* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Genes, Chromosomes, and Numbers
Epigenetics of human development wikipedia , lookup
Genomic imprinting wikipedia , lookup
Vectors in gene therapy wikipedia , lookup
Polycomb Group Proteins and Cancer wikipedia , lookup
Designer baby wikipedia , lookup
Genome (book) wikipedia , lookup
Y chromosome wikipedia , lookup
Hybrid (biology) wikipedia , lookup
Microevolution wikipedia , lookup
X-inactivation wikipedia , lookup
Genes, Chromosomes, and Numbers • Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes. • Typically, a chromosome can contain a thousand or more genes along its length. Website upload 2014 1 Diploid and haploid cells • In the body cells of animals and most plants, chromosomes occur in pairs. • A cell with two of each kind of chromosome is called a diploid cell and is said to contain a diploid, or 2n, number of chromosomes. (46 in humans) • Organisms produce gametes that contain one of each kind of chromosome. • A cell containing one of each kind of chromosome is called a haploid cell and is said to contain a haploid, or n, number of chromosomes. (23 in humans) Website upload 2014 2 Karyotype • A karyotype is a photographic inventory of an individual’s chromosomes. • A human female karyotype is shown below. Website upload 2014 3 diploid = 2 copies 2n Human female karyotype 46 chromosomes 23 pairs XX Website upload 2014 4 diploid = 2 copies 2n Human male karyotype 46 chromosomes 23 pairs XY Website upload 2014 5 Homologous chromosomes • The two chromosomes of each pair in a diploid cell are called homologous chromosomes. • Each pair of homologous chromosomes has genes for the same traits. Sister chromatids Website upload 2014 6 Paired chromosomes • Homologous chromosomes – both chromosomes of a pair carry “matching” genes • control same inherited characters • homologous = same information diploid eye color eye color 2n (brown?) (blue?) 2n = 4 homologous double stranded homologous chromosomes Website upload 2014 7 chromosomes Cell division / Asexual reproduction • Mitosis – produce cells with same information • identical daughter cells – exact copies • clones – same number of chromosomes • same genetic information Website upload 2014 Aaaargh! I’m seeing double! 8 Asexual reproduction • Single-celled eukaryotes – yeast – Paramecium – Amoeba • Simple multicellular eukaryotes – Hydra • budding What are the disadvantages of asexual reproduction? What are the Website upload 2014 advantages? 9 How about the rest of us? • What if a complex multicellular organism (like us) wants to reproduce? – joining of egg + sperm • Do we make egg & sperm by mitosis? No! What if we did, then... 46 egg + 46 92 sperm zygote Website upload 2014 Doesn’t work! 10 How do we make sperm & eggs? • Must reduce 46 chromosomes 23 – must half the number of chromosomes – haploid 46 23 meiosis egg 46 zygote 23 46 23 23 fertilization sperm Website upload 2014 gametes 11 Meiosis makes sperm & eggs • 46 chromosomes to 23 chromosomes – half the number of chromosomes 23 46 meiosis 46 diploid egg 23 Website upload 2014 sperm haploid12 Why Meiosis? • When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells. • Imagine if mitosis were the only means of cell division. • IF the parent organism has 14 chromosomes, it would produce gametes that contained a complete set of 14 chromosomes. • The offspring would have cell nuclei with 28 chromosomes, and the next generation would have cell nuclei with 56 chromosomes, and so on. Website upload 2014 13 Why Meiosis? • There must be another form of cell division that allows offspring to have the same number of chromosomes as their parents. • This kind of cell division, which produces gametes containing half the number of chromosomes as a parent’s body cell, is called meiosis. • Meiosis reduces the chromosome number from diploid to haploid. Website upload 2014 14 Why Meiosis? • Meiosis consists of two separate divisions, known as meiosis I and meiosis II. • Meiosis I begins with one diploid (2n) cell. • By the end of meiosis II, there are four haploid (n) cells. • These haploid cells are called sex cells— gametes. Website upload 2014 15 Why Meiosis? • Male gametes are called sperm. • Female gametes are called eggs. • When a sperm fertilizes an egg, the resulting zygote once again has the diploid number of chromosomes. • This pattern of reproduction, involving the production and subsequent fusion of haploid sex cells, is called sexual reproduction Website upload 2014 16 The Phases of Meiosis (I) Interphase Interphase • The cell replicates its chromosomes. • After replication, each chromosome consists of two identical sister chromatids, held together by a centromere. Website upload 2014 17 The Phases of Meiosis (I) Prophase I Prophase I • The chromosomes coil up and a spindle forms. • As the chromosomes coil, homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad. Website upload 2014 18 The Phases of Meiosis (I) Prophase I Prophase I • The chromatids in a tetrad pair tightly • In fact, they pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process known as crossing over. Website upload 2014 19 The Phases of Meiosis (I) Prophase I •Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time. •Genetic recombination results from crossing over during prophase I of meiosis. –This increases variation further Website upload 2014 20 The Phases of Meiosis (I) Metaphase I Metaphase I • During metaphase I, the centromere of each chromosome becomes attached to a spindle fiber. • The spindle fibers pull the tetrads into the middle. Website upload 2014 21 The Phases of Meiosis (I) Anaphase I Anaphase I • begins as homologous chromosomes, each with its two chromatids, separate and move to opposite ends of the cell. • This critical step ensures that each new cell will receive only one chromosome from each homologous pair. Website upload 2014 22 The Phases of Meiosis (I) Telophase I • Telophase I • Events occur in the reverse order from the events of prophase I. • The spindle is broken down, the chromosomes uncoil, and the cytoplasm divides to yield two new cells. Website upload 2014 23 Meiosis 1 overview • 1st division of meiosis Copy DNA during Divide 1 Line Up 1 interphase prophase I metaphase I 4 chromosomes diploid 2n gamete double stranded telophase I Website upload 2014 2 chromosomes haploid 24 1n The Phases of Meiosis (II) • The phases of meiosis II • The second division in meiosis is simply a mitotic division of the products of meiosis I. • During prophase II, a spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes. Website upload 2014 Prophase II 25 The Phases of Meiosis (II) Metaphase II • Metaphase II • The chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator. Website upload 2014 26 The Phases of Meiosis (II) • Anaphase II • Begins as the centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles. Website upload 2014 Anaphase II 27 The Phases of Meiosis (II) • Telophase II • Finally nuclei, reform, the spindles break down, and the cytoplasm divides. • At the end of meiosis II, four haploid cells have been formed from one diploid cell. Telophase II • These haploid cells will become gametes, transmitting the genes they contain to offspring. Website upload 2014 28 The Phases of Meiosis • At the end of meiosis II, four haploid cells have been formed from one diploid cell. • These haploid cells will become gametes, transmitting the genes they contain to offspring. Website upload 2014 29 Meiosis II overview Bye Bye 2 telophase II telophase I Line Up 2 • 2nd division of meiosis – looks like mitosis 2 chromosomes haploid 1n metaphase II gametes Website upload 2014 4 30 Meiosis = reduction division • Meiosis – special cell division in sexually reproducing organisms – reduce number of chromosomes • 2n 1n • diploid haploid –half – makes gametes • sperm, eggs Website upload 2014 31 Meiosis & mitosis • Meiosis to make gametes – sperm & egg • Mitosis to make copies of cells – growth – repair – development Website upload 2014 32 Comparison of Mitosis & Meiosis • MITOSIS MEIOSIS Website upload 2014 33 Sexual reproduction lifecycle 2 copies diploid 2n 1 copy fertilization haploid 1n We’re mixing things up here! meiosis 1 copy haploid 1n A good thing? Website upload 2014 34 Putting it all together… meiosis fertilization mitosis + development gametes 46 23 meiosis egg 46 23 23 23 zygote fertilization sperm 46 46 46 46 4646 46 46 46 Website upload 2014 mitosis mitosis & development 35 Meiosis Provides Genetic Variation • Cells that are formed by mitosis are identical to each other and to the parent cell. • Crossing over during meiosis I, however, provides a way to rearrange gene combinations. • Thus, variability is increased. • Random arrangement of chromosomes and the genetic information they carry is called genetic recombination. Website upload 2014 36 What are the advantages of asexual reproduction? What are the DISadvantages of asexual reproduction? Any Questions?? What are the advantages of sexual reproduction? Website upload 2014 What are the DISadvantages of sexual reproduction? 37