Post on 26-Dec-2015
transcript
Structure & Function of DNA
By C. Kohn, Waterford WI
What are genes?
You now know that genes encode for specific traits like eye color, ear lobes, and milk production.
A gene is simply a section of DNA that creates the proteins responsible for a specific trait.
Genes are found in DNA; chromosomes are made of DNA
Structure of DNA
DNA has several key components A Phosphate Molecule A Sugar Molecule A Nitrogenous Base (A,T,G,C)
Classification
The sugar and phosphate molecules comprise the ‘skeleton’ or ‘backbone’ of DNA
The nitrogenous base is used to encode the actual information on the gene needed to create the protein (a base is the C,G, T, or A)
Nucleotides
A nucleotide is a subunit (or building block) of DNA consisting of a base, a phosphate, and a ribose sugar.
Nucleotide
Bases
DNA has four nitrogenous bases Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
All information encoded in DNA exists through different combinationsof these four letters.
Bases (cont)
The DNA bases always exist in the same kinds of combinations A always pairs with T G always pairs with C “Great Combinations, Always Together”
Bases (cont)
A-T and G-C combos must occur for two main reasons 1. This is the only way they will fit inside
the framework of the DNA molecule 2. This is the only way that their binding
sites will match up
Pyrimidines vs. Purines
The bases are grouped into two categories Pyrimidines
Purines
Pyrimidines vs. Purines
Two pryrimidines would be too small to fit inside the structure of DNA
Two purines would be too big to fit inside the structure of DNA
Too small
Too big
Base Bonding
C-G and T-A are also necessary because of binding sites T and A have 2 binding sites C and G have 3 binding sites▪ They wouldn’t match up any other way
Specific Combos
Because of size, G and A would be too big together, and C and T would be too small together
Because of binding sites, G only matches up with C and T only matches with A
Why does this matter?
Knowing these facts are HUGE! This feature enables the structure of
DNA to enable its function In other words, because of
the G-C, T-A combination, DNA can be read and replicated.
Review
DNA has 3 main components A phosphate molecule A ribose sugar A Base (C,T,G,or A)
A phosphate, sugar, and base together is called a nucleotide, the building block of DNA
C-G and A-T are only possible because… This is the only way they fit inside DNA This is the only way their bonding sites match
up
To make a protein…
To make a protein, we have to make a copy strand of DNA and send it to a ribosome The copy strand is called mRNA
RNA vs. DNA
While our genetic information is encoded in double-stranded DNA, copies of this information are encoded in single-stranded RNA.
RNA is a primitive version of DNA. DNA and RNA are very similar; the key
differences are that… 1. RNA can be single stranded 2. RNA replaces a T with a U (uracil) 3. Also, the sugar is slightly different (extra -
OH molecule)
Transcription
The process of creating an mRNA copy of DNA is called Transcription. Think of “transcript” of a TV program – it’s just a copy
Transcription has three stages: 1. Initiation – DNA is unwound by helicase enzyme and a
polymerase enzyme binds to the DNA strand 2. Elongation – nucleotides are added by polymerase to the
developing mRNA strand 3. Termination – polymerase and mRNA are released from
the DNA strand; the strand is re-closed
Transcription involves two key enzymes: Helicase: the enzyme the opens the DNA strand Polymerase: the enzyme that creates the mRNA copy
Transcription Animation
C TG A C TG A C TG A
G C T A G C T A G C T A
C UG A C G A C G AU U
Step 1: Helicase opens and unwinds the DNA strandStep 2: Polymerase adds a complementary base for each nucleotideStep 3: The newly created mRNA strand goes to a ribosome to be readStep 4: The DNA strand is closed and re-wound
Key Note: 5 3
Transcription always occurs in a 5 > 3 direction.
The sugar molecule has 5 carbon atoms The 5th carbon atom is
‘inside’ the nucleotide, while the 3rd carbon atom is at the ‘lower’ edge▪ NOTE – there is no top or
down in DNA, so use these terms carefully!
Just remember: 5 > 3
5
3
Translation
Once an mRNA copy has been made, the next step is Translation.
Translation is when the information in the mRNA is ‘translated’ into the creation of a protein by a ribosome, or rRNA.
How Translation Works
The mRNA copy strand’s base letters are read in groups of three
E.g. if our mRNA strand was AUGGCAAAGGACCAU
it would be read as AUG GCA AAG GAC CAU
Each group of three is called a codon. i.e. AUG is a codon; GCA is a codon; etc.
1 Codon = 1 Amino Acid
Each codon codes for a specific amino acid. An amino acid is the building block of a
protein For example, GGG codes for Glycine
AUA codes for Serine CUA codes for Leucine
Each codon will specific which amino acid is added next in order to create a protein
Translation Animation
C UG A C G A C G AU U
Arginine
Serine
IsoleucineAsparagine
Arg
Ser
Iso
Asp
Protein
tRNA
Amino acids are determined by the strand of mRNA and brought to the ribosome by tRNA tRNA will only bind to a complementary
codon; e.g. ACG will bind the UGC–form of tRNA.
Amino Acids Proteins
A protein is a long string of amino acids.
The type of amino acids in a protein, and their order, determine the function of the protein
For example, insulin is shown here at the right
As you can see, it is simply a long chain of amino acids
The order and type of amino
acids is the primary
structure.
The arrangement of amino acids will create either a
helix spring or a pleated sheet.
The combination of springs and sheets is the
tertiary structure of a protein.
The final functional protein is
the quarternary structure.
Transcription occurs in the
nucleus. Translation
occurs in the ribosomes.
DNA and mRNA are a part of
transcription. mRNA, rRNA,
and tRNA are a part of
translation.
Transcription involves making the mRNA copy of
DNA.
Translation involves using
the mRNA copy to make a functional
protein out of amino acids in the ribosome.
DNA –> RNA –> Protein -> Traits