Presentation on theme: "BASIC PLANT GENETICS. Structures Controlling Inheritance Chromosomes - contained in nucleus - carry most of the genetic information - number/cell usually."— Presentation transcript:
Structures Controlling Inheritance Chromosomes - contained in nucleus - carry most of the genetic information - number/cell usually 2n, or diploid - sex cells are 1n, or haploid - chromosome numbers are known for most plant species
Chromosome Makeup DNA – deoxyribonucleic acid RNA – ribonucleic acid Various proteins DNA replicates and transmits genetic information throughout the cell
DNA Makeup Nucleotide A DNA/RNA subunit made up of a sugar, a phosphate, and a nitrogenous base Repeating nucleotides vary by which base is used DNA bases are: Cytosine(C), Guanine(G), Adenine(A), Thymine(T)
DNA Makeup (cont) Two spiral strands comprise DNA “Backbone” of strands is the repeating sugar-phosphate linkage (identical) Bases attach to sugars (vary) Phosphate-sugar-base = nucleotide Nucleotides of two strands are joined at bases by hydrogen bonds Bases are specific for bonding
Base Bonding Adenine with Thymine Cytosine with Guanine Referred to as “complementary pairs” Because the hydrogen bond between bases is relatively weak, DNA can “unzip” at this point to facilitate replication
RNA Makeup Similar to DNA Makeup Important variations: RNA is a single strand Sugar is Ribose Base Uracil replaces Thymine RNA is complementary to DNA Three forms of RNA: Messenger, Transfer, Ribosomal
RNA Forms Messenger (mRNA) Copies DNA (transcription) Carries copy out to cytoplasm Moves to ribosomes
GENES Gene definitions gene: (cistron) Structurally, a basic unit of hereditary material; an ordered sequence of nucleotide bases that encodes one polypeptide chain (via mRNA).
GENES (cont) The gene includes, however, regions preceding and following the coding region (leader and trailer) as well as (in eukaryotes) intervening sequences (introns) between individual coding segments (exons).
GENES (cont) Gene “facts” Part of chromosome Determine characteristics Too small to be seen Thousands per plant cell Some act independent, some together Genes on the same chromosome are said to be “linked”
GENES (cont) Linkage Genes move from one cell generation to the next as a unit Linkage may be broken during meiosis
HOMOLOGOUS CHROMOSOMES Definition: Chromosome pairs that have alleles for the same genes Alleles occupy the same position (loci) on homologous chromosomes and affect the same trait Genes may have two or more alleles Allelic genes can be dominant or recessive to each other
MITOSIS Definition: The process of nuclear division in which chromosomes are first duplicated, followed by the separation of daughter chromosomes into two genetically identical nuclei
MITOSIS (cont) Figures 1 through 6 illustrate the stages of mitosis in onion (Allium cepa) as viewed with light microscopy.
Microtubule dynamics in mitosis & cytokinesis A Interphase: Cortical microtubules are aligned within the inside of the cell wall. B Pre-prophase: Microtubules form a band around the equatorial region of the cell marking the plane of future cytokinesis. C Prophase: Cytoplasmic microtubules disappear and a mitotic spindle of microtubules is formed. D Metaphase: Chromosomes are aligned on an equatorial plate of the spindle. E Anaphase: Interzonal fibers extend from one pole to the other. Chromatids are moved to opposite poles of the spindle. Dictyosomes (two are shown) begin to produce vesicles which will fuse laterally initiating the cell plate. F Telophase: The spindle microtubules disappear and a phragmoplast is formed whose component microtubules are concentrated at the periphery of the cell plate which grows centrifugally towards the parent cell wall. The phragmoplast microtubules remain at the edge of the cell plate until it reaches the parent cell wall, and then they disappear. Illustration from: Ledbetter & Porter, 1970, Introduction to the Fine Structure of Plant Cells, Springer-Verlag
MITOSIS (cont) Results of Mitosis: Vegetative cells usually contain two sets of homologous chromosomes – the 2n or diploid number Daughter cells are genetically identical to the mother cell
MEIOSIS Definition: Nuclear division in which chromosomes are doubled and then divided twice
MEIOSIS (cont) Meiosis facts: occurs in the flower in plants, meiosis forms spores In angiosperms, forms pollen and egg the daughter nuclei from meiosis have half the number of chromosomes of the parent nucleus (1n or haploid) Crossing over of homologous chromosomes can occur
FERTILIZATION Definition: the fusion of sperm and egg in sexual reproduction Definition (Angiosperms): Double Fertilization: the process by which one sperm cell fertilizes the egg to form a zygote and another sperm cell fertilizes the polar nuclei to form a primary endosperm nucleus
Double Fertilization (cont) Double fertilization: One of the two sperm nuclei fertilizes the egg cell; the other fertilizes the central cell Pollen tube discharges sperm into one of the synergids A typical picture of the double fertilization process. The pollen tube (pt) enters through the micropyle, one of the synergids (s) discharges its contents. Then the sperm nuclei traverse the synergid, one enters the egg cell (e) and the other enters the central cell (cc). Thereafter, their nuclei (unlabeled arrows) fuse with the egg nucleus (yellow) and the polar nuclei (red). ii = inner integument; n = nucellus
MUTATIONS Definition: a sudden, heritable change appearing in an individual as the result of a change in genes or chromosomes
MUTATIONS (cont) Mutation statements: Mutations can and do occur during replication of DNA Altered genes may result in changes in plant characteristics Most mutations go unnoticed Many mutations are subtly harmful Some provide a source of genetic variability for developing new cultivars
MUTATIONS (cont) Hereditary modifications from chromosome number or structure change Doubling of chromosomes Addition or subtraction of chromosomes Structural change in chromosome
POLYPLOIDY Plant has more than two sets of homologous chromosomes in their vegetative cells Normal Diploid (2n) Triploid (3n) Tetraploid (4n) etc. Common in cultivated crops like oats, wheat, and tobacco (Table 14-1, text)
CLASSICAL GENETICS View video: Understanding Basic Genetics Complete Study Guide Complete Lessons 1, 2, 3, 4, and 5 from your DNA websight http://www.dnaftb.org/dnaftb/
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