CH's 16-17
Chapter 16: Molecular Inheritance
Molecular Structure of DNA and How it Contains the Information of Genetic Inheritance
Key Proteins in DNA Replication
DNA vs Chromatin vs Chromosomes
DNA is your "genetic endowment" meaning that you inherited traits from both of your parents and that genetic information is stored here
DNA Replication- the process by which DNA copies itself
early in the 20th century, finding the molecules that genetic information are made up of was a huge challenge
until the 1940's many thought that proteins contained genetic information
in 1928, Frederick Griffith discovered that the genetic information of dead, pathogenic bacteria can be inherited by living, nonpathogentic bacteria in their area and still infect the host.
this process was called transformation
transformation- a change in the genotype and phenotype by an external source
scientists also studied bacteriophages to prove that DNA was the genetic material
In 1952, Alfred Hershey and Martha Chase showed that DNA is the genetic material of bacteriophages
DNA is a polymer of nucleotides
3 Components
Nitrogenous base
pentose sugar (deoxyribose)
phosphate group
The base can be one of 4 bases
Adenine (A) purine
Thymine (T) pyrimidine
Guanine (G) purine
Cytosine (C) pyrimidine
A pairs with T and G pairs with C because a purine must always pair with a pyrimidine
In 1950, Erwin Chargaff discovered that the composition of DNA is different for different species
After most scientists accepted tha DNA was genetic material, the next challenge was finding its 3D shape
this was discovered by James Watson and Francis Crick
the shape of DNA is a double helix
this is backbone of DNA
they run antiparallel to each other
DNA strands are complementary to each other and thus contain the information for creating another complementary strand
That's how DNA replicates itself
there are 3 modeels for DNA replication
conservative- parental strands still pair after replication
semiconservative- the parental strand splits and pairs with the daughter strands and then in the second generation two out of the four granddaughter stands contain the parental genetic information
dispersive- parental strand is split evenly among all daughter strands for all generations
this is the one that turned out to be true
each somatic cell has 46 DNA molecules (1 molecule per chromosome)
the process of replication is pretty much the same for both prokaryotes and eukaryotes
Replicating DNA 101
begins at the origins of replication
Helicases unwind the DNA strand
single-strand binding proteins bind to the strand to keep it from repairing
Topoisomerase relieves the strain on parental DNA by breaking, swiveling, and rebinding. The strain is caused by the unwinding.
the short chain of RNA produced during synthesis is called a primer (this is synthesized by primase)
the primer is the synthesized by DNA Polymerase III
the resulting DNA strand is called the leading strand and the other strand is called the lagging strand (synthesized in series of segments (Okazaki Fragments))
once the synthesis of the Okazaki fragments is complete, DNA ligase joins the sugar phosphate backbone to the strand of DNA
DNA polymerase looks for errors and deletes the error and replaces it with the correct letter (There are also mismatch repairs that are made by other enzymes) Errors are normally corrected (errors that aren't corrected are called mutations). One common error repair system is the nucleotide excision repair system
the end of replication can only be completed by telomeres
we would end up with missing chromosomes in the gametes if the telomeres kept degrading so, telomerase lengthens telomeres to prevent this from happening
chromosomes- carriers of genetic information (DNA)
chromatin- DNA +protein inside the nucleus
DNA- genetic information
heterochromatin can NOT be transcribed
euchromatin can be transcribed
interphase chromosomes are attached to the nuclear lamina
Chapter 17: Gene Expression
Transcription- the synthesis of RNA using DNA
RNA Processing
Translation- synthesis of a polypeptide using mRNA
Gene Expression- the process by which DNA direct protein synthesis to express certain phenotypes
two stages for coding of genes in to proteins
transcription
translation
History of Genes and Proteins
Archibald Garrod was the first to connect genes, enzymes, and phenotypes in 1902
he also said the genetic issues are linked to an inability to make a certain enzyme
George Beadle and Boris Ephrussi discovered that fruit fly mutations in the eye are caused by a lack of a certain enzyme being made in the 1930's
one gene one enzyme hypothesis
this hypothesis has since been edited
not all proteins are enzymes
one gene one polypeptide hypothesis
result is mRNA that carries genetic information protein synthesizing parts of the cell
takes place in the ribosomes
turns mRNA into amino acids
DNA -> RNA -> Protein = Central Dogma
triplet codes- groups of 3 DNA bases that code for certain codons
the strand of DNA that is transcribed is called a template strand
the mRNA created here is complementary to the DNA
mRNA nucleotide triplets are called codons (written in the 5' to 3' direction)
codons can also be the nucleotide triplets along the non-coding DNA strand (aka: the coding strand since it's similar to the mRNA)
amino acids that are created in translation run in the 5' to 3' direction
Marshall Nirenberg discovered the first codon in 1961
all of the codons were found by the mid-1960's
there are 3 stop codons and one start codon
Start: AUG
Stop: UAA, UAG, and UGA
RNA Polymerase- tears apart the two strands of DNA and replaces complementary DNA with complementary RNA (assembles in a 5' to 3' direction)
the promoter initiates transcription and the terminator stops transcription
transcription unit- where DNA is transcribed into RNA
three steps
initiation- polymerase binds to the promoter and unwinds DNA, then, RNA synthesizes at the start point
elongation- RNA continues to elongate as it transcribes the DNA
termination- RNA transcript is released and polymerase detaches from the DNA
The mRNA ends
the 5' end gets a 5' cap (a modified form of guanine
occurs after transcription of the first 20-40 nucleotides
a poly-A tail (50-250 adenine) is added to the 3' end
the end caps, help ribosomes attach to the 5' end, protect the RNA, and facilitate the exit of mRNA from the nucleus
Splicing- when parts of the RNA are removed and the rest is reconnected
introns- non-coding sequences of nucleotides
extons- the coding sequences of nucleotides
spliceosomes carry out this process
sometimes functions as a ribozyme
acts as a translator and translates mRNA codons into amino acids
anticodons are nucleotide triplets that are complementary to mRNA (written in the 3' to 5' direction
aminoacyl-tRNA synthase makes sure the tRNA molecule is carrying the correct amino acid
some tRNA's can bind to more than one codon because the first and third bases can be swapped
this is called wobble
three sites
P site- holds tRNA
A site- holds the tRNA that's holding the next amino acid in the sequence
E site- the exit for tRNA
translation also has intiation, elongation, and termination
the polypeptide chains formed during translation are then folded into its secondary and tertiary structures
polypeptides
sent to specific locations
free ribosomes go free in the cytosol
bound ribosomes are attached to the ER
polypeptide codes the ribosome's location
marked by a signal peptide which in turn is recognized by a signal-recognition particle
a ribosome can make one in less than a minute
Mutations- changes to the genetic information of a cell
point mutation- changes of a nucleotide
small scale mutations
nucleotide-pair substitution- one pair of nucleotides is wapped out for another
silent doesn't change the resulting protein
missense changes the amino acid but has little effect on the resulting protein
most common
nonsense prematurely ends translation but putting in a stop codon
frameshift mutation- inserts or deletes a nucleotide pair
way more disastrous than a point mutation
mutagens- physical and chemical agents that interact with DNA