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DNA
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How are the structures of DNA and RNA related to their functions?
Coding for Traits
Genetic information is encoded in the order of nucleotide bases in DNA. The order of bases in DNA is
analogous to the order of letters in written text. Just as the English language uses 26 letters arranged in
patterns to form words that convey meaning, DNA uses a 4-letter alphabet to carry genetic information.
DNA is a very long molecule that contains intermittent “coding” segments called genes. In general, each gene
codes for a different protein. An organism’s DNA contains the genes to code for all of the proteins that the
organism needs to produce structures and carry out life processes.
Genes are made up of sequences with three-base segments called codons. Different codons code for
unique amino acids. However some amino acids may be coded for by more than one codon. For example the
amino acid leucine is coded for by six different codons. Other codons signal the stop or start of the gene.
Amino acids are the monomers that make up proteins. There are 20 different amino acids. Proteins can contain
hundreds of amino acids. The order of amino acids in a protein determines the protein’s structure and function.
To make a protein the gene containing the DNA code for that protein must first be converted into
an RNA code.
Transcription and Translation
The cell copies the information in a gene making complementary messenger RNA strands—a process
called transcription. These messenger RNA strands are then used as the code to assemble amino acids in the
correct order to form a protein. This second part of the process is called translation.
During transcription, the enzyme RNA polymerase catalyzes the copying of base codes in a gene from DNA to
a complementary strand of messenger RNA (mRNA). This complimentary mRNA strand then travels through the
cytoplasm to structures called ribosomes. Ribosomes are made of another kind of RNA called ribosomal RNA
(rRNA). Protein synthesis occurs at the ribosomes.
A third type of RNA, transfer RNA (tRNA), carries amino acids to the ribosomes and attaches them in the order
coded on the mRNA molecule. The amino acids are linked together with peptide bonds to form a protein. At
this point the original DNA code has been translated by the RNA into a protein sequence. This is why this
second part of the process is called translation. Transcription and translation are covered in more detail in the
concept “Transcription and Translation”.
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How do cells make copies of DNA?
Cell Division
Cells pass along their genetic information each time they divide. This ensures that every non-reproductive cell
in an organism contains a complete copy of the organism’s genetic code. In asexual reproduction, an organism
passes its complete genetic code to offspring. In sexual reproduction, about half of an organism’s genes are
passed to its offspring. In order to pass genetic information, the cell copies its DNA in the process of
replication. DNA replication is a biological process that occurs in every living organism.
Replication
DNA replication begins when the enzyme helicase unwinds a segment of DNA and separates the two strands of
the double helix, forming a structure called a replication fork. Each strand of the double helix serves as a
template for duplicating the sequence of bases found in the original molecule of DNA.
A different enzyme, DNA polymerase, catalyzes the synthesis of complementary strands of DNA by attaching
the correct deoxyribonucleotide having the correct complementary base (A if the template reads T, G if the
template reads C, etc.) to the new strand of DNA being made.
The synthesis of new DNA proceeds differently on each complementary strand. DNA polymerase can add
nucleotides only to a free 3' end of a growing segment, never to a 5' end, so DNA synthesis can occur in the 5'
to 3' direction only. As a result, only one strand—the leading strand—can be continuously assembled. Synthesis
of the other strand—the lagging strand—occurs in segments. The enzyme DNA ligase joins these segments,
called Okazaki fragments, by their sugar-phosphate backbones to form a continuous strand of DNA.
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In contrast to the leading strand, which proceeds continuously, assembly of a complementary strand
proceeds in small segments on the lagging strand.
Questions: (Answer on your own paper in complete sentences. You should have one paragraph for each
question)
1. How are DNA and RNA structures related to their functions? (think about what jobs DNA and RNA
have, and why they might need to be different shapes to be able to do those jobs).
2. How do cells make copies of DNA? (explain the process)