How Genes Work
How Genes Work
– We now understand that the genetic code is a set of
instructions for the production (or synthesis) of
proteins from amino acids
– The genetic code is found in the nucleus of cells, on
long strands called chromosomes.
• Chromosomes: strands of DNA in he nucleus of a
• A chromosome is made up of a protein core and strands
of nucleic acid.
• This nucleic acid is called deoxyribonucleic acid (DNA)
and carries the code.
–DNA is like a ladder with the ends
twisted in opposite directions.
• The rungs of the ladder carry the
• They are made up of pairs of bases
joined or bonded in the middle.
• Each base, along with the portion of
the side of the ladder to which it is
connected is called a nucleotide.
• Nucleotide: the basic building block of DNA
and RNA, made up of a sugar, a phosphate,
and one of four bases.
• Only 4 bases are involved:
– Adenine (A)
– Thymine (T)
– Cytosine (C)
– Guanine (G)
• They are only paired in A-T or T-A and G-C or C-
– Therefore, if we know the sequence of bases
on one side of the DNA helix, we can
correctly predict the sequence on the other
–The first function of this paring is to
enable the DNA molecule to make
copies of itself during cell division.
• During cell division, the helix unwinds,
and each strand, with its now unpaired
bases, picks up the proper
complementary bases, which are in
solution in the cell.
• This is called replication.
–Replication: the copying of the
genetic code during cell division.
• We refer to each consecutive sequence of three
DNA bases as a codon.
– Codon: the 3-base sequence that codes for a
specific amino acid.
– Codons are like words made up of three letters
– A string of words—or a genetic sentence—stands
for a chain of amino acids.
– Therefore, a sequence of condons codes for the
linking together of amino acids to make a pattern.
This process is called protein synthesis.
• Protein synthesis: the process by which the genetic
code puts together proteins in the cell.
• During protein synthesis only a small portion of
the DNA molecule is unwound.
– Messenger ribonucleic acid (mRNA) is
assembled against one strand of the unwound
– mRNA: the molecule that carries the genetic
code out of the nucleus for translation into
– The mRNA transcribes the gene by matching
complementary based to those exposed in the
coding strand of DNA, except that uracil (U)
replaces thymine (T).
• After the mRNA has transcribed the code, it
leaves the nucleus of the cell and moves to
specialized structures in the cell called
ribosomes, where the messenger is decided
and translated into an actual protein.
– Another type of RNA called transfer RNA (tRNA)
reads the three-letter codes, or codons, as
instructions for assembling a chain of amino acids.
– tRNA: RNA that lines up amino acids along
mRNA to make proteins.
– Now, this process is long and complex and it
beyond the scope of this class to get in to.
– Suffice it to say, it is still a long way from a protein
to a trait or the measurable, physical or chemical
characteristics of an organism.
• It does this by unwinding and splitting at the
bond between the bases A, T, G, and C, as in
the first step in protein synthesis.
– Each strand now picks up nucleotides with the
proper complementary bases and thus replaces its
– There are now two pair of each chromosome.
– When the cell divides, each daughter cell receives a
full set of chromosome pairs.
– This process is known as mitosis.
• Mitosis: the process of cell division that results in
two copies of the original cell.
• Some single-celled organisms reproduce in the
same fashion, but other single-celled organisms
and nearly all multicellular organisms reproduce
sexually, that is by the joining of specialized
cells from two parents.
– Such specialized sex cells are called gametes
(sperm and eggs).
• Gametes: the cells of sexual reproduction, which
contain only half the chromosomes of a normal cell.
– Zygote is the fertilized egg before cell division
• Zygote: the fertilized egg before cell division.
• The process of producing gametes is
– Meiosis: the process of cell division in
which gametes are produced.
– When the male fertilizes the gamete from
the female, the zygote once again had 2 of
each chromosome and 2 of each gene.
– But, because the members of each pair
come from different parents, the
combination of alleles of each gene may
well be different from that of either parent.