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DNA &
THE GENETIC CODE DON’T PANIC!
THIS SECTION OF SLIDES IS AVAILABLE AT CLASS WEBSITE
Recommended reading: “The Double Helix: A Personal Account of the Discovery of the Structure of DNA, by James D. Watson
Deoxy Nucleic
Deoxy Nucleic l Complex long molecule l “Double helix” with 2 identical
strands running in opposite directions (“anti-parallel”)
l Composed of four “bases” • Adenine (A) • Thymine (T) • Cytosine (C) • Guanine (G)
l The sequence of bases corresponds to the “code” l Genetic Information Carrier
l Used by all life to store and transmit all hereditary information (some use RNA)
A T C G A G C T
“All That Can Grow”
DNA: DeoxyriboNucleic Acid
The bases
A single nucleotide (“monomer”)
A polynucleotide (“polymer)
A T
A T
C G
G C
A T
A T
C G
G C
All the hereditary information is contained in the order of the bases along either strand of double helix è the hereditary library! • Each base = “letter”
• Each 3 base set = “word” è codon
• Large segments of many codons = “sentence” è gene è physical trait
l Contains sets of instructions – Each complete set of instructions = – Each gene is a segment of the DNA chain
l Contains sets of instructions – Each set of instructions = – Each gene is a segment of the DNA chain
Each pair of chromosomes contains one chromosome from the mother and one from the father
l Contains sets of instructions – Each set of instructions = – Each gene is a segment of the DNA chain
l Entire set of genes = – Humans have 3 billion bases – Spread across all 46 chromosomes (2
“copies” of 23 types; 1 copy is from the mother and the other is from the father, so the 2 copies are not exactly the same –why not all siblings look the same!)
– About 21,000 genes in total
l Every cell has complete genome (spread across the 23 chromosome pairs)
l Rules for how to read it:
l 98% of human DNA is non-coding! (“Non-functional” for protein synthesis—left over from evolutionary dead-ends?)
l DNA molecules are very large – Human chromosome 1 is a single DNA molecule
containing 243 million base pairs!
Note: DNA is only located in chromosomes for part of life cycle of a cell, in particular, just before cell division.
During the “interphase,” DNA is more distributed around the nucleus.
A T
A T
C G
G C
A T C G A G C T
“All That Can Grow”
l Accurate replications
l A ó T and C ó G
most complex & versatile “macromolecules”
à Cell structure (fingernails, hair,..)
à Enzymes (make reactions faster à digestion)
Composed of l 70 known, only ~20 used in life l “Left-handed” only Amine
group (NH2)
Carbolic acid group (COOH)
Long chains of amino acids are called polypeptides or proteins. Short chains (<30 or so) are usually just called peptides.
contains the information needed to make “stuff” from the amino acids. A part of one strand of the DNA becomes the template for the RNA (a process called “transcription”)
feeds selected instructions to ribosomes (molecule factories) about what amino acids must be located and carried to the ribosome to make proteins.
“worker”- build structure, function as enzymes to catalyze chemical reaction in the cell important for maintaining life function, such as metabolism.
l 4 letters (U,C,G,A) l 3 bases = codon = “word” l Each word = one amino acid or an instruction
to “start reading” or “stop reading”
Using the genetic code, RNA strands are translated to specific sequences of amino acids which are the building blocks of proteins. These RNA strands are created using DNA strands as the template (a process called “transcription”)
(transfer RNA)
(messenger RNA)
DNA tells mRNA what must be made.
At the factory (ribosome), mRNA sends tRNA out for the parts (various amino acids).
tRNA brings parts back to factory, where they are assembled into proteins.
Using the genetic code, RNA strands are translated to specific sequences of amino acids which are the building blocks of proteins. These RNA strands are created using DNA strands as the template (a process called “transcription”)
l DNA replication very accurate – One error per billion bases copied
l Errors form the basis of mutations.
l Most mutations are neutral, some are bad, some are beneficial, some are both.
l Good mutations get propagated by natural selection.
à different shaped red blood cells
stiff and sticky, can block blood flow
resistance to malaria
The Tree of Life Modern version based on detailed genetic analysis (DNA)
Scientist can determine, or sequence, the genomes of organisms on Earth and look for similarities.
Modern version based on detailed genetic analysis (DNA):
microbes microbes
“Last Universal Common Ancestor” LUCA
Tree of life points toward common ancestor
In the lab, usually make equal numbers of L and R handed Molecules, but for life on earth:
• Most amino acids are left handed • Most sugars are right handed
(“Handedness”)
l All life uses similar cells
l All life uses same 20 amino acids, while over 70 are known
l All life uses left-handed amino acids l All life uses DNA
l Tree of life points toward common ancestor
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