develops into the digestive tract). d) Body Cavity Formation: the body cavity (coelom) forms from the splitting & expansion of the mesoderm that emerges from the walls of the archenteron. V. Chromosomal Mutations Figure 7 : Human Karyotype - A Karyotype is a picture of all of the chromosomes within a cell, in which the chromosomes of the cell are arranged in the following manner … a) The chromosomes are arranged in homologous pairs, each pair containing a maternal & corresponding paternal chromosome.
Figure 8 .1: Effects of Aneuploidy ● Aneuploidy: a) 2n+1 = b) 2n-1 = Figure 8.2: Effects of Aneuploidy: Lethal vs Nonlethal ● Aneuploid zygotes that are most likely to survive usually involve trisomies of chromosomes 13, 18, & 21. This may be explained by the small size of these chromosomes & their relatively small amounts of genes. Trisomies & monosomies of all other autosomes are LETHAL early in embryonic development.
VI. Aneuploidy & Disease ● Down Syndrome: 1 in 700 children born & results from trisomy of chromosome 21. Nondisjunction of chromosome 21 occurs more often during oogenesis. Results in unique facial features, short stature, heart defects, susceptibility to respiratory infection, & mental retardation. Affected individuals have shorter life spans & are usually sexually underdeveloped & sterile. Nondisjunction of chromosome 21 occurs more frequently in women 45 years or older.
● Klinefelter Syndrome: results from an extra X chromosome in males, producing XXY individuals. Affected individuals
have severely underdeveloped testis & are always sterile. In addition, this disorder also leads to breast enlargement, paucity of body hair, high-pitched voice & the formation of other feminine contours. Are usually capable of normal sexual function, but many, if not most, are unable to produce sufficient amounts of sperm for conception. Klinefelter syndrome males with more than two X chromosomes usually have extreme symptoms & are often mentally retarded. ● Turner Syndrome: results from monosomy of the X chromosome in females (XO). Are short of stature, averaging 4’7” in adults. Exhibit abnormally small, widely spaced breasts; have broad, shield-shaped breasts & turned out elbows. Ovaries do not develop & individuals are sterile, but usually of normal intelligence. VII. Chromosomal Structural Mutations Figure 9 : Deletions ● Chromosomal Deletions result in the permanent loss of a section of a chromatid due to errors during crossing over. Alleles on the deleted segment are not expressed , resulting in a reduction in the production of vital gene products. Figure 9 .1: Duplication ● Duplications occur when a section of a chromatid exists more than once as a result of errors during crossing over (see “unequal” crossing-over).
VIII. Pre-Natal Chromosome Testing Figure 10 : Amniocentesis vs CVS
- Amniocentesis is performed between the 14th- 16 th^ weeks of pregnancy, whereby amniotic fluid surrounding the fetus is collected. The fetal cells suspended in the fluid are cultured & ruptured, whereupon the chromosomes are photographed & arranged in a karyotype to be analyzed. - CVS is performed between the 8th^ - 10 th^ weeks of pregnancy, whereupon a piece of the placenta is collected, its cells ruptured, & chromosomes photographed & arranged in a karyotype. Both procedures are performed on women 35 & older (likelihood of fetal chromosomal disorders are greater).
