DNA
LS 5.3
What is DNA?
■ Deoxyribonucleic Acid
– The hereditary material
■
This is what you get from your parents, and what is passed to offspring during
reproduction
■ A type of nucleic acid (recall 4 main biological molecules)
■ A characteristic of all living things
– The smallest bacteria has DNA
■
DNA is very similar between organisms
–
Evidence for common ancestry
■ Contains the instructions for the organism
– Called genes
DNA, Chromosomes and Genes
■ DNA is the hereditary material
– Usually long and stringy (chromatin)
■
Wraps around proteins during cell division (chromosomes)
■ A segment of DNA that contains instructions is called a gene
Discovery of DNA
■ People knew something existed that
passed traits on, but didn’t know what it
was.
– Mendel called them factors
– Darwin said something would be
found that explains how traits are
passed
■ In the early 1900’s, DNA was suspected to
be the hereditary material
■ Rosalind Franklin and Maurice Wilkins
– Took X-ray photographs of DNA
■ James Watson and Francis Crick
– Took Franklin and Wilkins’ work and
discovered the structure of DNA
Structure of DNA
■ DNA is a chain of smaller units
– Like links on a chain
– Looks like a twisted ladder
■
Called a double helix
■ The smaller units are called nucleotides
■ A nucleotide is made up of 3 parts
– The backbone is made of:
■
A sugar, called deoxyribose
■
A phosphate group
– The interior (rungs of the ladder) have:
■
Nitrogen bases
Nitrogen Bases
■ 4 of them
– Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
– Their sequence determines the gene
■ Bases come in pairs
– They are held together by bonds
■
They give the DNA molecule its shape
– A-T (apple tree)
– C-G (car garage)
DNA Replication
■ Occurs whenever the cell splits (mitosis: recall S-phase)
■ DNA unzips (just like a zipper) and splits apart
– Each ½ provides a template to form another identical molecule
■ Example: Find the complementary strand
– AGTCGA
– TCAGCT
RNA
■ Another nucleic acid
■ Ribonucleic Acid
■ Similar to DNA, but key differences
– Where DNA has 2 sides, RNA is single-sided
– Where DNA has deoxyribose as its sugar, RNA has ribose
– Where DNA has Thymine (T), RNA replaces it with Uracil (U)
■
T DOES NOT EXIST IN RNA!
Types of RNA
■ mRNA (messenger RNA)-carries message from nucleus to ribosomes
■ rRNA (ribosomal RNA)-RNA in a ribosome that reads the code
■ tRNA (transfer RNA)-carries amino acid to ribosome
Making a protein
■ Proteins are chains made of units called amino acids
– The sequence of amino acids determines the protein’s shape
■
The shape determines the job
■ The sequence of amino acids is determined by the gene
– Better definition for gene: a segment of DNA that codes for a specific protein
■ mRNA is made from a strand of DNA (Called transcription)
– Practice: What RNA strand would form from this DNA strand?
■
■
–
ATGCGTA
UACGCAU
Because it is small, mRNA can leave the nucleus
■
It travels to a ribosome, where rRNA bonds to it
■ tRNA carries amino acids to the ribosome, and place them in the correct spot
(translation)
– The amino acids bond together, forming the protein
Central Dogma of Biology
■ Dogma-an idea known to be true that is central to the field
■ DNA  RNA  Proteins  Traits
Mutations
■ Change in nucleotide sequence on DNA
■ Different types
– Nucleotides can be deleted
– A wrong nucleotide can be added
– The wrong nucleotide can replace the correct one
– Chromosome parts can be broken
■ Can be natural, or can be the result of a mutagen (something in the environment
that causes a mutation)
– Smoking, alcohol, disease, excess sunlight, asbestos, etc.
Results of Mutations
■ Most mutations are neutral
– You’d never even know it happened
■
The cell repairs it, or it occurs in noncoding (junk) DNA
■ Other mutations are bad
– Can lead to disease, such as cancer
■ Occasionally, mutations are beneficial
– Give something to the offspring that they did not previously have
■
Example: Making an animal’s coat look a little more similar to its surroundings
–
This is what natural selection acts on (more to come next chapter)