dna – molecular genetics. dna structure – review…… -dna is made of monomers called...
TRANSCRIPT
DNA – Molecular Genetics
DNA STRUCTURE – Review……
- DNA is made of monomers called nucleotides.
- Nucleotides consists of - 5 carbon sugar- Phosphate group- Nitrogenous base (A,T,C,G)
- Backbone- made of covalent bonds between bonds between the phosphate and sugar
- “Rungs of Ladder” – made of pairs of nitrogenous bases.
A-T C-G
Nitrogenous Bases In DNA, there are 4 types of bases
STRUCTURE-
- Sides of Ladder – Sugar/Phosphate backbone
- Rungs of Ladder – Base Pairs
- Two strands of DNA are connected through weaker Hydrogen bonds that form between bases
- Only certain bases can form these hydrogen bonds with each other
-They are called complementary
How many hydrogen bonds between
C-G?
A-T?
Would A-G be possible????
CHARGAFF’S RULE
Amount of Adenine = The amount of Thymine
Amount of Guanine = The amount of Cytosine
If a sample of DNA contained 16% adenine……….?
PURINES = PYRIDAMINES
What does DNA need to be able to do?
Be copied– every time a cell divides
DNA Replication
Be “read” – every time we need to make a protein.
Transcription
DNA REPLICATION
Overview Semi-conservative
Molecule opens up Each side is a template to build a
complementary side End up with two DNA molecules, each
one half old and half new.
Step 1 Unzipping the DNA Enzyme called helicase which breaks the
hydrogen bonds between the base pairs. OPENS THE MOLECULE UP!
Step 2 Complementary Base Pairing Each of the DNA strands can now act as
templates to construct complimentary sides. Enzyme DNA polymerase attaches
complementary nucleotides to each of the open strands
Semi-Conservative Replication
- Each of the resulting DNA molecules are identical to each other and each contains one original strand and one new one.
Not that simple though…… Antiparallel strands….
The two sides of the DNA molecule run in opposite directions. The 5 and 3 refer to the orientation of the ribose molecule
Because they are anti-parallel….
The copying can only happen in one direction along the
35 side of the template.
Leading strand
- Built continuously as fork opens
Lagging Strand
- Built in fragments,
- Needs patching together by ligase
Animations
Overview
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/micro04.swf::DNA%20Replication%20Fork
Detailed http://www.johnkyrk.com/DNAreplication.html
Replication Bubbles
DNA is LOOOOOOOOONNNNNNGGGGGG!
In order to speed-up copying, replicate in several places at once.
RNA vs. DNA
3 Main Differences Single Stranded Sugar in RNA is ribose In RNA, thymine is
replaced by uracil
Why use these replacements????? CHEAPER!!!!
3 types of RNA
Messenger (mRNA) carries the RNA copy of the DNA gene out of the nucleus to the ribosomes.
Ribosomal (rRNA) make-up portion of the ribosome.
Transfer (tRNA) bring individual amino acids to the ribosome for assembly into polypeptides.
2 key steps to making proteins
Transcription
- Copy DNA code on mRNA
Translation
Reading the mRNA molecule code and turning it into a polypeptide chain (linking together amino acids)
Transcription
Goal: Copy DNA code (gene) onto an mRNA molecule.
Steps:
1. Helicase opens DNA molecule at specific section (gene)
2. Enzyme RNA polymerase binds to a promoter site (specific base sequence) just before the gene sequence
3. RNA polymerase moves along gene sequence and creates a complementary RNA strand
Transcription
Animation: http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf
When RNA polymerase reaches terminator sequence (specific base sequence), new RNA molecule released from DNA molecule
mRNA molecule travels to the cytoplasm where the message will be translated into polypeptides
YOU TRY IT!
Transcription
Difference in cell types?
In eukaryotes, mRNA transcript has to be modified before leaving the nucleus to be translated. RNA EDITING!
SIGNIFIGANCE – One gene can code for many different proteins!!!! Introns – What gets cut out (stay in the nucleus) Exons – What gets left it (Exits the nucleus)
In prokaryotes, the mRNA is ready to go No Editing Needed!
RNA EDITING
Exons vs. Introns
Translation
Goal: mRNA Polypeptide (will be a protein once the 3D folding occurs)
Review
Proteins made by linking together amino acids
20 different amino acids
Sequence of amino acids going to determine protein properties (shape).
So how can a code of 4 letters (bases) code for 20 different amino acids?
The Genetic Code
In the genetic code, three bases will code for one amino acid
Why 3? 4^3 = 64 different combinations with nucleotide triplets, compared with 4^ 2 = 16 different combinations with pairs. Would 2 bases be enough???
The three bases that code for a specific amino acid are called codons
ONE CODON = ONE AMINO ACID!!!!
Translation
Codons- You are responsible for knowing 4:
AUG
UAA, UAG, UGA
Translation - Interpreting the Code
Sequence of mRNA GGUACGUCCCCA
Read as GGU-ACG-UCC-CCA
Translation (the details)
The players
mRNA – code for making polypeptide
rRNA – ribosome - assembles polypeptide
tRNA – transfers amino acids to ribosome for assembly
All RNA made by the NUCLEOLUS
tRNA and rRNA structure/function
tRNA is a folded RNA strand
- amino acid on one end- anti-codon on the other end
rRNA makes up part of the ribosomes - ribosomes made of rRNA and protein- Consists of two subunits
Translation in action…..
Best to watch a video first
Simple http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/tr
anslation.swf
Complex
http://highered.mcgraw-hill.com/olc/dl/120077/micro06.swf http://www.johnkyrk.com/DNAtranslation.html
Overall Process Initiation
Start codon read, first tRNA binds to mRNA bringing methionine
Ribosome assembles
Elongation See animations for detail Chain builds
Termination Stop codon reached Whole assembly falls apart Polypeptide formed
Mutations
Mistakes happen!
Sometimes during DNA replication, bases can get inserted, removed or switched!!
Changes in the DNA base sequence are called MUTATIONS!!!
Can be single nucleotides – Point Can be whole chunks of DNA- Chromosomal
Point Mutations
One base in the sequence is affected
Substitution – one base switched out for another
Insertion – base put into code
Deletion – base removed from code
Point - Substitution
If this happens on a gene portion of the DNA molecule, it can result in a change in one of the amino acids in a polypeptide sequence.
Point - Insertion/Deletion
When a base is inserted or deleted, much bigger changes
Changes like this are called frame shift mutations shift the reading frame of the genetic code. (how
we read it)
These mutation affect all amino acids that follow the mutation point
Example of frameshift
Point - Substitutions
Missense mutations new nucleotide alters the codon so as
to produce an altered amino acid in the protein product. Can cause big changes in
polypeptide if new amino acid in chain is chemically different than the one it replaced.
Can be neutral if new amino acid is chemically similar to one it replaced.
Point - Substitutions (cont.) Nonsense mutations
With a nonsense mutation, the new nucleotide changes a codon that specified an amino acid to one of the STOP codons (TAA, TAG, or TGA).
Significance?
The earlier in the gene that this occurs, the more truncated (shortened) the protein product and the more likely that it will be unable to function.
Point - Silent mutations
Most amino acids are encoded by several different codons.
For example, if the third base in the TCT codon for serine is changed to any one of the other three bases, serine will still be encoded. Such mutations are said to be silent because they cause no change in their product
Chromosomal Mutations
Involve large-scale changes to structure or number of a chromosome!
Have much greater consequences than point mutations WHY???
MANY GENES AFFECTED!!!!
Mutations-
So all mutations are bad right?????