Human Genetics - Part III (Fall 2023) PDF

Human genetics- Part III Nermin Eissa, Ph.D. College of Health Sciences Abu Dhabi University Fall-2023  Learning Outcomes: Distinguish between meiosis and mitosis with regard to the number of divisions and the number and chromosome conten...

Human genetics- Part III Nermin Eissa, Ph.D. College of Health Sciences Abu Dhabi University Fall-2023  Learning Outcomes: Distinguish between meiosis and mitosis with regard to the number of divisions and the number and chromosome content of the resulting cells. Contrast the events of meiosis I and meiosis II with the events of mitosis. Explain how nondisjunction produces monosomy and trisomy chromosome conditions. Describe the causes and consequences of trisomy 21. Describe the effects of deletions, duplications, inversions, and translocations on chromosome structure. 2 ©2020 McGraw-Hill Education  Comparison of Meiosis and Mitosis 2 Comparison of meiosis and mitosis: DNA replication takes place only once prior to both meiosis and mitosis. Meiosis requires two nuclear divisions, mitosis only one. Meiosis produces four daughter cells, mitosis two. Daughter cells of meiosis are haploid (n); of mitosis, diploid. 3 ©2020 McGraw-Hill Education  A Comparison of Meiosis and Mitosis Access the text alternative for these images 4 ©2020 McGraw-Hill Education  Comparison of Meiosis and Mitosis 3 The daughter cells of meiosis are not genetically identical to each other or to the parent cell; the daughter cells of mitosis are. The specific differences between these nuclear divisions can be categorized according to occurrence and process. 5 ©2020 McGraw-Hill Education  Occurrence Meiosis occurs only in the reproductive organs and produces the gametes. Mitosis is more common; it occurs in all tissues during growth and repair. 6 ©2020 McGraw-Hill Education  Process: Comparison of Meiosis I with Mitosis 1 Events that distinguish meiosis I from mitosis: Homologous chromosomes pair and undergo crossing-over during prophase I of meiosis but not during mitosis. Paired homologous chromosomes align at the metaphase plate during metaphase I in meiosis; individual chromosomes align at the metaphase plate during metaphase in mitosis. 7 ©2020 McGraw-Hill Education In mitosis In meiosis  Process: Comparison of Meiosis I with Mitosis 2 Homologous chromosomes (with centromeres intact) separate and move to opposite poles during anaphase I of meiosis. Centromeres split, and sister chromatids, now called chromosomes, move to opposite poles during anaphase in mitosis. The events of meiosis II are like those of mitosis except that in meiosis II the nuclei are haploid. 9 ©2020 McGraw-Hill Education  Comparison of Meiosis I with Mitosis Table Comparison of Meiosis I with Mitosis. Meiosis I Mitosis Prophase I Prophase Pairing of homologous chromosomes No pairing of chromosomes Metaphase I Metaphase Homologous duplicated Duplicated chromosomes at equator chromosomes at equater Anaphase I Anaphase Homologous chromosomes Sister chromatids separate, becoming separate. daughter chromosomes, which move to the poles. Telophase I Telophase Two haploid daughter cells Two daughter cells, identical to the parent cell 10 ©2020 McGraw-Hill Education  Comparison of Meiosis II with Mitosis Table Comparison of Meiosis II with Mitosis. Meiosis II Mitosis Prophase II Prophase No pairing of chromosomes No pairing of chromosomes Metaphase II Metaphase Haploid number of duplicated Duplicated chromosomes at equator chromosomes at equator Anaphase II Anaphase Sister chromatids separate, becoming Sister chromatids separate, becoming daughter chromosomes, which move daughter chromosomes, which move to to the poles. the poles. Telophase II Telophase Four haploid daughter cells Two daughter cells, identical to the parent cell 12 ©2020 McGraw-Hill Education  Check Your Progress List the similarities and differences between meiosis I and mitosis. List the similarities and differences between meiosis II and mitosis. Explain why a close examination of metaphase can indicate whether a cell is undergoing mitosis or meiosis. 13 ©2020 McGraw-Hill Education  Chromosome Inheritance 2 Normally an individual receives 22 pairs of autosomes and two sex chromosomes. Each pair of autosomes carries alleles for particular traits. The alleles can be different. That is, one can contain instructions for freckles and the other may not. 14 ©2020 McGraw-Hill Education  Changes in Chromosome Number 1 Some individuals are born with either too many or too few autosomes or sex chromosomes. Caused by nondisjunction during meiosis I or meiosis II. During meiosis I, the homologous chromosomes fail to separate correctly. During meiosis II, the daughter chromosomes fail to separate. 15 ©2020 McGraw-Hill Education  Changes in Chromosome Number 2 Trisomy—one type of chromosome is present in three copies (2n + 1). Monosomy—one type of chromosome is present in a single copy (2n − 1). That is, if an egg with 22 chromosomes (versus 23) is fertilized with a normal sperm. 16 ©2020 McGraw-Hill Education  The Consequences of Nondisjunction of Chromosomes during Oogenesis  Down Syndrome: An Autosomal Trisomy Down syndrome (trisomy 21)—the most common autosomal trisomy. Three copies of chromosome 21. A woman over 40 is more likely to have a Down syndrome child. Characteristics: short stature; eyelid fold; flat face; stubby fingers; a wide gap between the first and second toes; a large, fissured tongue; a round head; intellectual disability. 18 ©2020 McGraw-Hill Education  Down Syndrome 19 ©2020 McGraw-Hill Education (photo): ©CNRI/SPL/Science Source  Changes in Chromosome Structure 1 Changes in chromosome structure. Environmental agents such as radiation, organic chemicals, or viruses can cause chromosomes to break. Ordinarily, when breaks occur the two broken ends reunite. But sometimes the broken ends do not rejoin in the same pattern as before. 20 ©2020 McGraw-Hill Education  Changes in Chromosome Structure 2 Changes in chromosome structure, continued. Include deletions, duplications, inversions, and translocations of chromosome segments. Deletion—a part of a chromosome breaks off. Even when only one member of a pair of chromosomes is affected, a deletion often causes abnormalities. 21 ©2020 McGraw-Hill Education  The Various Types of Chromosomal Mutations Access the text alternative for these images 22 ©2020 McGraw-Hill Education  Changes in Chromosome Structure 3 Changes in chromosome structure, concluded. Duplication—the presence of a chromosomal segment more than once in the same chromosome. Inversion—a segment of a chromosome is inverted. Most do not present problems because all of the genes are present. 23 ©2020 McGraw-Hill Education  More Changes in Chromosome Structure Translocation—the movement of a segment from one chromosome to another nonhomologous chromosome. In 5% of cases, a translocation that occurred in a previous generation between chromosomes 21 and 14 is the cause of Down syndrome. 24 ©2020 McGraw-Hill Education  Human Syndromes 1 Deletion syndromes. Williams syndrome—chromosome 7 loses an end piece. Turned-up nose, wide mouth, small chin, and large ears. Poor academic skills but excellent verbal and musical abilities. The gene for elastin is missing; this affects the cardiovascular system and causes skin to age prematurely. 25 ©2020 McGraw-Hill Education  A Chromosomal Deletion Access the text alternative for these images 105 ©2020 McGraw-Hill Education (b): ©The Williams Syndrome Association