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Chapter 16: the molecular basis of inheritance/ Chapter 17: Gene expression
Chapter 16: the molecular basis of inheritance/ Chapter 17: Gene expression
DNA is the genetic material (16.1)
In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA
DNA is a double helix consisting of two antiparallel sugar-phosphate chains
Nitrogenous bonds in DNA
Cytosine and Guanine go together because they are able to form three hydrogen bonds
Thymine and Adenine go together because they are able to form two hydrogen bonds
Nitrogenous bases form hydrogen bonds with specific pairs
Nitrogenous bases in DNA
Guanine
Cytosine
Adenine
Thymine
A chromosome consists of a DNA molecule packed together with proteins (16.3)
figure 16.15
DNA polymerases add nucleotides only to the free 3′ end of a growing strand; therefore, a new DNA strand can elongate only in the 5′ to 3′ direction
Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork
figure 16.16
To elongate the other new strand, called the lagging strand, DNA polymerase must work in the direction away from the replication fork
The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase
figure 16.14
DNA polymerases catalyze the synthesis of new DNA at a replication fork (with the release of two phosphates) Most DNA polymerases require a primer and a DNA template strand
figure 16.17 (DNA replication steps)
The leading strand is synthesized continuously in the 5 prime to 3 prime direction by DNA polymerase II
Primase begins synthesis of the RNA primer for the fifth Okazaki fragment
molecules of single-strand binding protein stabilize the unwound template strands
DNA polymerase III detaches and begins adding DNA nucleotides to the 3 prime end of the fragment 5 primer in the replication fork
Helicase unwinds the parental double helix
DNA polymerase I replaces fragments with DNA nucleotides, after last addition the backbone is left with a free 3 prime end
DNA ligase joins the 3 prime end of fragments to the 5 prime ends of fragments
telomeres
Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends called telomeres
Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules
Many proteins work together in DNA replication and repair (16.2
)
Primase
Synthesizes an RNA primer at 5 prime end of leading strand and at 5 prime end of each Okazaki fragment of lagging strand
DNA polymerase III
Using parental DNA as a template, synthesizes new DNA strand by adding nucleotides to an RNA primer or a pre-existing DNA strand
Topoisomerase
Relieves overwinding strain ahead of replication forks by breaking, swiveling, and rejoining DNA strands
DNA polymerase I
removes RNA nucleotides of primer from 5 prime end and replaces them with DNA nucleotides added to 3 prime end of adjacent fragment
Single-strand binding protein
Binds to and stabilizes single-stranded DNA until it is used as a template
DNA ligase
Joins Okazaki fragments of lagging strand; on leading strand, joins 3 prime end of DNA that replaces primer to rest of leading strand DNA
Helicase
Unwinds parental double helix at replication forks
DNA replication in prokaryotes and eukaryotes
prokaryotes
In prokaryotic cells, there is only one point of origin, replication occurs in two opposing directions at the same time, and takes place in the cell cytoplasm
eukaryotes
Eukaryotic cells on the other hand, have multiple points of origin, and use unidirectional replication within the nucleus of the cell.
Transcription is the DNA-directed synthesis of RNA: a closer look (17.1)
elongation
A gene can be transcribed simultaneously by several RNA polymerases
Nucleotides are added to the 3′ end of the growing RNA molecule
As RNA polymerase moves along the DNA, it untwists the double helix
termination
The mechanisms of termination are different in bacteria and eukaryotes
In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification
initiation
Promoters signal the transcription start point
AUG is always the starting codon. A promoter is a specific nucleotide sequence in the DNA of a gene that binds RNA polymerase, positioning it to start transcribing RNA at the appropriate place.
RNA splicing
RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence
In some cases, RNA splicing is carried out by spliceosomes
Mutations of one or a few nucleotides can affect protein structure and function (17.4)
Point vs. frame shift (including insertion/deletion)
Point mutations are changes in just one nucleotide pair of a gene
Insertions and deletions are additions or losses of nucleotide pairs in a gene
Nonsense
: change an amino acid codon into a stop codon; most lead to a nonfunctional protein
Missense vs. silent
Missense: still code for an amino acid, but not the correct amino acid
Silent: have no effect on the amino acid produced by a codon because of redundancy in the genetic code
Translation is the RNA-directed synthesis of a polypeptide: a closer look (17.3)
what must happen
a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase is made
a correct match between the tRNA anticodon and an mRNA codon must be made
steps
Elongation
During elongation, amino acids are added one by one to the C-terminus of the growing chain
Each addition involves proteins called elongation factors
Elongation occurs in three steps: codon recognition, peptide bond formation, and translocation
termination
Elongation continues until a stop codon in the mRNA reaches the A site of the ribosome
The A site accepts a protein called a release factor
The release factor causes the addition of a water molecule instead of an amino acid
This reaction releases the polypeptide, and the translation assembly comes apart
Initiation
The start codon (AUG) signals the start of translation
First, a small ribosomal subunit binds with mRNA and a special initiator tRNA
Then the small subunit moves along the mRNA until it reaches the start codon
what happens next?
Polypeptides destined for the ER or for secretion are marked by a signal peptide
polyribosomes
Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome. Polyribosomes enable a cell to make many copies of a polypeptide very quickly
tRNA structure
T arm
ribosome recognition
anticodon
three base pair sequence complementary to mRNA codon
D arm
aminoacyl-tRNA synthetase recognition
CCA tail
amino acid attachment