Chapter 18 Cell Division Chromosome Inheritance Chromosomes Humans have 46 chromosomes that are in 23 pairs in a cell s nucleus o Pairs of chromosomes are called homologous chromosomes o Autosomes are the 22 pairs of chromosomes that control traits unrelated to gender somatic o Sex chromosomes are the 1 pair that contains the genes that control gender Somatic body cells have 46 2n paired chromosomes and are called diploid Sex cells have only 23 n unpaired chromosomes and are called haploid Karyotype Chromosomal pattern present in nucleus Each chromosome is composed of 2 sister chromatids each containing 1 DNA molecule 1 chromosome 2 DNA molecules Y determines de gender Cell cycle and how it s controlled Cell cycle has 2 parts 1 Interphase o G1 stage cell doubles its organelles cell grows in size o S stage DNA replication occurs o G2 stage proteins needed for division are synthesized 2 Cell division o Mitosis karyokinesis nuclear division o Cytokinesis cytoplasmic division The cell cycle control Checkpoints can delay the cell cycle until certain conditions are met There are 3 major checkpoints o G1 if passed the cell is committed to divide o G2 cycle pauses to verify DNA replication o M cycle pauses to verify spindle assembly and chromosome attachment Chromosome structure in mitosis Chromosomes contain both DNA and proteins collectively called chromatin Chromosomes that are dividing are made up of 2 identical parts called sister chromatids The sister chromatids are held together at a region called the centromere The spindle in mitosis Centrosome the microtubule organizing center of the cell Centrioles short cylinders of microtubules that assist in the formation of spindle fibers Aster an array of microtubules at one of the poles ends of the cell Mitosis A diploid cell makes and divides an exact copy of its nucleus It is used in cell growth and cell repair Mitosis occurs in body cells It comprises 4 phases 1 Prophase 2 Metaphase 3 Anaphase 4 Telophase 1 Prophase 2 Metaphase 3 Anaphase Chromosomes condense and become visible The nuclear envelope fragments The nucleolus disappears Centrosomes move to opposite poles Spindle fibers appear and attach to the centromeres Chromosomes line up at the middle of the cell equator Spindle becomes fully formed Sister chromatids separate at the centromeres and move towards the poles 4 Telophase and cytokinesis Chromosomes arrive at the poles Chromosomes become indistinct chromatin again Nucleoli reappear Spindle disappears Nuclear envelope reassembles 2 daughter cells are formed by a ring of actin filaments cleavage furrow Meiosis 2 nuclear divisions occur to make 4 haploid cells It is used to make gametes female egg male sperm Occurs in sex cells Has 8 phases 4 in each meiosis I II Meiosis I from homologues chromosomes o Prophase I Homologous chromosomes pair synapsis undergo crossing over with each other i e exchange of genetic information Homologous pairs line up at the equator o Metaphase I o Anaphase I o Telophase I Homologous chromosomes separate and move toward opposite poles 2 daughter cells result each with 23 duplicated chromosomes What is crossing over Crossing over is the exchange of genetic information between non homologous sister chromatids during synapsis This occurs during prophase I of meiosis and increases genetic variation Meiosis II Prophase II Metaphase II Anaphase II Telophase II Chromosomes condense again Chromosomes align at the equator Sister chromatids separate to opposite poles 4 daughter cells result each with 23 unduplicated chromosomes The production of sperm and egg Spermatogenesis The process of making sperm in males A continual process after puberty About 400 million sperm are produced per day Oogenesis The process of making eggs in females During meiosis 1 egg and 3 polar bodies are formed Polar bodies act to hold discarded chromosomes and thus disintegrate Normally 1 egg per month is produced and 500 are produced during the entire reproductive cycle Mitosis vs Meiosis Growth and repair of cells Occurs in body cells Results in 2 diploid genetically identical 1 division cells Meiosis II is similar to Mitosis because the y both separate sister chromatids No crossing over no genetic variability Changes in chromosome number Nondisjunction Formation of gametes Occurs in sex cells Results in 4 haploid genetically 2 divisions different cells Meiosis I homologues cells separate Meiosis II sister chromatids separate Occurs when both members of a homologous pair go into the same daughter cell during meiosis I OR when sister chromatids fail to separate in meiosis II Results of nondisjunction could be mortal Monosomy cell has only 1 copy of a chromosome e g Turner syndrome only 1 X chromosome e g Down syndrome 3 copies of chromosome 21 Trisomy cell has 3 copies of a chromosome Change in sex chromosome number Turner syndrome XO short stature broad shouldered with folds of skin on the neck underdeveloped sex organs and breasts Klinefelter syndrome XXY underdeveloped sex organs breast development large hands and long arms and legs Poly X females XXX XXXX XXX tend to be tall and thin but do not usually have mental XXXX have severe mental retardation Jacobs syndrome XYY tall persistent acne speech and reading retardation problems Barr bodies and X inactivation protein production in females Females have 2 copies of X genes males have 1 Thus each gene in females would lead to double So in female embryos 1 X chromosome is randomly inactivated condensed Barr body in each cell Changes in chromosome structure Deletion loss of a piece of the chromosome e g Williams syndrome Translocation movement of chromosome segments from one chromosome to another nonhomologous chromosome Alagille syndrome Duplication presence of a chromosome segment more than once in the same chromosome Inversion a segment of a chromosome is inverted 180 degrees
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