CH:16-17 (Life's Operating instructions (DNA begins at particular…
Life's Operating instructions
DNA is the genetic material
Shape of DNA is Double Helix
how a molecule of DNA is copied
Sugar-Phosphate Backbone with nitrogen bases
Theses bases are:
DNA Nucleotide is the Sugar-Phosphate backbone and the Nitrogen
DNA begins at particular sites called origins of replication
1) At each end of a replication bubble is a replicationfork
Y-shaped region where the parental strands of DNA
are being unwound
2) Helicases are used in making them available as template strands
enzymes that untwist the double helix at the replication forks, separating the two parental strands
3) single- strand binding proteins bind to the unpaired DNA strands, keeping them from re-pairing
The untwisting of the double helix causes tighter twisting and strain ahead of the replication fork
4) Topoisomerase are used during this process to help relieve the strain by breaking, swiveling, and rejoining DNA strands
5) DNA synthesis is actually a short stretch of RNA, not DNA.
This RNA chain is called a primer and is synthesized by the enzyme primase
Primase starts a complementary RNA chain with a single RNA nucleotide and adds RNA nucleotides one at a time
6) DNA polymerases catalyze the synthesis of new DNA by adding nucleotides to the 3′ end of a preexisting chain
7) When replication begins, the two parent DNA strands are separated. One of these is called the leading strand
it is replicated continuously in the 3' to 5' direction
The other strand is the lagging strand, and it is replicated discontinuously in short sections
These segments of the lagging strand are called Okazaki fragments, after Reiji Okazaki,
8) DNA polymerases proofread new DNA, replacing incorrect nucleotides.
In mismatch repair, enzymes correct errors that persist
Nucleotide excision repair is a process by which nucleases cut
out and other enzymes replace damaged stretches of DNA
The ends of eukaryotic chromosomal DNA get shorter with each
round of replication
The presence of telomeres, repetitive sequences at the ends of linear DNA molecules, postpones the erosion of genes.
Telomerase catalyzes the lengthening of telomeres in germ cells.
The chromosome of most bacterial species is a circular DNA molecule with some associated proteins, make up the nucleoid
The chromatin making up a eukaryotic chromosome is composed of DNA, histones, and other proteins
The histones bind to each other and to the DNA to form nucleosomes
the most basic
units of DNA packing
Histone tails extend outward from each
bead-like nucleosome core
Additional coiling and folding lead ultimately to the highly condensed chromatin of the metaphase chromosome
Chromosomes occupy restricted areas in the interphase
In interphase cells, most chromatin is less compacted
but some remains highly condensed (heterochromatin)
Euchromatin, but not heterochromatin, is
generally accessible for transcription of genes.
The Flow of Genetic Information
Beadle and Tatum’s studies of mutant strains of Neurospora led to the one gene–one polypeptide hypothesis.
During gene expression, the information encoded in genes is used to make specific polypeptide chains RNA molecules
Transcription is the synthesis of RNA complementary to a
template strand of DNA
RNA synthesis is catalyzed by RNA polymerase
These links together RNA nucleotides complementary to a DNA template strand.
This process follows the same base-pairing rules as DNA
replication, except that in RNA, uracil substitutes for thymine
The three stages of transcription are initiation, elongation,
initiation: RNA Polymerase-Promoter Complex binds to the promoter gene in the DNA.
This also allows for the finding of the start sequence for the RNA polymerase.
elongation: process through which nucleotides are added to the growing RNA chain
As the RNA polymerase moves down the DNA template strand, the open complex bubble moves also
termination: RNA polymerase will keep transcribing until it gets signals to stop.
it happens once the polymerase transcribes a sequence of DNA known as a terminator.
Termination differs in bacteria and eukaryotes
A promoter, often including a TATA box
in eukaryotes, establishes where RNA synthesis is initiated
Transcription factors help eukaryotic RNA polymerase recognize promoter sequences, forming a transcription initiation complex.
Translation is the synthesis of a polypeptide
the amino acid sequence is specified by the nucleotide sequence in messenger RNA (mRNA).
A cell translates an mRNA message into protein using transfer
aminoacyl-tRNA synthetase binds tRNA to specific amino acids
a tRNA lines up via its anticodon at the complementary codon on mRNA.
A ribosome, made up of rRNAs and proteins, facilitates this coupling with binding sites for mRNA and tRNA
Ribosomes coordinate the three stages of translation through the A and P sites and exit through the E site
After translation, during protein processing, proteins may be
modified by cleavage or by attachment
sugars, lipids, phosphates,
or other chemical groups.
Free ribosomes in the cytosol initiate synthesis of all proteins, but
proteins with a signal peptide are synthesized on the ER
A gene can be transcribed by multiple RNA polymerases simultaneously
Also, a single mRNA molecule can be translated simultaneously
by a number of ribosomes, forming a polyribosome
Genetic information is encoded as a sequence of nonoverlapping nucleotide triplets, or codons
A codon in mRNA either is translated into an amino acid (61 of the 64 codons) or serves as a stop signal (3 codons)
Codons must be read in the correct reading frame.
Eukaryotic cells modify RNA after transcription
Eukaryotic mRNAs undergo RNA processing
includes RNA splicing, the addition of a modified nucleotide 5' cap to the 5'end, and the addition of a poly-A tail to the 3' end
The processed mRNA includes an untranslated region (5' UTR or 3' UTR) at each end of the coding segment.
Most eukaryotic genes are split into segments
They have introns interspersed among the exons
the regions included in
In RNA splicing, introns are removed and exons
RNA splicing is typically carried out by spliceosomes,
but in some cases, RNA alone catalyzes its own splicing
The catalytic ability of some RNA molecules, called ribozymes, derives from the inherent properties of RNA.
The presence of introns
allows for alternative RNA splicing.