Biology Notes: Chromosomes, Meiosis and the Production of Sex Cells

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1 Biology Notes: Chromosomes, Meiosis and the Production of Sex Cells Introduction to Meiosis Exclusively for reproduction purposes and occurs only once Occurs only in reproductive tissue germinal epithelial cells of gonads- testes/ ovaries of animals and anthers/ ovaries of plants Give rise to sex cells called gametes Allows important process of REDUCTION in number of chromosomes to prevent doubling up effect of fertilization Therefore 46 chromosomes are halved to 23- when the gametes fuse 23+23= 46 This chromosomal reduction happens before sexual reproduction DNA molecules only form an X once they have replicated Haploid and Diploid When chromosomes are halved, they are haploid When chromosomes are doubled they are diploid Therefore egg and sperm are haploid and when they fuse to a zygote they are diploid The Paternal and Maternal Chromosomes In a person s karyotype, each pair has a paternal chromatid and a maternal chromatid The daughter cells that come out of Meiosis must have one chromosome from each pair of chromosomes of the 46 This means the offspring will have a mixture of the parents maternal/ paternal chromosomes The selection of which is random, but one must be selected from each pair The Location and Structure of Chromosomes Chromatin network in nucleus Threadlike structures made up of DNA strands wound around histones Somatic cells (cells of the body) have the same chromosome number They occur in 2 sets- maternal/ paternal- each paternal has a matching maternal o =Homologous pair- similar size/ shape/ same genes in same place (locus) Somatic cells are therefore called diploid of 2n as they have 2 sets of 23 chromosomes When cells divide, threads of the chromatin network become shorter/ thicker- visible

2 So, what sets Meiosis Apart? occurs in the germinal epithelial cells within GONADS(reproductive structures)= gametes this reduction division happens ONLY ONCE: result is sperm/ egg or pollen grain/ egg cell Chromosome Number and Genes somatic cells are diploid because they have 2 sets of chromosomes human diploid cells have 46 chromosomes gametes have only one set of chromosomes= haploid prevents doubling up effect of fertilization in fertilization the gametes combine to form a zygote which is diploid Meiosis= gametes/ sex cells Interphase Precedes Meiosis it is a preparation phase which allows nuclear division During interphase, DNA replication occurs Two strands called chromatids are joined by a centromere

3 The Difference between Mitosis and Meiosis MITOSIS Occurs in somatic cells - Development of zygote to embryo - Growing regions (bone, skin, damaged area) Produced two identical daughter cells Needed for growth/ repair I-P-M-A-T Differences: One cell division Two daughter cells same number chromosomes Somatic cells formed genetically similar to parent cell MEIOSIS Occurs in reproductive organs Testes/ ovaries (animal) anthers/ovaries (plant) Occurs only once Diploid to haploid (half number of chromosomes-prevent doubling up) Produces gametes(animal) or spore (plant) Enables fertilization to occur Produces four daughter cells that have genetic variation Has 2 stages - 1= reduction to 2 haploid cells. 2= reduction to 4 haploid cells Two cell divisions Four daughter cells formed-haploid Gametes are formed genetically different to parent cell The Process of Meiosis Meiosis 1: reduction division which results in two cells being formed each with half the number of chromosomes of parent cell (ie: haploid) Meiosis 2: copying division which involves two haploid cells each dividing by mitosis to from four haploid cells STAGE ONE Interphase- the DNA replicates and is in its replicated state- one chromosome replicates to two sister chromatids joined by a centromere. So 46 chromosomes goes to 46 pairs of 92 chromatids 1. Early Prophase a. Chromosomes become visible- threads of chromatin network short/ thick b. Chromosomes of homologous pairs lie alongside each other c. They form Bivalents- a pair of homologous chromosomes that have lined up together-( 4 sister chromatids) 2. Late Prophase a. Spindle forms between poles b. Nuclear membrane disappears

4 3. Crossing Over a. Chromatids touch and swap genes at points called chiasmata (1= chiasma) b. Chromosomes of the homologous pairs join and exchange segments- genetic variation 4. Metaphase a. Homologous pairs line up on equator- random assortment occurs 5. Anaphase a. Spindle fibres contract and pull the homologous pairs (Bivalents) apart- one chromosome of each pair moves to an opposite pole 6. Telophase a. One complete set of chromosomes reaches each pole and two new cells form b. Each cell has one set of chromosomes= haploid STAGE TWO OF MEIOSIS 1. Prophase a. Each chromosomes has 2 chromatids joined by a centromere b. Spindles begin to form between the poles in each cell 2. Metaphase a. Chromosomes line on equator 3. Anaphase a. The centromeres split b. Spindles pull the chromatids of each chromosome apart c. The chromatids move towards the poles 4. Telophase a. The chromatids (now called daughter chromosomes as they are individuals) reach the poles b. Nuclear membrane reforms around each new nucleus 5. Cytokinesis (splitting of cytoplasm) occurs at the end of Telophase a. 4 daughter cells with a haploid amount of chromosomes LEARNS COMPLIMENTARY DIAGRAMS

5 Why is Meiosis Important? 1. Chromosome number is halved- prevents chromosome number from doubling each generation 2. Meiosis ensure genetic variation a. Crossing over: exchange of genetic info ensures gametes carry different gene combos therefore different offspring from same parent b. Segments of chromatids of homologous chromosomes are exchanged c. Chiasmata develop at a point where 2 pairs cross- mixing of genes d. Metaphase 1: homologous chromosome pairs are arranged randomly on equator- Random Assortment= variation of genes, the Law Of Independent Assortment e. Metaphase 2: chromatids line up randomly of equator- random assortment 3. Therefore, crossing over, metaphase 1 and 2 = genetic variation Human Karyotypes Show number, size and shape of chromosomes during metaphase of mitosis Complete diploid set of chromosomes within one cell of the individual From nuclei of white blood cells Treated with drug that arrests mitosis in metaphase- visible chromosomes Dyed/ photographer and a computer arranged them in homologous pairs 22 pairs of autosomes(ordinary body chromosomes) one pair of gonosomes (sex chromosome- XX f XY m) GENOME: complete set of genes on all the chromosomes of an organism- all the genetic info

6 Chromosome Non- Disjunction Homologues fail to separate during Anaphase 1 or sister chromatids fail to separate during Anaphase 2 Result: one gamete has 2 copies of 1 chromosome while the other has no copy of that chromosome If either of these gametes unite with another in fertilization= aneuploidy (abnormal chromosome number) Trisomic cell: has one extra chromosome (2n+1) eg trisomy 21 Polyploidy: 3 homologous chromosomes instead of 2 Monosomic cell: one missing chromosome (2n- 1) usually lethal LOOK AT CHROMOSOME NON- DISJUNCTION DIAGRAM Chromosome Alteration Down syndrome (trisomy 21) Extra copy of chromosome 21, 47: xx/xy: 21+ 1:700 Alters phenotype drastically/ moderately Characteristic facial features/ short stature/ heart defects/ mental retardation Respiratory disease/ shorter lifespan Prone to Alzheimer s/ leukaemia Sexually under developed/ sterile Alterations in Chromosome Structure- When Chromosomes BREAK! 1. Deletion- portion of chromosome lost in cell division- missing genes Eg Cri du chat 2. Duplication- a fragment joins the homologous chromosome- repeated genes Eg Fragile X 3. Translocation- fragment of chromosome is moved from one to another- alters phenotype as genes are placed in new environment

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