Mitosis Meiosis Making Copies and Making Babies Meiosis
Mitosis & Meiosis Making Copies and Making Babies
Meiosis �What would happen if sexually reproducing organisms didn’t reduce their chromosome number prior to conception? �Parent with 1 chromosome would have offspring with 2 chromosomes �Next generation would have 4 chromosomes, next would have 8, then 16, then 32, 64, 128, 256, 512…
Meiosis �Gamete cells (sperm & egg) have only half the chromosomes of their parent �Somatic cells (not gametes) are diploid � 2 copies of each chromosome (one from each parent) �called “homologous” chromosomes
Homologous Chromosomes �have genes for the same traits in the same location �The version of those genes may be different (e. g. red hair from mom and brown from dad) but still determine the same trait (hair colour)
Homologous Chromosomes homologous chromosomes centromere sister chromatids
Homologous Chromosomes �position of a gene on a chromosome is called its locus (plural = loci) �chromosomes of a homologous pair carry genes for the same trait at the same locus
Genes & Alleles �genes for a specific trait on each homologous chromosome might not be the same �different forms of the same gene are called alleles
Diploid vs. Haploid 2 copies of each chromosome (one from mom and one from dad) Only 1 copy of each chromosome i. e. gametes Meiosis creates haploid gametes
Stages of Meiosis �similar to mitosis but some significant differences �broken down into Meiosis I and Meiosis II �In Meiosis I, cells become haploid �In Meiosis II the sister chromatids separate into haploid daughter cells
Before Meiosis I �During interphase the cell replicates its genetic information (just like in mitosis) �This is referred to as the pre-meiotic S phase �replicated chromosomes are joined by a centromere to which the spindle fibers attach
Meiosis I �Meiosis I is broken down into the following stages �Prophase I �Metaphase I �Anaphase I �Telophase I & Cytokinesis
Meiosis II �Meiosis II is broken down into the following stages �Prophase II �Metaphase II �Anaphase II �Telophase II & Cytokinesis
Prophase I Similarity to mitosis: � the chromosomes condense & the nuclear membrane dissolves
Prophase I Different from mitosis: � homologous chromosomes pair up �known as “synapsis” �forms a tetrad �Non-sister chromatids exchange pieces of themselves in a process called “crossing-over” or genetic recombination �allows for genetic variation to exist in a species
Crossing Over
Metaphase I �homologous chromosomes are attached �this pair moves together to the metaphase plate at the equator of the cell
Anaphase I �homologous chromosomes are each pulled by their centromere to opposite poles of the cell �sister chromatids remain attached
Telophase I & Cytokinesis �Homologs are now separate �Each pole is now haploid �Each chromosome still has 2 chromatids �Cytokinesis occurs simultaneously �result is two haploid daughter cells, each with half the number of chromosomes of the parent cell
Interphase? �Some cells may have no interphase after meiosis I �Some cells may have a short one �Some cells may have a very long period of dormancy before meiosis II occurs �In any case, there is NO replication of DNA! (No Sphase)
Prophase II �spindle begins to appear again �Spindle fibers attach to chromosomes at their centromere and begin to move to the metaphase plate
Metaphase II �chromosomes line up on the metaphase plate or equator (Just like mitosis!)
Anaphase II �Centromeres of sister chromatids finally separate �sister chromatids of each pair are now independent chromosomes sister chromatids move toward opposite poles
Telophase II & Cytokinesis �Nuclei begin to form at opposite poles of the cells �Chromosomes unwind and elongate �Cytokinesis occurs simultaneously �End result of the entire process is 4 daughter cells with half (haploid) number of chromosomes
Meiosis Animation http: //www. youtube. com/watch? v=D 1_m. QS_FZ 0&feature=related
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