BASIC PLANT GENETICS

1. BASIC PLANT GENETICS

2. 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

3. Chromosome Makeup • DNA – deoxyribonucleic acid • RNA – ribonucleic acid • Various proteins DNA replicates and transmits genetic information throughout the cell

4. 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)

6. 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

8. 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

11. 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

12. RNA Forms • Messenger (mRNA) • Copies DNA (transcription) • Carries copy out to cytoplasm • Moves to ribosomes

15. RNA Forms (cont) • Transfer (tRNA) • Reads mRNA (translation) • Brings Amino Acids to ribosomes • Protein is synthesized

17. RNA Forms (cont) • Ribosomal (rRNA) • A chief component of Ribosomes • The site of protein synthesis

19. 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).

20. 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).

22. 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”

23. GENES (cont) • Linkage • Genes move from one cell generation to the next as a unit • Linkage may be broken during meiosis

24. 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

25. 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

28. MITOSIS (cont) • Figures 1 through 6 illustrate the stages of mitosis in onion (Allium cepa) as viewed with light microscopy.

30. 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

31. MEIOSIS • Definition: Nuclear division in which chromosomes are doubled and then divided twice

32. 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

37. 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

39. 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

40. MUTATIONS • Definition: a sudden, heritable change appearing in an individual as the result of a change in genes or chromosomes

41. 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

42. MUTATIONS (cont) • Hereditary modifications from chromosome number or structure change • Doubling of chromosomes • Addition or subtraction of chromosomes • Structural change in chromosome

43. 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)

44. 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/