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Chapter 16 & 17 Inheritance & Gene Expression (DNA (GENETIC…
Chapter 16 & 17 Inheritance & Gene Expression
DNA
In 1953, James Watson and Francis Crick introduced an elegant double-helical model.
Hereditary information is encoded in DNA and reproduced in all cells of the body
DNA program directs the development of biochemical, anatomical, physiological, and behavioral traits
DNA is copied during DNA replication, and cells can repair their DNA
DNA is the genetic material
T. H. Morgan’s group showed that genes are located on chromosomes.
2 components of chromosomes—DNA and protein—became candidates for the genetic material
role of DNA in heredity was first discovered by studying bacteria and the viruses that infect them
TRANSFORM BACTERIA
genetic role of DNA began with research by Frederick Griffith in 1928
with two strains of a bacterium, one pathogenic and one harmless
transformation
, now defined as a change in genotype and phenotype due to assimilation of foreign DNA
GENETIC MATERIAL
DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group
The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C)
In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next
This evidence of diversity made DNA a more credible candidate for the genetic material
RULES
Two findings became known as Chargaff’s rules
The base composition of DNA varies between species
In any species the number of A and T bases is equal and the number of G and C bases is equal
The basis for these rules was not understood until the discovery of the double helix
SCIENTIFIC INQUIRY
Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical
The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases
The pattern in the photo suggested that the DNA molecule was made up of two strands, forming a double helix
VIRAL DNA
bacteriophages
(or phages), are widely used in molecular genetics research
A virus is DNA (sometimes RNA) enclosed by a protective coat, often simply protein
DNA is the genetic material of a phage known as T2
two components of T2 (DNA or protein) enters an E. coli cell during infection
injected DNA of the phage provides the genetic information
DNA REPLICATION AND REPAIR
relationship between structure and function is manifest in the double helix
specific base pairing suggested a possible copying mechanism for genetic material
the two strands of DNA are complementary
each strand acts as a template for building a new strand in replication
the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules
(a) Parental molecule
(b) Separation of parental strands into templates
(c) Formation of new strands complementary to template strands
semiconservative model
replication predicts that when a double helix replicates
ONE derived or “conserved” from the parent molecule) and one newly made strand
the dispersive model (each strand is a mix of old and new)
conservative model (the two parent strands rejoin)
ORIGIN OF REPLICATION
At the end of each replication bubble is a
replication fork
, a Y-shaped region where new DNA strands are elongating
Helicases
are enzymes that untwist the double helix at the replication forks
Single-strand binding proteins
bind to and stabilize single-stranded DNA
Topoisomerase
relieves the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands
SYNTHESIZING
DNA polymerases require a primer to which they can add nucleotides
The initial nucleotide strand is a short RNA
primer
This is synthesized by the enzyme
primase
Primase can start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template
The primer is short (5–10 nucleotides long), and the 3′ end serves as the starting point for the new DNA strand
Enzymes called DNA polymerases catalyze the synthesis of new DNA at a replication fork
the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork
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
Proofreading and Repairing DNA
DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides
mismatch repair of DNA, repair enzymes correct errors in base pairing
nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA
TELOMERES
Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends
do not prevent the shortening of DNA molecules
postpone the erosion of genes near the ends of DNA molecules
the shortening of telomeres is connected to aging
enzyme called telomerase catalyzes the lengthening of telomeres in germ cells
shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions
CHROMOSOMES
The bacterial chromosome is a double-stranded, circular DNA molecule associated with a small amount of protein
Eukaryotic chromosomes have linear DNA molecules associated with a large amount of protein
In a bacterium, the DNA is “supercoiled” and found in a region of the cell called the nucleoid
In the eukaryotic cell, DNA is precisely combined with proteins in a complex called
chromatin
Chromosomes fit into the nucleus through an elaborate, multilevel system of packing
Proteins called
histones
are responsible for the first level of packing in chromatin
Unfolded chromatin resembles beads on a string, with each “bead”
nucleosome, the basic unit of DNA packaging
Loosely packed chromatin is called
euchromatin
During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into
heterochromatin
GENETIC INFORMATION
Gene expression, the process by which DNA directs protein synthesis
includes two stages: transcription and translation
Proteins are the links between genotype and phenotype
DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins
BASIC PRINCIPLES
RNA is the bridge between genes and the proteins for which they code
Transcription
is the synthesis of RNA using information in DNA
Transcription produces
messenger RNA (mRNA)
Translation
is the synthesis of a polypeptide, using information in the mRNA
Ribosomes
are the sites of translation
THE GENETIC CODE
There are 20 amino acids, but there are only four nucleotide bases in DNA
triplet code: a series of nonoverlapping, three-nucleotide words
the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript
TRANSCRIPTION
primary transcript is the initial RNA transcript from any gene prior to processing
central dogma is the concept that cells are governed by a cellular chain of command
DNA → RNA → protein
Transcription is the first stage of gene expression
MOLECULAR COMPONENTS
RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and joins together the RNA nucleotides
The RNA is complementary to the DNA template strand
RNA polymerase does not need any primer
RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine
SEQUENCE
The DNA sequence where RNA polymerase attaches is called the promoter
In bacteria, the sequence signaling the end of transcription is called the terminator
The stretch of DNA that is transcribed is called a transcription unit
STAGES
Initiation
Elongation
Termination
BINDING AND INITIATION
Promoters signal the transcription start point and usually extend several dozen nucleotide pairs upstream of the start point
Transcription factors mediate the binding of RNA polymerase and the initiation of transcription
The completed assembly of transcription factors and RNA polymerase II bound to a promoter is calledd a transcription initiation complex
A promoter called a
TATA box
is crucial in forming the initiation complex in eukaryotes
RNA
During RNA processing, both ends of the primary transcript are altered
mRNA ends
Each end of a pre-mRNA molecule is modified in a particular way
The 5′ end receives a modified nucleotide 5′ cap
The 3′ end gets a poly-A tail
These modifications share several functions
They seem to facilitate the export of mRNA to the cytoplasm
They protect mRNA from hydrolytic enzymes
They help ribosomes attach to the 5′ end
RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence
These noncoding regions are called intervening sequences, or introns
The other regions are called exons
SPLICING
RNA splicing is carried out by spliceosomes
Spliceosomes
consist of a variety of proteins and several small RNAs that recognize the splice sites
The RNAs of the spliceosome also catalyze the splicing reaction
RIBOZYMES
catalytic RNA molecules that function as enzymes and can splice RNA
rendered obsolete the belief that all biological catalysts were proteins
ENZYME SIMILARITY
It can form a three-dimensional structure because of its ability to base-pair with itself
Some bases in RNA contain functional groups that may participate in catalysis
RNA may hydrogen-bond with other nucleic acid molecules
TRANSLATION
During translation, the mRNA base triplets, called codons, are read in the 5′ → 3′ direction
Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide
the RNA-directed synthesis of a polypeptide
mRNA to protein through the process of translation
MOLECULAR COMPONENTS
A cell translates an mRNA message into protein with the help of transfer RNA (tRNA)
tRNAs transfer amino acids to the growing polypeptide in a ribosome
Translation is a complex process in terms of its biochemistry and mechanics
tRNA
Each tRNA molecule enables translation of a given mRNA codon into a certain amino acid
Each carries a specific amino acid on one end
Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA
Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon
ACCURACY
Accurate translation requires two steps
First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase
Second: a correct match between the tRNA anticodon and an mRNA codon
RIBOSOMES
Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis
The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)
Bacterial and eukaryotic ribosomes have significant differences
Some antibiotic drugs specifically target bacterial ribosomes without harming eukaryotic ribosomes
THREE BINDING SITES
The P site holds the tRNA that carries the growing polypeptide chain
The A site holds the tRNA that carries the next amino acid to be added to the chain
The E site is the exit site, where discharged tRNAs leave the ribosome
POLYPEPTIDE
The three stages of translation:
Initiation
Elongation
Termination
All three stages require protein “factors” that aid in the translation process
Energy is required for some steps.
Polypeptides destined for the ER or for secretion are marked by a signal peptide
A signal-recognition particle (SRP) binds to the signal peptide
The SRP escorts the ribosome to a receptor protein built into the ER membrane
SYNTHESIS
Polypeptide synthesis begins
SRP bind to signal peptide
SRP bind to receptor protein
SRP detaches and polypeptide synthesis resumes
Signal-cleaving enzyme cuts off signal peptide
Completed polypeptide folds into final conformation
TERMINATION ON TRANSLATION
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
Ribosome reaches a stop
codon on mRNA.
Release factor promotes
hydrolysis.
Ribosomal subunits and other components dissociate