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

NucleotideA DNA/RNA subunit made up of a sugar, a

phosphate, and a nitrogenous baseRepeating nucleotides vary by which base

is usedDNA 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 = nucleotideNucleotides of two strands are joined

at bases by hydrogen bondsBases 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 strandSugar is RiboseBase Uracil replaces ThymineRNA is complementary to DNAThree forms of RNA:

Messenger, Transfer, Ribosomal

RNA Forms

Messenger (mRNA)

Copies DNA (transcription)Carries copy out to cytoplasmMoves to ribosomes

RNA Forms (cont)

Transfer (tRNA)

Reads mRNA (translation)Brings Amino Acids to ribosomesProtein is synthesized

RNA Forms (cont)

Ribosomal (rRNA)

A chief component of RibosomesThe 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 chromosomeDetermine characteristicsToo small to be seenThousands per plant cellSome act independent, some togetherGenes on the same chromosome are

said to be “linked”

GENES (cont)

LinkageGenes move from one cell generation

to the next as a unitLinkage may be broken during

meiosis

HOMOLOGOUS CHROMOSOMES

Definition: Chromosome pairs that have alleles for the same genesAlleles occupy the same position (loci)

on homologous chromosomes and affect the same trait

Genes may have two or more allelesAllelic 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 flowerin plants, meiosis forms sporesIn angiosperms, forms pollen and eggthe 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 DNAAltered genes may result in changes

in plant characteristicsMost mutations go unnoticedMany mutations are subtly harmfulSome provide a source of genetic

variability for developing new cultivars

MUTATIONS (cont)

Hereditary modifications from chromosome number or structure changeDoubling of chromosomesAddition or subtraction of

chromosomesStructural change in chromosome

POLYPLOIDY

Plant has more than two sets of homologous chromosomes in their vegetative cellsNormal 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

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