BASIC PLANT
GENETICS
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

RNA Forms (cont)

Transfer (tRNA)
Reads mRNA (translation)
 Brings Amino Acids to ribosomes
 Protein is synthesized

RNA Forms (cont)

Ribosomal (rRNA)
A chief component of Ribosomes
 The site of protein synthesis

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
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Understanding Basic Genetics
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