Standard Set 4. Genetics (Molecular Biology)
All cells contain DNA as their genetic material. The role of DNA in organism is twofold: first, to store
and transfer genetic information from one generation to the next, and second, to express that genetic
information in the synthesis of proteins. By controlling protein synthesis, DNA controls the structure and
funcÂ¬tion of all cells. The complexity of an organism determines whether it may have several hundred
to more than twenty thousand proteins as a part of its makeup.
Proteins are composed of a sequence of amino acids linked by peptide bonds (see Standard 10.c for
chemistry in this chapter). The identity, number, and seÂ¬quence of the amino acids in a protein give
each protein its unique structure and function. Twenty types of amino acids are commonly employed in
proteins, and each can appear many times in a single protein molecule. The proper sequence of amino
acids in a protein is translated from an RNA sequence that is itself encoded in the DNA.
4. Genes are a set of instructions encoded in the DNA sequence of each
organism that specify the sequence of amino acids in proteins characteristic of
that organism. As a basis for understanding this concept:
4. a. Students know the general pathway by which ribosomes synthesize proteins, using
tRNAs to translate genetic information in mRNA.
DNA does not leave the cell nucleus, but messenger RNA (mRNA), compleÂ¬mentary to DNA,
carries encoded information from DNA to the ribosomes (tranÂ¬scription) in the cytoplasm. (The
ribosomes translate mRNAs to make protein.) Freely floating amino acids within the cytoplasm are
bonded to specific transfer RNAs (tRNAs) that then transport the amino acid to the mRNA now located
on the ribosome. As a ribosome moves along the mRNA strand, each mRNA codon, or sequence of
three nucleotides specifying the insertion of a particular amino acid, is paired in sequence with the
anticodon of the tRNA that recognizes the sequence. Each amino acid is added, in turn, to the growing
polypeptide at the specified posiÂ¬tion.
After learning about transcription and translation through careful study of expository texts, students can
simulate the processes on paper or with representative models. Computer software and commercial
videos are available that illustrate aniÂ¬mated sequences of transcription and translation.
4. b. Students know how to apply the genetic coding rules to predict the sequence of amino
acids from a sequence of codons in RNA.
The sequence of amino acids in protein is provided by the genetic information found in DNA. In
prokaryotes, mRNA transcripts of a coding sequence are copied from the DNA as a single contiguous
sequence. In eukaryotes, the initial RNA transcript, while in the nucleus, is composed of exons,
sequences of nucleotides that carry useful information for protein synthesis, and introns, sequences that
do not. Before leaving the nucleus, the initial transcript is processed to remove introns and splice exons
together. The processed transcript, then properly called mRNA and carrying the appropriate codon
sequence for a protein, is transported from the nucleus to the ribosome for translation.
Each mRNA has sequences, called codons, that are decoded three nucleotides at a time. Each codon
specifies the addition of a single amino acid to a growing polypeptide chain. A start codon signals the
beginning of the sequence of codons to be translated, and a stop codon ends the sequence to be
translated into protein. Students can write out mRNA sequences with start and stop codons from a given
DNA sequence and use a table of the genetic code to predict the primary sequences of proteins.
4. c. Students know how mutations in the DNA sequence of a gene may or may not affect the
expression of the gene or the sequence of amino acids in the encoded protein.
Mutations are permanent changes in the sequence of nitrogen-containing bases in DNA (see Standard 5.
a in this section for details on DNA structure and nitrogen bases). Mutations occur when base pairs are
incorrectly matched (e.g., A bonded to C rather than A bonded to T) and can, but usually do not,
improve the product coded by the gene. Inserting or deleting base pairs in an existing gene can cause a
mutation by changing the codon reading frame used by a ribosome. Mutations that occur in somatic, or
nongerm, cells are often not detected because they cannot be passed on to offspring. They may,
however, give rise to cancer or other undesirable cellular changes. Mutations in the germline can produce
functionally different proÂ¬teins that cause such genetic diseases as Tay-Sachs, sickle cell anemia, and
Duchenne muscular dystrophy.
4. d. Students know specialization of cells in multicellular organisms is usuÂ¬ally due to
different patterns of gene expression rather than to differÂ¬ences of the genes themselves.
Gene expression is a process in which a gene codes for a product, usually a proÂ¬tein, through
transcription and translation. Nearly all cells in an organism contain the same DNA, but each cell
transcribes only that portion of DNA containing the genetic information for proteins required at that
specific time by that specific cell. The remainder of the DNA is not expressed. Specific types of cells
may produce specific proteins unique to that type of cell.
4. e. Students know proteins can differ from one another in the number and sequence of amino
Protein molecules vary from about 50 to 3,000 amino acids in length. The types, sequences, and
numbers of amino acids used determine the type of protein produced.
4. f.* Students know why proteins having different amino acid sequences typically have
different shapes and chemical properties.
The 20 different protein-making amino acids have the same basic structure: an amino group; an acidic
(carboxyl) group; and an R, or radical group (see Standard Set 10, â€œOrganic and Biochemistry,â€�
in the chemistry section of this chapter). The protein is formed by the amino group of one amino acid
linking to the carboxyl group of another amino acid. This bond, called the peptide bond, is repeated to
form long molecular chains with the R groups attached along the polymer back-bone.
The properties of these amino acids vary from one another because of both the order and the chemical
properties of these R groups. Typically, the long protein molecule folds on itself, creating a three-
dimensional structure related to its funcÂ¬tion. Structure, for example, may allow a protein to be a highly
specific catalyst, or enzyme, able to position and hold other molecules. The R group of an amino acid
consists of atoms that may include carbon, hydrogen, nitrogen, oxygen, and sulfur, depending on the
amino acid. Amino acids containing sulfur sometimes play an important role of cross-linking and
stabilizing polymer chains. Because of their various R groups, different amino acids vary in their chemical
and physical proper-ties, such as solubility in water, electrical charge, and size. These differences are
reflected in the unique structure and function of each type of protein.