Molecular Genetics
Chromosomes found in the nucleus carry the hereditary material -- DNA
DNA (deoxyribonucleic acid)
DNA controls cellular activity by influencing the production of enzymes.
Structure of DNA Molecules:
DNA is a very long chain polymer made up of thousands of repeating units called
nucleotides.
Nucleotide Unit is composed of a phosphate group, a pentose sugar, and a
nitrogenous base.
The Nitrogenous Bases are; adenine (A) thymine (T) guanine (G) cytosine (C)
Watson and Crick (early 1950's)
-- determined the structure of the DNA molecule
** Consists of two chains of nucleotide units in a twisted ladder-like structure.
(resembles a spiral staircase)
This spiral staircase is called an alpha helix. (37 degree turn)
-- The sides of the ladder are made up of alternating deoxyribose sugar -phosphate group units.
-- The rungs of the ladder are made of 2 nitrogenous bases per rung linked
together by a weak hydrogen bond.
-- Only 2 combinations of base pairs can form the rungs of the DNA molecule.
Adenine - Thymine (A-T)
Guanine - Cytosine (C-G)
** This specific matching up of the nitrogenous bases is called complementary
base pairing.
** DNA is able to replicate itself.
[How does DNA form duplicates of itself during mitosis and meiosis?]
1. The double stranded DNA molecule unwinds and
unzips between the weak hydrogen bonds between
the nitrogenous base pairs.
2. Free nucleotides present in the nucleus attach
themselves by forming new hydrogen bonds with
the exposed bases in the single chain. The only
base which can reattach is the same type of base that was originally joined to it.
A view of DNA replication
(Usefulness of the Watson-Crick Model)
1. Explained how mitosis produces exact DNA copies
for each daughter cell -- thus the genetic information passes on unchanged.
2. Explained how DNA acts as a code directing the making of enzymes and other
proteins by a cell, thus directing cellular activities.
Directs the synthesis of RNA which makes proteins.
RNA (ribonucleic acid)
** RNA is also a polymer formed by a sequence of nucleotides.
How does RNA differ in structure from DNA?
1. The RNA molecule is a SINGLE nucleotide
strand, not a double strand as in DNA.
2. The sugar molecule in RNA is RIBOSE -not deoxyribose as in DNA.
3. The base URACIL (U) takes the place of thymine. (T)
Synthesis of RNA
-- The various types of RNA are made from the template (pattern) of the DNA
molecule.
Types of RNA
1. mRNA (messenger RNA) made in nucleus
2. tRNA (transfer RNA) on the cytoplasm
3. rRNA (ribosomal RNA) in the ribosomes
How are proteins synthesized from a DNA template? (model)
1. DNA serves as a template for the synthesis of mRNA from RNA nucleotides in
the nucleus.
transcription: transfer of the genetic message from DNA to mRNA
2. mRNA molecules carrying a specific code determined by the base sequence
of the DNA template move from the nucleus to the cytoplasm.
3. Strands of mRNA carrying codons transcribed from DNA, move to the
ribosomes in the cytoplasm.
codon: a triplet in a DNA molecule
triplet: grouping of three nitrogenous bases in DNA or RNA molecules
(each codon will code for a specific amino acid)
Translation: communication between mRNA, rRNA and tRNA to create a protein
from mRNA sequence. (steps 4-8)
4. mRNA strands become associated with rRNA on
the ribosomes. (rRNA -- arranges date between mRNA and tRNA)
5. Different triplets of nitrogenous bases in tRNA
molecules pick up specific amino acids in the cytoplasm and carry them to
mRNA at the ribosomes. (tRNA -- I want aa.)
6. Amino acids are put into position on the ribosome with instructions from the
triplet codes of tRNA and mRNA.
7. With the aid of enzymes and ATP (energy), the
amino acids are bonded to form a polypeptide chain (protein) on the ribosome.
An overview of the entire protein synthesis (transcription and translation)
process.
8. This protein formation is what directs metabolic activity in any cell.
** One gene codes for one polypeptide chain.
gene = the sequence of nucleotides in a DNA molecule necessary to synthesize
a polypeptide
** Since the sequence of nucleotides in DNA determines the sequence of
nucleotides in messenger RNA, DNA ultimately determines the sequence of
amino acids in specific proteins. The specificity of enzymes is dependent on their
protein makeup, and, since the individuality of a cell is largely a function of the
enzymes it possesses, it is evident that DNA determines the individuality and
function of an organism.
The work of a cell is carried out by the many different kinds of molecules it
assembles, mostly proteins. Proteins are long, folded molecules made up of up
to 20 different kinds of amino acids which interact to produce specific protein
shapes.
The specific shape of the protein (exs. enzymes and hormones) determines the
specific function of that protein.
Offspring resemble their parents because they inherit similar genes that code for
the production of proteins that form similar structures and perform similar
functions.
How are cell functions regulated?
1.) Gene regulation allows only the selective expression of certain individual
genes.
2.) The regulation of certain genes controls the activity and production of certain
proteins.
** All this gene regulation allows cells and organisms to respond to their
environment and control their growth and division.
Body cells of an individual can be very different from each other, even though
they have descended from a single cell (zygote) and have essentially the same
genetic instructions. This is because different parts of these instructions are
used in different types of cells, influenced by the cells environment and
developmental history.
Genetic Research
1. Cloning: producing a group of genetically identical offspring from the cells
of an organism
** This technique shows great promise in agriculture. Plants with desirable
qualities can be rapidly produced from the cells of a single plant.
2. Genetic engineering: (recombinant DNA)
-- transfer of genetic information from one organism to another
-- includes the transfer of entire genes and gene splicing
** A cell can synthesize a new chemical coded for by its new gene(s)
-- examples include interferon, insulin, and growth hormone.
** Genetic engineering can correct genetic defects & produce agriculturally more
efficient plants and animals.
restriction enzymes -- used to cut segments of DNA in one organism so they can
be transferred into another organism
Characteristics produced by the segments of DNA may be expressed when
these segments are inserted into new organisms such as bacteria.
Inserting, deleting, or altering DNA segments can alter genes. An altered gene
may be passed on to every cell that develops from it.
human genome project --- has allowed humans to know the basic framework of
their genetic code
Knowledge of genetics is making possible new fields of health care. Genetic
mapping is making it possible to detect and possibly correct, defective genes that
may lead to poor health.
A down side to this is that health insurance agencies and other organizations
may use this genetic information against individuals.
Substances from genetically engineered organisms may reduce the cost and
side effects of replacing body chemicals. Human insulin produced in bacteria is
already an example of this.