By Wayne Handlos, Ph.D.
< Chloroplast DNA in Pelargonium x hortorum
^ Cells of Elodea with chloroplasts
It is the season of wonder – and should we be surprised that our beloved zonal geraniums should add
to the wonders of the season? In addition to providing wreath-like images (of the chloroplast DNA) we find that the
zonal cultivar ‘Irene’ has the largest of these wreath-like structures among all the green plants examined so far.
What am I writing about??? All green plants are green because of the presence of the chemical compound called
chlorophyll. (This compound comes in several forms, which I will ignore here.) At the heart of the chlorophyll molecule
is the element magnesium. (Do you ever give your plants a dose of Epsom salts? If so, you
are providing them with magnesium.) Chlorophyll is one of the compounds found in plants, which is involved in the
process of photosynthesis. Photosynthesis is the process by which energy from light (usually sunlight) is used to
break down water, release oxygen gas and combine carbon dioxide and hydrogen to synthesize simple sugars (in
which more energy from sunlight is stored). Simple sugars are the basis for all food on the face of the Earth.
(Have you thanked your green plants today for the oxygen and food they have provided you? Even the meat we eat
and the milk we drink contains stored sun’s energy which the animals obtained by eating (green) plants. (Have you
heard the expression: “All flesh is grass”?)
Photosynthesis occurs in green plant cells (any green cell whether in the leaf or elsewhere in the plants) in small
structures called chloroplasts. These complex, microscopic structures contain all the substances and cellular machinery
required to carry out the various chemical processes involved in capturing energy from (sun)light, breaking down water
and making simple sugars. Without becoming totally technical, chloroplasts contain DNA. Remember DNA is the genetic
material that passes from generation to generation and carries the information that determines many of our physical
(and other) characteristics. In animals and green plants DNA is usually found in the nucleus in the form of chromosomes
(See Mitosis and Meiosis). However, a small amount of DNA is located outside the nucleus in chloroplasts
(in green plants) and in the mitochondria (other small structures inside the cell) of both plants and animals.
The chloroplast DNA is what this article is all about.
From: T.W. Chumley et al., 2006. The Complete Chloroplast
Genome Sequence of Pelargonium x hortorum: Organization
and Evolution of the Largest and Most Highly Rearranged
Chloroplast Genome of Land Plants.
The genes are labeled on the outer circle. The inner circle
shows the numbers of the DNA segments and shows where
and how they are duplicated and inverted.
Studies have been made of the DNA of a number of green plant species. These studies covered plants throughout the plant kingdom
from simple, single celled green algae, mosses and liverworts, ferns, gymnosperms (conifers and such plants), to flowering
plants. Among the plants studied Pelargonium x hortorum ‘Irene’ was found to have the largest chloroplast DNA molecule of all
the plants which have been studied! The chloroplast DNA of green plants ranges in size from about 100,000 base pairs
(important chemical jargon related to the number of chemical units within the molecule) to about 150,000 base pairs. Pelargonium
x hortorum comes in at 217,000 base pairs making it Number One in size of chloroplast DNA in the plant kingdom. The
chloroplast DNA of most plants contains between 120 and 150 genes (units of inherited information) but P. x hortorum ‘Irene’
has 220 genes! Scientists have been impressed with the uniqueness of Pelargonium in this regard. [If you know anything about
genetics and chromosomes you may know that duplication or repetition of genes can occur – especially in plants. This multiplication
or duplication of genes explains the longer length of the DNA molecule of the Pelargonium chloroplast. In addition, inversions
(reversal of the direction of the genes within a molecule) have added to the uniqueness of the chloroplast DNA of Pelargonium.]
In addition, inheritance of chloroplast (and mitochonrial) DNA is normally through the mother (female line). The egg cell
(normally the larger reproductive cell) contains (pro)plastids and mitochondria but the male’s sperm cell usually contributes only
a nucleus to the reproductive process and the new offspring Studies have been done which indicate that chloroplast inheritance
in Pelargonium is unusual because it may be biparental, i.e. both parents may contribute to the chloroplast constitution of our
plants. Wow! Haven’t we always thought Pelargoniums are special. Little did we know.
From: R.K. Jansen et al. 2006. Phylogenetic analyis of Vitis
(Vitaceae) based on complete genome sequences.
BMC Evol Biol Apr.9;6:32.
The chloroplast DNA of Vitis vinifera (grape) is
over 160,000 base pairs long and contains 138
Chloroplast DNA ofWelwitschia