Download DNA Structure and Function

Module A: Unit 2, Lesson 6 - DNA Structure and Function
What is DNA?
• The genetic material in cells is contained in a molecule called deoxyribonucleic
acid, or DNA.
• Scientists describe DNA as containing a code. A code is a set of rules and
symbols used to carry information.
• To understand how DNA functions, you first need to learn about the structure of
the DNA molecule.
How was DNA discovered?
• Many scientists from all over the world contributed to our understanding of DNA.
• Some scientists discovered the chemicals that make up DNA, and others learned
how these chemicals fit together.
• Still others determined the three-dimensional structure of the DNA molecule.
What does DNA look like?
• Experiments and imaging techniques have helped scientists to infer the shape of
DNA.
• The structure of DNA is a twisted ladder shape called a double helix.
• The two sides of the ladder are made of sugars and phosphate groups.
• The rungs of the ladder are made of pairs of bases.
• A base, a sugar, and a phosphate group make a building block of DNA called a
nucleotide.
• There are four different nucleotides in DNA.
• The bases in nucleotides are paired, or complementary.
• Adenine always pairs with thymine (A-T).
• Cytosine always pairs with guanine (C-G).
• The order of the nucleotides in DNA is a code that carries information.
• Genes are segments of DNA that relate to a certain trait.
• The code in the nucleotide order has information about which proteins the cells
should build.
• The types of proteins that your body makes help determine your traits.
How are copies of DNA made?
• The cell makes copies of DNA molecules through a process known as
replication.
• During replication, the two strands of DNA separate.
• The bases on each side of the molecule are used as a pattern for a new strand.
• As bases on the original molecule are exposed, complementary nucleotides are
added.
• When replication is complete, there are two identical DNA molecules.
When are copies of DNA made?
• Before a cell divides, it copies its DNA.
• Our cells can replicate DNA in just a few hours, because replication begins in
many places along a DNA strand.
• Many groups of proteins are working to replicate your DNA at the same time.
What are mutations?
• Mutations are changes in the number, type, or order of bases on a piece of DNA.
• There are three main kinds of mutations: deletions, insertions, and substitutions.
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In a deletion mutation, a base is left out.
In an insertion mutation, an extra base is added.
The most common mutation, substitution, happens when one base replaces
another.
• Mutations can happen by random error, and also by damage to the DNA molecule
by physical or chemical agents called mutagens.
• Cells make proteins that can fix errors in DNA, but sometimes the mistake is not
corrected.
• The mistake then becomes part of the genetic code.
• A genetic disorder results from mutations that harm the normal function of the
cell.
• Some genetic disorders are inherited, or passed on from parent to offspring.
• Other disorders result from mutations during a person’s lifetime. Most cancers fall
in this category.
What is the role of DNA and RNA in building proteins?
• Some of the information in the DNA is copied to a separate molecule called
RNA, or ribonucleic acid.
• RNA is used to build proteins.
• Like DNA, RNA has a sugar-phosphate backbone and the bases adenine (A),
guanine (G), and cytosine (C).
• Instead of thymine (T), RNA contains uracil (U).
• Three types of RNA have special roles in making proteins.
• When a cell needs to make a protein, it makes an RNA copy of a section of the
DNA. This is called transcription.
• In transcription, DNA is used as a template to make a complementary strand of
messenger RNA (mRNA).
• The information in the mRNA is then used to build proteins. This is called
translation.
• In translation, the mRNA passes through a protein assembly line within a
ribosome.
• A ribosome is a cell organelle made of ribosomal RNA (rRNA) and protein.
• As mRNA passes through, transfer RNA (tRNA) delivers amino acids to the
ribosomes.
• The order of the bases codes for which amino acid is attached.
• The amino acids are joined together to form a protein.