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biology chapter 16 and 17 (17 (1rst transcription (fig 17.8) (tools of…
biology chapter 16 and 17
17
the discovery that genes code for proteins
one gene one enzyme
1902 Archibald Garrod suggested genes dictated enzymes
several decades later research supported his hypothesis
neurospora experiment
Beadle and Edward Tatum mutated a breadmold so it was missing a single gene, and found that it could not produce a specific enzyme
one gene one protein
the hypothesis was made more general so it said one gene one protein, then one gene one polypeptide
genetic code
codons
three nucleotide bases code for one nucleic acid
cracking the code
molecular biologists cracked the code of life in the 1960s, starting with simple codons such as UUU
to read codons correctly you must start at the right point in the reading frame
1rst transcription (fig 17.8)
tools of transcription
promotor -sequence that initiates transcription
unwinds DNA and finds complimentary RNA neucleotides
terminator -sequence that ends transcription
transcription unit -DNA downstream from promotor
elongation
polymerase continues to move down strand and build RNA
DNA rewinds afterward
termination
RNA transcript is released and RNA polymerase detaches from DNA
initiation
RNA polymerase binds to promotor, unwinds DNA and synthesizes RNA at start point
this is done in the neucleus
2nd RNA processing
mRNA ends are each given an end cap, to protect and help attach to ribosomes
RNA splicing -large portions of rna is not used to code for polypeptides, this is removed by a splicosome
the functional and evolutionary importance of introns
exons -all other sections of RNA
different exons code for different parts (or domains) of proteins
introns -sectiond that do not code for amino acids and are removed
alternative rna splicing -genes can make different polypeptides depending on how it is spliced
ribozymes -sometimes a splicosome is made up of RNA and sometimes am RNA strand's own introns can act as a splicosome for itself
this is also done in the neucleus, only in eukaryotic cells
3rd translation
tools of translation
transfer RNA -a molecule made of RNA that has an anticodon that reads for a specific amino acid that it carries (fig 17.16)
aminoacyl tRNA synthetases -enzymes that help attach amino acids to their correct transfer RNAs
ribosomes
facilitates the matching of tRNA anticodons with mRNA codons
made of two subunits made of protein and RNA
building a polypeptide
elongation (fig 17.20)
ribosomes have three sites that tRNA moves through
They are moved by the mRNA as it slides through the ribosome
then it moves to the P site
when another tRNA joins the ribosome, the original tRNA leaves through the E site
the tRNA with the right anticodon binds to the A site
a peptide bond is formed between the amino acid and the growing chain
termination
when the stop codon on the mRNA comes, a release factor (a protein shaped like tRNA) comes instead of tRNA
it frees the polypeptide chain it causes the ribosomal subinits and mRNA to detach
initiation
mRNA binds to a small ribosomal subunit and a transfer RNA attaches to the start sequence (AUG)
a large ribosomal subunit joins
as mRNA moves through the ribosome it can enter other ribosomes and be used to make more than one polypeptide at once
targeting polypeptides to specific locations
a signal recognition particle can point an active ribosome to the endoplasmic reticulum, causing the polypeptide to be made into it
the is done on free ribosomes in the cytoplasm or attached to the endoplasmic reticulum
mutations
types
frameshift - causes the all the following codons to move and makes all of them unreadable
insertions -addition of a neucleotide
deletions -loss of a nucleotide
point mutations
silent -a substitution of a neucleotide that causes the codon to still code for the same amino acid
missense -changes one amino acid to another one
nonsense -changes to a stop codon, resulting in a shorter polpeptide
16
chromosomes
interphase chromatin
heterochromatin is highly condensed lumps that re not accessible to DNA transcription machinery
interphase chromatin may not be formlessly scrambles, but structured
attached to the nuclear lamina, nuclear matrix, and nuclear envelope
chromatin packing
30 nanometer fiber
the nucleosomes are coiled and wrapped together into a fiber
looped domains 300nm fiber
30nm fibers are looped and stacked together into an even thicker fiber
nucleosomes
DNA is wrapped around bundles of 6 histones
histones
there is as much histone as DNA in chromatin
DNA double helix
the typical wound piece of DNA
unknown step
the looped domains fold in a way not fully understood, compacting the chromatin into a chromosome
metaphase chromosome
the fully compacted x that is seem during mitosis of meiosis
DNA replication and repair
semiconservative model -base paring to a template strand
for DNA replication Watson and Crick guessed that DNA was split into its two strands and each strand was used as a template
experimentation by Mathew Meselson and Franklin Stahl supported their guess
getting started
origins of replication -replication proteins recognize specific sequence of nucleotides and open a replication bubble. replication proceeds in both directions
helocase unwinds and separated DNA
single stranded binding proteins stabilize unwound strands
Primase uses the strand as a template to synthesize RNA primers
Topoisomerase prepares DNA for unwinding
synthesizing a new DNA strand
DNA polymerase adds DNA nucleotides to primers, synthesizing a new strand
antiparalell elongation
DNA can only be replicated from the original strand's 3' end to its 5' end
Leading strand. This strand can be replicated continuously toward the point where the original strand is being unzipped
lagging strand this strand can't be continuously replicated because the original strand is being unzipped in the wrong direction
instead it is made in many short sections called Okazaki fragments
DNA pol 1 replaces the many rna primers with dna
dna ligase attaches the many fragments
DNA replication complex
the many proteins form one DNA replication machine
DNA might move through this machine, as in the trombone model
proofreading and repairing DNA
permanently altered DNA nucleotides become mutations
the vast majority of which are neutral or harmful
many dna polymerases proofread each nucleotide as it is atatched
enzymes proofread and repair DNA contunuously
DNA can altered during replication, from exposure to X rays or during chemical changes to the cell
the ends of DNA molecules
because of the lagging strand not being able to replicate without a 3' end, DNA strands grow shorter with each replication
telomeres simple repetitions of neucleotides on the ends of DNA that help prolong its length
telomerase helps lengthen telomeres
it is used by cells that replicate many times such as cancer or tumor cells
story of DNA discoverys
DNA can transform bacteria
(1928) Frederick Griffith found that nonpathogenic strains of
Streptococcus pneumoniae
could become pathogenic if mixed with the remains of pathogenic strains
the chemical component of dead cells that caused this change was later discovered to be DNA
viral DNA programs cells
(1952) Alfred Hershey and Martha Chase used viruses and bacteria to show that it was nucleic acid and not protein that coded for genetic information
more evidence
Erwin Chargriff found that DNA is different among different species, he also found that there is roughly equal ratios of each of the bases (Adenine, Thymine, Guanine Cytosine)
discovering a model
the first to come up with a complete model were James Watson and Francis Crick
with the help of Maurice Wilkins, they used an X ray to discover the double helix, they also figured out the base pair partnerships
DNA is made of a phosphate backbone attached to a sugar, which is attached to a neucleotide. tow of these connected at the neucleotides form a ladder, which is then wound into a helix
certain arrangements of neucleotides code for certain amino acids