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Biology Ch 16-17 (Ch 16.2 (Synthesizing DNA Strand (DNA polymerases…
Biology Ch 16-17
Ch 16.2
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Synthesizing DNA Strand
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Primase can start an RNA chain from scratch and adds RNA nucleotides using parental DNA as a template
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The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells
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As each monomer joins the DNA strand, via a dehydration reaction, it loses two phosphate groups as a molecule of pyrophosphate
Antiparallel Elongation
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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 of DNA the DNA polymerase synthesizes a leading strand continuously moving toward replication fork
To elongate the other new strand, called the lagging strand, DNA polymerase must work in the direction away from the replication fork
he lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase
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DNA Replication Complex
The proteins that participate in DNA replication form a large complex, a “DNA replication machine”
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Recent studies support a model in which DNA polymerase molecules “reel in” parental DNA and extrude newly made daughter DNA molecules
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Ch 17.4
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Building a Polypeptide
Initiation
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First, a small ribosomal subunit binds with mRNA and a special initiator tRNA
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Proteins called initiation factors bring in the large subunit that completes the translation initiation complex
Elongation
During elongation, amino acids are added one by one to the C-terminus of the growing chain
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Elongation occurs in three steps: codon recognition, peptide bond formation, and translocation
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The ribosome and mRNA move relative to each other, codon by codon
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Termination
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This reaction releases the polypeptide, and the translation assembly comes apart
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Targeting Polypeptides
Two populations of ribosomes are evident in cells: free ribosomes (in the cytosol) and bound ribosomes (attached to the ER)
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Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell
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Ch 17.2-17.3
Transcription
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RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and joins together the RNA nucleotides
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RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine
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In bacteria, the sequence signaling the end of transcription is called the terminator
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Ch7.5
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Subsitutions
A nucleotide-pair substitution replaces one nucleotide and its partner with another pair of nucleotides
Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code
Missense mutations still code for an amino acid, but not the correct amino acid
Nonsense mutations change an amino acid codon into a stop codon; most lead to a nonfunctional protein
Insertions and Deletions
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Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation
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Gene
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A gene can be defined as a region of DNA that can be expressed to produce a final functional product that is either a polypeptide or an RNA molecule
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Ch 16.3
Chromosomes
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The bacterial chromosome is a double-stranded, circular DNA molecule associated with a small amount of protein
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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
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Unfolded chromatin resembles beads on a string, with each “bead” being a nucleosome, the basic unit of DNA packaging
They are composed of two each of the four basic histone types, with DNA wrapped twice around the core of the eight histones
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Nucleosomes, and especially their histone tails, are involved in the regulation of gene expression
Chromosomes changes
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At interphase, some chromatin seems to be organized into a 10-nm fiber, but much is compacted into a 30-nm fiber, through folding and looping
Interphase chromosomes occupy specific restricted regions in the nucleus, and the fibers of different chromosomes do not become entangled
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During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin
Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions
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