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Pyrimidines
nitrogen bases with 1 ring (Thymine and Cytosine)
Purines
Nitrogenous bases with 2 rings (Adenine and Guanine)
The strands of DNA are
antiparallel
3’ stand of DNA has
no phosphate on the end
5’ strand of DNA has
a phosphate on the end
What holds the strands together?
hydrogen bonds
the bonds between phosphates and sugars are called
phosphodiester bonds
DNA in prokaryotes
circular chromosomes, is held in cytoplasm, and is made up of plasmids and double strands
DNA in eukaryotes
multiple linear chromosomes, is held in the nucleus, wrapped in proteins called histones, and is made up of plasmids and double strands
Base pairing allows each strand to
serve as a template for a new strand
DNA replication occurs in what phase?
S phase
Each double helix consists of
a parent/template strand and a new DNA strand
Semi-conservative process
every double helix has a new and old strand
Helicase
unwinds part of the DNA double helix
Topoisomerase
helps relieve the strain of unwinding by breaking, swiveling, and rejoining DNA strands
DNA polymerase
connects nucleotides together to make a strand
RNA polymerase
adds a few nucleotides of RNA to get the process started
ligase
connects DNA fragments together
During DNA replication, the first step is
DNA helicase unwinds the DNA strands
When the DNA helicase unwinds the DNA strand, topoisomerase
relaxes supercoiling in front of replication fork
Complementary nucleotides match with
the nucleotides on the original DNA molecule, which are connected to make a new strand
RNA polymerase adds a few nucleotides so that
DNA polymerase can start its function
DNA polymerase connects the nucleotides but can only add
nucleotides to a 3’ end of a nucleotide
Leading strand
once an RNA primer is added, DNA polymerase can continuously add nucleotides in the 5’ and 3’ direction
Lagging strand
made in Okazaki fragments that are later joined together by ligase
When RNA primers are removed, they are replaced by
DNA nucleotides
Telomeres
non-coding and repetitive sequences on the ends of chromosomes that serve as a protective cap
Aging process is believed to be due to
chromosomes getting shorter with each replication
Telomerase
adds DNA bases at 5’ end and have high activity in stem cells and cancers but not in most somatic cells
mRNA
carries information from DNA to ribosome
tRNA
carries amino acids to the ribosome
rRNA
building blocks of ribosomes
microRNA
small RNA molecules that bind to other RNA molecules to degrade them
The nucleotide sequence in DNA makes
complementary sequences in mRNA
In transcription, RNA polymerase uses a single strand of DNA to make
mRNA
The single strand that is used to make mRNA is called
the template strand; AKA noncoding, minus, or antisense strand
What happens before the mRNA strand leaves the nucleus?
a Poly-A tail and GTP cap is added
What is the purpose of the Poly-A tail and GTP cap?
it protects the ends of the mRNA strand from degrading
Alternative splicing
different versions of mRNA resulting from combining different exons
Where does translation occur?
occurs at ribosomes
In prokaryotes, translation occurs as the mRNA is
being transcribed
In eukaryotes, translation occurs
right after transcription
Translation: Initiation
small ribosomal sub-units binds to mRNA and an initiator tRNA, then a larger ribosomal sub-unit attaches
Translation: Elongation
the ribosome with the attached sub-units move down the mRNA in the 5’-3’ direction. for each codon, a tRNA with a corresponding anticodon brings an amino acid to the ribosome. After, the amino acid is added to the receding one by a peptide bond
If the anticodon on the tRNA is not complementary to the codon on the mRNA, it must
wait for a tRNA with a complementary anticodon to arrive
Translation: Termination
elongation continues until the ribosome reaches a stop codon in the mRNA, to which a protein called a release factor causes the polypeptide chain to separate from the ribosome
What processes happen after translation?
the polypeptide folds up based on the arrangement of its amino acids. Some polypeptides combine with others to make bigger proteins, and may be packaged at the ER or modified and packaged in the golgi
Protein synthesis in eukaryotes
transcription in cytoplasm, no mRNA editing, and transcription and translation occur simultaneously
Protein synthesis in prokaryotes
transcription in nucleus, mRNA is edited prior to translation, and translation occurs after transcription
Mutagen
external factors such as radiation and reactive chemicals
Point mutation
a base is changed but the number of bases stays the same (substitution)
Nonsense mutation
codes for a stop codon and the remainder of codons will not be read
Missense mutation
codes for a different amino acid
If a mutation results in an amino acid with a different property than the original one,
the protein will have a different shape and function
Silent mutation
codes for the same amino acid
Frameshift mutation
the insertion or deletion of a base shifts the reading frame, changing all the codons after mutations
Even with the same DNA,
not all cells in a multicellular organism look the same or do the same things
Histones
protein that DNA is wrapped around
Histone acetylation
acetyl groups are added to histones, which prevent them from binding the DNA as tightly in order to make room for proteins to bind for transcription
DNA methylation
methyl groups can attach to DNA bases, preventing transcription
Repressors prevent RNA polymerase from
binding to the DNA, preventing transcription
When RNA polymerase binds to DNA, it
increases transcription
mRNA degradation
nucleus enzymes break down mRNA; lifespan varies
What cell organelle modified proteins?
rough ER
MicroRNA can bind to mRNA, causing it to
degrade or blocking it from being translated
Operon
group of genes of related function
When transcribed, one mRNA is made containing the
sequence for all the genes in the operon
Repressible operon
usually functions in anabolic pathways; when the end product is present, transcription is repressed to allocate resources to other uses
Inducible operon
usually functions in catabolic pathways; produces enzymes only when the nutrients are available to avoid making useless proteins
General characteristics of gene regulation in prokaryotes:
operons, only in cytoplasm, no histones, no introns, regulation only at transcriptional level
General characteristics of gene regulation in eukaryotes:
individual genes, in nucleus and cytoplasm, histones present, regulation at epigenetic, post/transcriptional, post/translational levels
Restriction enzymes
cuts DNA into segments at specific sequences
Polymerase Chain Reaction (PCR)
process of making many copies of DNA for analysis
PCR: 1- Denaturation
heat briefly separates DNA strands
PCR: 2- Annealing
cools to allow primers to form hydrogen bonds with ends of target sequence
PCR: 3- Extension
DNA polymerase adds nucleotides to 3’ end of each primer
Gel electrophoresis
electricity is used to separate DNA fragments of different sizes
Small, self-replicating circular DNA molecules
plasmids
contains additional genes from another source
recombinant plasmids
Viruses must infect a host cell to
reproduce
Viruses consist of
a protein coat called a capsid, genetic material, and a lipid envelope with glycoproteins on the surface
The glycoproteins on the lipid envelope surface functions to
help bind the virus to the cell
Lytic infection
virus attaches, injects genetic material, host cell makes virus parts, and cell bursts, repeat
In a lysogenic infection, after genetic material is injected in, it combines with the host cell’s genetic material to form a
prophage
Each time the host cell divides, the prophage is replicating, passing
viral genetic material to new cells
In lysogenic infections, viruses are ___, meaning they do not cause symptoms
dormant
In retroviruses, they use RNA to make ___ with the enzyme Reverse Transcriptase
DNA
Note
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