University of Zakho
Faculty of science
DNA Genes and cell division
Department of Biology
Plant Botany I
theory (1)
The discovery of DNA structure by James Watson and Francis Crick was a major
breakthrough in science (1953). They proposed a model of a double helix that has
become the most famous and intensely studied molecule in science. DNA, genes, and
chromosomes and how they can control the activities of a cell. Also, you will learn
about cell division and the life of a dividing cell during the stages of the cell cycle. A
nucleus contains DNA associated with
proteins that make up the chromosomes.
Plant genes are found in the chromosomes,
and DNA is the genetic material.
Functions of the genes:
(1) Transmission of the genetic information.
(2) Expression of the genetic information.
What are genes, and why are they important?
The DNA makes up genes and genes are organized into chromosomes. A gene is a
sequence of DNA that codes for a gene product, usually a protein (genes also code for
RNA as a product). Most recently, it has been shown that a gene can actually code for
several proteins, so our definition might state "single or several proteins", or RNA. Also,
some segments of DNA have a regulatory role in transcription and form no known
gene product. As you can see defining a gene can be confusing, but most biologists
agree that a gene is the basic unit of heredity.
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University of Zakho
Faculty of science
Department of Biology
Plant Botany I
theory (1)
Keep in mind two roles of genes that give them importance in nature:
1. Genes are the genetic information passed to the next generation of offspring cells
following division of the parental cell. Genes are instructions passed to offspring
cells as parental cells divide. This role is really that of heredity, as genes, which carry
the genetic information, are transmitted from parent to offspring. Genes are the
units of inheritance.
2. In cells, genes are the molecular blueprints for the production of proteins; these
proteins control the structure, development, and activities of a cell as it grows and
develops. DNA directs the synthesis of RNA, which in turn directs the assembly of
amino acids into proteins. The sequence of DNA ---> RNA ---> protein has
been called the dogma of protein synthesis since it occurs in all organisms including
plants and animals.
We now know that chromosomes are made of genes that control the cell. In turn,
genes are made of DNA. It is important to know basic chromosome and DNA structure
to understand the critical functions of genes. Remember that structure-function
relationships are of major importance in botany. Chromosomes and DNA provide a
good example of this relationship at the molecular level.
Chromosomes examined by electron microscopy appear as small beads on a thin
string. The beads are actually called nucleosomes and are the basic structural unit of
all eukaryotic chromosomes. Nucleosomes probably help keep the long DNA molecules
untangled and organized inside the nucleus. Also, nucleosomes may help control which
genes are expressed. (Nucleoside = Base + Sugar ----- Nucleotide = Base + Sugar +
Phosphate).
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University of Zakho
Faculty of science
Department of Biology
Plant Botany I
theory (1)
Nucleosomes consist of:
(1) DNA
(2) Chromosomal proteins called histones.
DNA appears to wrap around four kinds of histones, forming a nucleosome that
looks like a small bead (fig. 1) and most certainly contains the DNA of numerous genes.
Nucleosomes are separated by spacer DNA, which is associated with a fifth histone that
apparently binds nucleosomes together in a regular, repeating array. The entire DNA
plus histone association, including the nucleosomes, is called Chromatin. Chromatin is
folded into loops that extend from the main axis of the chromosome
The discovery of DNA structure
The structure of DNA was first correctly recognized by James Watson and Francis
Crick of Cambridge University they are therefore credited with first describing the
structure of DNA, the most significant discovery in biology of the twentieth century. In
1962, Watson and Crick, with associate Maurice Wilkins, received a Nobel Prize for
their landmark effort.
Watson and Crick's model of DNA was a double helix that is, a two-stranded spiral
(fig.2). Watson and Crick actually built a model of a DNA molecule by putting together
pieces of metal to represent the chemical components of DNA. The nitrogen-containing
bases of each nucleotide show specific pairing; that is, thymine pairs only with
adenine, and cytosine pairs only with guanine. This pairing joins the two strands made
of the deoxyribose sugar and phosphate that form the backbone of a DNA molecule.
The variability of the base sequence in DNA explains the enormous diversity of
genes and organisms in nature, for it is the sequence of these bases that determines
genetic variability. The impact of the Watson - Crick Model was immediate. The model
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University of Zakho
Faculty of science
Department of Biology
Plant Botany I
theory (1)
hinted that different genes could exist as base sequences of different lengths. The
structure of DNA also explains its role in directing RNA synthesis inside the nucleus as
DNA serves as a template for the assembly of RNA.
Fig 1 : Diagrams and electron micrographs of DNA and histone proteins in chromatin
Fig2 : The double-helical structure of the DNA
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University of Zakho
Faculty of science
Department of Biology
Plant Botany I
theory (1)
How Do Genes Work?
In cells, proteins are made in the cytosol in ribosomes, not in the nucleus,
where most of the plant's genes (DNA) occur. This observation was one of the first
clues that genes do not make proteins directly and that RNA is involved in the process.
This observation is a start to answer our question about how genes work.
RNA is transcribed in the nucleus and translated in the cytosol.
The directed assembly of amino acids into a protein is a multi-step process that
begins when the DNA of a gene in the nucleus is used as a template for making RNA.
This process of RNA synthesis is called transcription. RNA fresh off the DNA template is
often referred to as the primary transcript, or pre-RNA, because, while still in the
nucleus, RNA is chemically modified, then spliced ‫ َرب ََط‬and reassembled
‫َوأعادََتجميع‬before export as mature RNA into the cytosol.
There are three main kinds of RNA transcribed in the cell nucleus on a DNA template.
1. Messenger RNA (mRNA) is the actual message coded from DNA that directs the
synthesis of a protein in a ribosome, and carries the DNA message from gene to
ribosome.
2. Transfer RNA (tRNA) binds to amino acids and brings (transfers) them to a
ribosome as specified by mRNA.
3. Ribosomal RNA (rRNA) combines with ribosomal proteins, forming a ribosome, the
workbench of protein synthesis.
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University of Zakho
Faculty of science
Department of Biology
Plant Botany I
theory (1)
In every cell-plant, animal, fungus, bacterium-all three kinds of RNA are essential
for protein synthesis. The process of linking amino acids into a protein in a ribosome is
called translation, or protein synthesis. The directions for translation are contained in
mRNA. Once translation is complete, proteins are often assembled into multi chain
units or otherwise modified before becoming functional proteins. Overall, the way
genes work (gene expression) to transfer genetic information from a DNA template
into the structure of a protein can be summarized as follows:
Nuclear DNA (genes)
Transcription ‫َالنسخة‬
Pre-mRNA
Editing, then export to cytosol
Mature mRNA
Translation ‫َالترجمة‬
Protein
Assembly, modification
Functional protein (here an enzyme)
Catalysis
Chemical reaction that determines a trait
DNA ultimately directs the synthesis of the red pigment, which serves as an example
of the expression of a genetic trait-red color in the petal.
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