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Date: ___________
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Making Proteins – The Central Dogma of Biology Handout
The Central Dogma of Biology represents the molecular flow of information in living
organisms. It is simply the idea that information flows from DNA to RNA to Protein, and
that proteins are responsible the traits of an organism. DNA is the genetic information in
cells, storing all of the blueprints for building the organism. The process by which this
information is used to build an organism occurs in two main steps: transcription, in which
a copy of a gene’s nucleotide sequence is made, and translation, in which that copy is
used to direct the production of a specific protein.
RNA is Ribonucleic Acid
Within the cell, DNA is the ultimate source of all genetic information and is protected by being kept inside
the nucleus. RNA is another nucleic acid, highly similar to DNA, which is used as a messenger service in the
cell to send information from the DNA to the
cytoplasm, where proteins are produced. There are a
number of important similarities and differences
between DNA and RNA. DNA and RNA are both organic
polymers built from nucleotide monomers, and each
uses the bases adenine (A), cytosine (C), a guanine (G).
Where DNA uses the base thymine (T), RNA uses the
base uracil (U). In addition, while DNA is stored in the
nucleus as a double-stranded double helix, RNA is a
single-stranded molecule that leaves the nucleus to go
to the cytoplasm. Just like in DNA, it is the sequence of
nucleotides (the nucleotide bases) that represents the
information stored in a molecule of RNA and a
different sequence provides different information.
Transcription: Copying in the Language of Nucleic Acid
In order to make a protein from the instructions written in the sequence of DNA, cells must first transcribe
the sequence of DNA into a molecule of messenger RNA, or mRNA. This process is called transcription.
Transcription begins with the action of a group of proteins that bind to a sequence in the DNA that
indicates a gene sequence is nearby. These proteins then unwind the DNA double helix and an enzyme
called RNA polymerase is moves along one of the two DNA strands, known as the template strand. As it
moves, the RNA polymerase “reads” the DNA sequence of nucleotides and builds a complementary mRNA
according to the rules of base pairing. As with DNA, cytosine (C) always binds to guanine (G). However,
because RNA uses uracil (U) instead of thymine (T),
when the RNA polymerase reads an adenine (A) in the
DNA, it uses a uracil (U) to complementary pair with the
adenine and adds the uracil (U) to the growing RNA
polymer. Once the mRNA copy is complete, the DNA
winds back together, and the single-strand of mRNA
leaves the nucleus. In the cytoplasm, this mRNA then
serves as the information for an organelle called the
ribosome to build a protein.
Translation: Nucleic Acid Language into the Language of Amino Acids
Once the mRNA reaches the ribosome, the information encoded in the nucleotide sequence can be
translated into the language of amino acids and a protein can be made. This process of building protein
based on the instructions in an mRNA sequence is called translation. The ribosome itself is also made of
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Date: ___________
Period______
RNA (known as rRNA) and functions as an enzyme, catalyzing (speeding
up) the reaction than links amino acids together to build a protein.
During translation, the ribosome “reads” the mRNA message and builds
a chain of amino acids. The ribosome acts like a factory while mRNA
serves as the instruction that details which amino acids should be joined
together to form a protein. Amino
acids are called for based on groups of
three nucleotides called codons. Each
codon is like a word: its letters name a
particular amino acid. For example,
the codon GGU (in the mRNA
sequence) specifies the amino acid
glycine should be added to the
growing protein. In order to build the growing protein, the ribosome
works with a third type of RNA called transfer RNA, or tRNA. tRNA is
responsible for physically bringing the required amino acids to the
ribosome. Each tRNA has a structure that allows it to act as an adaptor: one end of the tRNA binds to a
single amino acid while the other end binds to the mRNA. The part that binds mRNA is called the anticodon
because it has complementary base pairing with an mRNA codon. When the tRNA containing the
appropriate anticodon binds with the mRNA codon, the tRNA releases the amino acid to the ribosome. The
ribosome then catalyzes the reaction that adds the amino acid to the growing protein molecule. These rules
by which mRNA codons specify amino acids are known as the genetic code.
Many mRNA molecules are hundreds to thousands of nucleotides long, and this sequence includes codons
that signal the ribosome to start and stop translating. The codon AUG is always used as a START sequence
and (almost) every protein made starts with the amino acid methionine (Met), and is known as the start
codon. After building the protein the ribosome eventually reads a stop codon that signals the end of
translation. The three possible stop codons are UAA, UAG, and UGA. After reading a stop codon, the
ribosome releases both the mRNA transcript and the completed protein.
Every step in the process to build proteins requires a significant amount of ATP to provide the
energy for the various chemical reactions that must occur. Accordingly, cells use sophisticated
ways to control which proteins are made, so as to only make those that will be used by the cell.
Mutations – Part of all of us
The word mutation has a lot of negative meaning in our common, spoken language, however in
the language of biology it means little more than a change in DNA sequence. Mutations can
involve large regions of a chromosome, or a single nucleotide pair, which is the case for the
disease Sickle Cell Anemia. When a mutation occurs within the DNA sequence of a gene, this leads
to a change in the mRNA sequence, which frequently leads to changes in the amino acid sequence
of the corresponding protein and therefore its structure and function. Mutations that change a
single DNA nucleotide for a different nucleotide are called substitution mutations. Mutations that
add a new nucleotide are insertion mutations. Mutations
that remove a nucleotide are deletion mutations. Although
mutations are often harmful, they can also change a protein
in a way that may be beneficial. In humans, there are 175
new mutations each generation. Overall, mutations are the
ultimate source of variation and genetic diversity in all living.