Download Dna: Hereditary molecules of life

DNA: HEREDITARY
MOLECULES OF LIFE
SBI 3U, Ms.Zafar
Every time a cell divides, an exact copy of its genetic
information must be passed on to each of the two new
cells produced. This is a huge task. Copies of molecules
that consist of 6 billion base pairs (the diploid set)
must be made, and each base pair must be “matched”.
At the rate of one base per second, this would take
190 years. However our cells can do it in minutes –
with an error rate of less than one in a million! How is
that possible?
What is DNA?
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Deoxyribonucleic acid
Primary carrier of genetic information
DNA has the instructions necessary to build all the
proteins an organism requires
Responsible for ensuring the genetic continuity of the
species, and therefore the continuity of life
Chromosomes
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DNA is passed from one generation to another in the form
of chromosomes
Eukaryotes – chromosomes only visible during cell division
X-shaped objects
Size and number of chromosomes vary in species – ex:
humans have 46, turkeys have 82, potatoes have 48 and
the adder’s tongue fern has 1200!
Recall: Proteins
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Composed of amino acids  how many kinds of
amino acids do we have?
Many roles  physical and chemical
Ribosomes translate sequence in RNA to make
amino acid chains/polypeptides  some of which
will later become functional proteins
Heredity & DNA
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Gregor Mendel: Austrian monk, who
spent his spare time breeding the
pea plant
Noticed that through the selective
cross-breeding of common pea
plants over many generations,
certain traits show up in offspring
without any blending of parent
characteristics!
Example: pea flowers are either
purple or white--intermediate colors
do not appear in the offspring of
cross-pollinated pea plants.
Heredity & DNA, continued …
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Mendel knew that a
heredity molecule
existed, and we now
know that hereditary
molecule as DNA
All information that is
needed for optimal cell
functioning is coded in a
cell’s DNA
DNA stored in nucleus
for eukaryotes (some in
mitochondria and
chloroplasts)
DNA stored in cytoplasm
for prokaryotes
Genes and Chromosomes
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Genes: coding regions of DNA  contain
instructions for building the proteins responsible for
each inherited trait
Alleles: several versions of each gene that give a
trait more than one potential physical appearance
Genes and Chromosomes, continued …
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DNA stored as chromosomes in nucleus  in the
chromosome, the DNA is wrapped around special
proteins called histones to form a complex
Several of these complexes are bundled into coils to
form thicker strands called chromatin  protects
DNA and helps to reduce its volume
Genes and Chromosomes, continued …
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Archae and bacteria do not have a nucleus  they
contain less genetic material and DNA is freefloating
Bacterial chromosomes are usually found in loops
Smaller, accessory loops of DNA, called plasmids
may also occur
The Genome
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The entire complement of hereditary information
that is contained in an organism is called its genome
It is the complete set of DNA, including all of the
genes
In humans, a copy of the entire genome – more than
3 billion DNA base pairs – is contained in all cells
that have a nucleus
Summary
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DNA is the molecule that carries genetic information
in all living things
A gene is a region of DNA that codes for the
building of a particular polypeptide
Eukaryotic DNA is wound around histone proteins
and organized into linear chromosomes. The
chromosomes are found inside the nucleus of each
cell
Summary, continued …
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The genome of most eukaryotes is in the form of
homologous sets of chromosomes in diploid
organisms
Prokaryotic cells usually have a single chromosome,
which is in the form of a loop of DNA and is not
associated with histones. Most of the genome is
stored in this chromosome, but smaller loops of
DNA, called plasmids may also be present