14.1 Genes specify proteins via transcription and translation 14.2 Transcription is the DNA-directed synthesis of RNA 14.3 Eukaryotic cells modify RNA after transcription 14.4 Translation is the RNA-directed synthesis of a polypeptide 14.5 Mutations of one or a few nucleotides can affect protein structure and function
The replication of DNA is
semiconservative
origin of replication
the area of DNA that replication begins at (has specific sequence of nucleotides that allows starting proteins to attach to)
replication fork
the Y-shape formed by separating the two strands of DNA
helicase is responsible for
unwinding the helix at the replication fork
single-strand binding proteins
bind to unpaired DNA to keep the strands from pairing again
topoisomerase
relieves strain caused by unwinding on/around replication fork (keeps DNA strands straight and avoids supercoiling)
primer
is a sequence of 5 - 10 nucleotides made by primase; synthesis can’t start on its own and builds from the 3’ end of the primer
DNA polymerase
synthesizes new DNA by attaching complementary base pairs to the template strands
DNA ligase
joins phosphate backbones of fragments into a continuous strand
Describe prokaryotic chromosomes
single, circular DNA molecule
Describe eukaryotic chromosomes
linear DNA and many proteins
levels of chromosome packing in eukaryotes
DNA (double helix)
proteins called histones (+ charge) bind to phosphate backbones of DNA (- charge)
chromatin folds so histones form bead-like nucleosomes
fold into chromatin fiber as histone tails interact with other nucleosomes
fiber folds into “looped domains”
looped domains coil into condensed chromosomes
Interphase chromatin can be slightly condensed and packaged as well, called
heterochromatin, which is inaccessible to transcription because of its folding
euchromatin
“true chromatin”, less compact and not condensed; able to be transcribed
genes affect phenotypes by dictating the production of
polypeptides (that make up proteins)
differences between RNA and DNA
ribose sugar instead of a deoxyribose sugar
contains uracil instead of thymine, which is less stable (and therefore unsustainable for DNA)
transcription
the synthesis of RNA using DNA as a template
translation
the synthesis of a polypeptide using info from mRNA, occurs at the ribosomes
bacterial cells can begin translation while mRNA is still being transcribed because
they do not have nuclei
central dogma
Crick’s theory stating genetic information flows in one direction: DNA —> RNA —> proteion
how many nucleotides make up a unit of code that corresponds to an amino acid?
three (triplet code)
compared to the template DNA strand, the RNA molecule is
complementary and antiparallel
codon
a set of three nucleotides of DNA that codes for one amino acid
why is the genetic code redundant but not ambiguous?
several codons may code for the same amino acid, but will only code for that amino acid
how does genetic code support theories of evolution?
the genetic code is nearly universal, which suggests a shared common ancestor
RNA polymerase
synthesizes RNA by attaching RNA nucleotides to template strand
assemble in 5’ to 3’ direction
able to start a chain from scratch (doesn’t need a primer like DNA polymerase)
transcription unit
section of DNA transcribed
phases of transcription
initiation - RNA polymerase binds to the promoter (eukaryotes: transcription factors form a transcription initiation complex and help RNA polymerase bind and initiate)
elongation - RNA polymerase reads DNA from 3’ to 5’ and synthesizes 5’ to 3’, adding bases to the 3’)
termination
bacteria - reaches a terminator sequence + releases the transcript w/out modification
eukaryotes - proteins cut off RNA from polymerase at a transcribed signal + pre mRNA goes through modification
modifications to eukaryotic pre mRNA
5’ methyl cap attached to 5’ end
poly-A tail added to 3’ end
facilitate export of mRNA from nucleus + help protect RNA from hydrolytic enzymes
help mRNA attach to ribosomes
RNA splicing
removal of RNA that was initially synthesized that occurs in the nucleus; non-coding sections are cut and not translated
introns vs exons
introns are intervening sequences and are removed; exons are coding regions that are expressed
alternative RNA splicing
exons from the same gene are arranged in different combos which = more possible proteins formed from the same transcription
spliceosome
protein complex that removes introns, degrades them, and joins exons on either side of the intron together
ribozyme
RNA molecules that function as enzymes
where do amino acids added to the polypeptide come from?
in the cell’s cytoplasm or are taken in from surrounding solution
the anticodon of tRNA complements the
codon on mRNA; corresponds to one type of amino acid
what does a tRNA do after it has added its amino acid and exits the ribosome?
is reused and picks up another amino acid in the cytosol
aminoacyl-tRNA synthases
enzymes that help the tRNA and its amino acid bind
ribosomes contain these three binding sites used for translation:
P site - holds tRNA adding to the polypeptide
A site - holds the next tRNA in line
E site - discharges tRNA after amino acid has been deposited
polypeptide will exit through an exit tunnel
the small subunit of the ribosome allows
attachment of mRNA
ribosomes mainly rely on this for their structure and function
rRNA (ribosomal RNA, which act as ribozymes)
phases of translation
initiation - mRNA binds to small subunit and an initiator tRNA; mRNA is scanned until start codon so the initiator tRNA can bind to it
elongation - mRNA is read from 5’ to 3’ and amino acids are added one by one; rRNA forms peptide bonds between amino acids of A and P site & removes bond to tRNA; tRNA is moved A —> P, P—> E
termination - stop codon reaches A site and release factors hydrolyze bond between polypeptide and tRNA; polypeptide is released through exit tunnel
post-translational modifications
chemical changes to amino acids, removal of amino acids, change to polypeptides length; must be modified to become functional proteins and do specific jobs
point mutation
changes in a single nucleotide pair of a gene
genetic disorders/hereditary diseases are
when mutations have adverse effects on the phenotype
small-scale mutations include
single nucleotide-pair substitution - replacement of one base and its complement
insertion/deletion - addition or loss of nucleotide pairs in a gene
silent mutation
are not harmful b/c of redundancy of genetic code/translate to the same amino acid
missense mutation
mutations that change an amino acid to another
nonsense mutation
codon for amino acid is changed into a stop codon, ending translation prematurely
mutagen
physical or chemical agents that interact with DNA and can cause mutations (ex. X-rays, UV radiation, nucleotide analogs)