BISC 100-Lecture 22: Meiosis


Flashcards on BISC 100-Lecture 22: Meiosis, created by Chelsi Souch on 10/08/2016.
Chelsi Souch
Flashcards by Chelsi Souch, updated more than 1 year ago
Chelsi Souch
Created by Chelsi Souch over 7 years ago

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SEXUAL REPRODUCTION • Three major events Meiosis Formation of gametes Fertilization
Sexual reproduction is performed by 1. Organisms that have 2 (usually) or more sets of chromosomes 2. Through meiosis they produce gametes that harbor just one set of these chromosomes. For example in humans we find: 46 (2n) chromosomes. i.e. 23 (1n) paternal and 23 (1n) maternal i.e. 23 pairs of chromosomes
Homologous Chromosomes They both * look alike * same length, * same centromere position * same banding pattern Each pair harbors the same set of software (GENES). Each gene that is found in one chromosome is found on the other at the same position or Locus (loci) We call these forms, different Alleles of the same gene ex. Human Height (Trait) tall vs short (Alleles or forms)
THE PROBLEM SEXUALLY REPRODUCING ORGANISMS HAVE ADDRESSED IF no mechanism to HALVE the number of chromosomes (i.e. 2n n) THEN the number of chromosomes would DOUBLE at each generation 46 Chromosomes 47,104 (HUMAN) Chromosomes THE PROBLEM SEXUALLY REPRODUCING ORGANISMS HAVE ADDRESSED 10 GENERATIONS LETHAL
Meiosis involves two successive divisions Phase one: Reduction division (Halving) Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 and cytokinesis I Phase two: Similar to mitosis Prophase II Metaphase II Anaphase II Telophase 1I and Cytokinesis
Prophase 1 Chromosomes are much more thread like than in Mitosis. Why would that be? Chromosomes that are more stretched out are more accessible. Why accessibility important? During this phase homologous chromosomes pair up into a structure called tetrad and cross over one another and shuffles the chromosome parts (recombinations) Crossing over can happen several times along the length of the chromosomes • Why allow breaking and shuffling? This is an exceptionally good way of creating new combinations of alleles • After crossing over chromosomes start to condense. • Why would that be? Now transport is more important than accessibility – Genetic recombination is the production of new combinations of genes due to crossing over – Crossing over involves exchange of genetic material between homologous chromosomes – Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over – Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids
Random fertilization
Meiosis reduces the chromosome number from diploid to haploid
Events in the nucleus during meiosis II Prophase II – Chromosomes coil and become compact Meiosis reduces the chromosome number from diploid to haploid – Events in the nucleus during meiosis II – Metaphase II – Duplicated chromosomes align at the cell equator – Anaphase II – Sister chromatids separate and chromosomes move toward opposite poles – Events in the nucleus during meiosis II – Telophase II – Chromosomes have reached the poles of the cell – A nuclear envelope forms around each set of chromosomes – With cytokinesis, four haploid cells are produced
Mitosis and meiosis have important similarities and differences Which characteristics are similar for mitosis and meiosis? – One duplication of chromosomes – Which characteristics are unique to meiosis? – Two divisions of chromosomes – Pairing of homologous chromosomes – Exchange of genetic material by crossing over Mitosis and meiosis have important similarities and differences – What is the outcome of each process? – Mitosis: two genetically identical cells, with the same chromosome number as the original cell – Meiosis: four genetically different cells, with half the chromosome number of the original cell
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