BCH1001 Genetics 1- Cell Division PDF

Biochemistry & Cell Biology BCH1001 Genetics 1 - Cell Division Dr Patricia Johnson [email protected]  Re-Cap Chromosomes contain a long double helix of deoxyribonucleic acid (DNA) wrapped around proteins. DNA carries the cell’s genetic info...

Biochemistry & Cell Biology BCH1001 Genetics 1 - Cell Division Dr Patricia Johnson [email protected]  Re-Cap Chromosomes contain a long double helix of deoxyribonucleic acid (DNA) wrapped around proteins. DNA carries the cell’s genetic information A gene is a section of DNA that infl uences one or more hereditary traits in an individual. Different versions of a specifi c gene are called alleles  Alleles: versions of the same gene A locus is the specifi c physical location of a gene or other DNA sequence on a chromosome, like a genetic street address Alleles and Genes https://www.youtube.com/watch?v=pv3Kj0 UjiLE  One of a pair of chromosomes with the same gene sequence in the same locations (loci), chromosomal length and centromere location are homologous chromosomes, or homologs.  The karyotype is the number and types of chromosomes present in an organism  A normal human karyotype contains 46 chromosomes (23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes), generally arranged in order from largest to smallest  By convention, the letter n stands for the number of distinct types of chromosomes in a given cell and is called the haploid number Cell Division  2 types of cell division  Mitosis  Meiosis  Mitosis occurs in majority of cells in the body, during cell growth and repair :  cells are constantly dividing : some cells divide ~ 24hrs other take much longer and some (brain cells) never divide  Meiosis only occurs in germ cells (reproductive cells) – cells of testis and ovary  Each cell contains 46 chromosomes, need to half this number and get 23 from each parent  Mitosis (2n - 2n)  Mitosis is divided into number of different phases: interphase, prophase, metaphase, anaphase, telophase  Cells mostly in interphase, once mitosis starts chromosome become distinct : (metaphase)  Each chromosome replicates to form pair of sister chromatids, held together by centromere  Centromeres divide and sister chromatids separate, one of each kind goes to a daughter cell  Mitosis produces 2 identical daughter cells with the same DNA content as parent cells, referred to as 2n or diploid Mitosis link…. http://www.youtube.com/watch?v=VlN7K1-9QB0  Meiosis  The purpose of meiosis is to shuffl e genetic information and halve the cellular chromosome number from 46 chromosomes to 23 chromosomes  In this way, when an egg and sperm cell combine during fertilization, the resulting embryo will inherit the appropriate amount of unique genetic information from each parent  Complex process as cells fi rst duplicate chromosomes, and then undergoes 2 divisions, meiosis I and meiosis II  One diploid cell will end up with 4 daughter cells, which contain only half the DNA content of their parents, 1n (haploid)  Meiosis not complete until fertilization occurs and results in a diploid individual  During Meiosis I chromosomes come together and line up side by side and exchange genetic material – cross- over  This gives rise to a different combination of genes on each chromatid and daughter cells are not identical Meiosis links http://www.youtube.com/watch?v=nWYpKD8CeRs&list=PLC43 C129B774EDC97 https://www.youtube.com/watch?v=VzDMG7ke69g Mitosis Vs Meiosis Mitosis Meiosis Somatic cells (any cell Germline cells other than a reproductive cell) (reproductive cells) 2 daughter cells 4 daughter cells Chromosomes Half no. of parental identical to parents chromosomes Two of each One of each chromosome, chromosome, Diploid Haploid Errors in Meiosis - Nondisjunction  Nondisjunction results when both homologues of the same chromosome pair going to the same pole rather than opposite poles  Generally occurs during the anaphase of cell division  One gamete has 2 copies of one chromosome and the other has no copy of that chromosome  The other chromosomes are  Abnormalities in sex  chromosomes Change in number of sex chromosomes results in many different disorders e.g.  Turner : female with XO (partly or completely missing an X chromosome)  Random, Occurs in 1 in 25,000 births, short, do not undergo puberty, usually normal intelligence and infertile even if given hormone treatment, prone to kidney and heart problems  Klinefelter : male with XXY  Occurs in 1 in 500 male births, sterile, underdeveloped sexually, may develop breast, language impairment, gender identity disorder  Triple-X : female with XXX or (XXXX)  Occurs in 1 in 1,500 births, most have no apparent abnormalities, may show early onset of menopause, learning delays, language impairment  Jacob : male with XYY  Occurs in 1 in 1,000 births, usually taller than average, tend to have speech and reading problems Change in autosome number  An individual with the appropriate number of chromosomes for their species is called euploid; in humans = 22 pairs of autosomes and one pair of sex chromosomes  An individual with an error in chromosome number is described as aneuploid, a term that includes monosomy (loss of one chromosome) or trisomy (gain of an extraneous chromosome)  Monosomic human zygotes missing any one copy of an autosome invariably fail to develop to birth because they have only one copy of essential genes  Change in autosome number  Trisomy : 3 copies of particular chromosome only 3  Down syndrome: trisomy 21 (usually from egg) affects 1:700 trisomies survive to birth rest usually die in utero children, characteristic physical effects (short stature, eyelid fold, palm crease, fi ssured tongue), learning diffi culties which may be severe (Gart gene)  Edward, trisomy 18 : affects 1:10,000 live births, almost every organ system affected, children generally do not live more than a few months  Patau, trisomy 13 : affects 1:5000 live births, serious eye, brain, circulatory defects as well as cleft palate, children rarely live more than a few months