(3) DNA deoxyribonucleic acid, base coding function, genes,
chromosomes and genome
Relative size:
DNA
molecule < gene < chromosome < genome
Reminders: A chromosome as a thread-like structure of
DNA, carrying genetic information in the form of genes.
A gene is a length of DNA that codes for a protein. An
allele as a version of a gene.
DNA
(deoxyribonucleic acid) is a large molecule essential for life and cell
replication and is another example of a natural polymer.
DNA, a natural polymer is made up of
string of repeating units (the monomer) called nucleotides.
The DNA forms two linked strands
coiled together in the shape of a double helix (more DNA
structure later).
DNA molecules hold all of an organism's genetic
material, that is all the chemical instructions for individual cells and
complex organisms to grow and develop.
All the instructions that are needed for an
organism to grow, develop and reproduce is encoded in the DNA.
The contents of your DNA directly determines what
your inherited characteristics are.
The DNA is organised into long coiled up sections
called chromosomes in the cell nucleus, and within the chromosomal DNA there are shorter
sections of DNA called genes, each of which codes for a particular
function e.g. code for the synthesis of an individual protein, eye colour,
hair colour, blood type.
The complete set of chromosomes for an organism is
called its genome.
In terms of size:
DNA molecule < genes < chromosomes
< genome
In other
words
<<
(BUT not to scale!!!)
Each chromosome consists of many short sections of
DNA called genes, one or more of which codes for one
characteristic of an organism e.g. blood group, eye colour or hair
colour.
The genes have the codes for making all the
different proteins, many of which are enzymes and how to make large
important molecules like haemoglobin.
Reminder: Your total DNA, that is the full molecular contents of
your genes, is called the genome.
The human genome contains between 50 000 and 100 000 genes (there
is still uncertainty on the final number), and around 22 000 code
for proteins.
There are two copies of each gene in the human genome, so they
work in pairs, but one gene in a pair tends to dominate over the
other.
Chromosomes normally come in pairs and the
both have the same type of genes in the same order along their
very long length (in the molecular sense!).
Reminder- one chromosome from each parent makes up
the pair, and human diploid cells have 46 chromosomes (23 pairs).
Apart from the nucleus, certain other parts of the
cell can contain DNA e.g. mitochondria (sites of respiration), known as
mitochondrial DNA.
Bacteria can contain free rings/strings of DNA
called plasmids.
Plasmids are not part of a bacteria's
chromosome, and do not help the functioning of the cell, but
e.g. they contain genes that help them develop resistance
against antibiotics, and so help in their survival.
DNA encodes genetic instructions for the development and
functioning of living organisms and viruses.
e.g. every protein molecule
needed by a living organism down to individual cell level is
synthesised by other molecules reading the genetic DNA code and combining the right
amino acids in the right order.
This means every different protein has its own
specific number and precise order of amino acids.
After synthesis, the protein molecule (polymer
chain of amino acids) folds into its own specific unique shape to
perform its own unique function e.g. an enzyme to catalyse a
specific biochemical reaction.
A section of DNA that codes for a particular
protein is called a gene and it is the order of the bases
in a gene that determines the order of amino acids in the
protein, hence its structure and function.
An example of a section of DNA is shown below and
Part 4 describes in detail
the synthesis of proteins
which is based on triplet codes of bases for the amino acids to
be assembled into protein molecules.
The DNA not only codes for all the necessary
proteins, it also determines what type a cell becomes e.g. blood
cell, brain cell, muscle cell, skin cell etc.
Proteins are polymers of amino acids.
DNA is a
polymer of nucleotides.
So amino acids and nucleotides are monomers.
Every protein has a specific structure for a
particular function including enzymes, and most be encoded in DNA.
Key points -
Summary of ideas
Based on
the syllabus-specifications for students taking the AQA, Edexcel and OCR
GCSE level biology examinations (~US grades 9-10).
Main points on DNA base
coding and the relative sizes of DNA molecules, genes, chromosomes, and
the genome, along with their importance in understanding genetics.
DNA Base Coding
DNA is composed of a sequence
of nucleotides, each containing a nitrogenous base,
a phosphate group, and a sugar molecule
(deoxyribose). The bases are:
-
Adenine (A)
-
Thymine (T)
-
Cytosine (C)
-
Guanine (G)
Base pairing rules:
These base pairs form the
double helix structure of DNA and encode genetic
instructions.
The sequence of bases determines the arrangement of
amino acids in proteins, following the triplet code,
where each three-base sequence (codon) specifies an amino
acid.
Relative Sizes: DNA
Molecule, Genes, Chromosomes, Genome
-
DNA molecule:
A single DNA strand is extremely long (up to 2 meters in a human cell)
but is tightly coiled to fit within the nucleus.
-
Gene:
A specific segment of DNA that codes for a protein or functional
RNA. It varies in size but consists of thousands of base pairs.
-
Chromosome:
A highly condensed structure of DNA. Humans have 46 chromosomes
(23 pairs).
-
Genome:
The complete set of genetic material in an organism. In
humans, this consists of approximately 3 billion base pairs.
The hierarchy is:
Base pairs < Gene < Chromosome <
Genome
Importance in
Understanding Genetics
-
Protein synthesis:
DNA base coding determines protein structure, essential for bodily
functions.
-
Genetic
inheritance: Variations in
genes contribute to inherited traits.
-
Evolution and
mutations: Mutations in
DNA can lead to genetic diversity and evolutionary changes.
-
Medical
applications:
Understanding the genome helps in diagnosing genetic disorders and
developing treatments.
-
Biotechnology:
Genetic engineering and CRISPR technology rely on DNA sequencing.
Summary of learning objectives and key words or phrases
Know and understand the function of DNA (deoxyribonucleic
acid) - the base coding function information.
Appreciate the relative size of DNA molecules, genes,
chromosomes, genome their functions.
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