Principles of Plant Genetics and Breeding

52 CHAPTER 3
Enhancer
region
subunits). All genes do not code for proteins, and further,
all genes in a cell are not actively transcribing
mRNA all of the time. Also, most enzymes are proteins,
but all proteins are not enzymes.
Protein structure
CCAAT box TATA box
Figure 3.19 Diagrammatic presentation of a typical eukaryotic gene showing the three basic regions – the upstream 5′
flanking regions, the transcriptional unit, and the downstream 3′ flanking region – and their constitution.
Polypeptides are precursors of proteins. Once produced,
they fold to assume 3D forms, the functional stage that
becomes proteins. There are four basic levels of protein
structure – primary, secondary, tertiary, and quaternary.
The primary structure of proteins is the sequence
of the amino acids in the linear backbone of the
polypeptide. The next fold (exemplified by the DNA
molecule), is an α-helix, a spiral chain of amino acids
stabilized by hydrogen bonds. The secondary structure
describes the arrangement of amino acids within certain
areas of the polypeptide chain. The tertiary structure is a
3D conformation of the entire chain in space. Proteins
with more than one polypeptide chain may exhibit the
quaternary protein structure through aggregations of
the polypeptides.
Regulation of gene expression
Gene regulation is a critical activity performed by
plants for proper growth and development. It is not
important for a gene just to be expressed, but its
expression must be regulated such that it is expressed
at the right time only and to the desired extent.
Regulation entails the “turning on” and “turning off”
5′ non-coding
sequence
Introns
3′ non-coding
sequence
Promoter region Exons Enhancer
region
Upstream 5′ flanking region Transcriptional unit Downstream
3′ flanking
region
of genes. It is through regulation of gene expression
that cellular adaptation, variation, differentiation, and
development occur. Some genes are turned on all the
time (called constitutive expression), while others are
turned on only some of the time (called differential
expression).
The underlying principle of gene regulation is that
there are regulatory molecules that interact with nucleic
acid sequences to control the rate of transcription or
translation. Six potential levels for regulation of gene
expression exist in eukaryotes – the regulation of: (i)
transcription; (ii) RNA processing; (iii) mRNA transport;
(iv) mRNA stability; (v) translation; and (vi) protein
activity. Transcription is temporarily and spatially
separated from translation in eukaryotes.
A typical eukaryotic gene is shown in Figure 3.19.
Unlike that of a monocistronic gene (lacks introns; has
one transcriptional unit and one translational unit) as
occurs in bacteria, eukaryotic genes are polycistronic
(split genes with introns). Genes that encode the primary
structures of proteins required by all cells for enzymatic
or structural functions are called structural
genes. In prokaryotes, these genes are organized into
clusters that are transcribed as a single unit (coordinately
controlled). The mRNA is called polycistronic mRNA,
coding for multiple proteins involved in the same regulatory
pathway (e.g., the lac operon).
There are two basic categories of gene regulation –
negative and positive (Figure 3.20). In negative regulation,
an inhibitor that is bound to a DNA (gene) must
be removed in order for transcription to occur. In
positive regulation, gene transcription occurs when an
activator binds to the DNA. One of the main ways in