Exam 3 Flashcards
Deoxyribonucleic acid (DNA)
Each nucleotide is made up of ribose, phosphate, and a nitrogen base
Purine nucleotides
Adenine and guanine, 2 rings, pair with pyrimidines
Pyrimidines
Cytosine and thymine, 1 ring, pair with purines
What holds the sugar phosphate backbone together?
Phosphodiester bonds
Chargaff’s rules
A+G=T+C
double helix structure
creates a major and minor groove, important for the proteins that bind to its ability to access the DNA
Nucleosomes
DNA is compacted 7x by winding it around these
They are the most basic units of compaction, made up of histone proteins.
5 main types of histones
H1, H2a, H2b, H3, H4
What histones make up nucleosomes?
2x2H2A, 2XH2B, 2XH3, 2XH4. 146 bp of DNA is wrapped around this core of proteins
What is the purpose of the H1 histone?
Serves as a locket that seals the DNA/histone complex
Chromatin
DNA + histones
Structure of DNA
Antiparallel double helix
Semiconservative replication
DNA is replicated by unwinding the 2 strands of the double helix and building up a new complementary strand on each of the separated strands of the original double helix
Conservative replication
copy the whole chromosome - daughter DNA completely new
Dispersive replication
randomly copy a bit here a bit there
Meselson and Stahl experiment
grew E. Coli with a heavy isotope of nitrogen - integrated into the genome. then switch to a lighter nitrogen isotope, look at different weights of DNA
Steps of DNA replication
- Initiation
- Elongation
- Termination
Bidirectionally
Initiation
Unwind the double helix at the right spot and keep it unwound
Helicase
Enzyme that breaks H-bonds and unwinds the DNA
Single-stranded binding proteins
Keep DNA strands from binding each other or itself and protect them from being chewed up
DNA gyrase (bacteria) / topoisomerase (eukaryotes)
Keeps DNA from tangling up as the replication fork moves along
Origin of replication
The location where DNA replication is initiated
Rolling circle replication
In circular bacterial chromosomes, replication occurs in both directions until they meet
Specific DNA sequences at the ori
AT-rich - AT is easier to break because they only have 2 hydrogen bonds
DnaA
A protein that binds to 9-mer region, forcing unwinding of the 13-mer region to form an open complex
DnaB and DnaC
DnaC delivers DnaB protein to the open complex to initiate helicase activity
Which regions are replicated 1st?
Regions of genes actively expressed (euchromatin) are replicated earlier than those with repressed genes (heterochromatin)
Elongation
DNA polymerase, the enzyme that add nucleotides can only add them to the 3’ end and since DNA is antiparallel, replication occurs in opposite directions at the fork. DNA is added 5 —> 3 (of the new strand)
Primer
DNA pol III cannot start a chain of nucleotides, it can only add to a chain, so it needs to add on to primers. The primer is a short sequence of nucleotides that binds to the template strand to form a short duplex
Primase
The enzyme that adds the primer to the DNA chain; it is a type of RNA polymerase
DNA polymerase I
Enzyme that removes the primer - 5’ to 3’ exonuclease activity
5’ to 3’ polymerase - adds DNA nucleotides
Ligase
Stitches DNA back together
DNA topoisomerase
Relaxes supercoiling - recognizes supercoil, cuts DNA, rotates it to linearize it
Proofreading activity
DNA polymerase III finds incorrect bases and excises them because of its 3’>5’ exonuclease activity
Telomeres
Because DNA is synthesized only in 5’>3’ after the primer is removed, the end of the lagging strand is missing a bit of DNA at the telomere. Functions as a protective cap by sheltering the 3’ overhang of the DNA, which would be “seen” as a DNA break
Telomerase
Extends the template strand, primer is removed —> new 3’ overhang, makes new telomeres every cell division, made of an RNA and a protein (ribonucleoprotein), cancer cells reactivate telomerase expression
Hayflick’s limit
Somatic cells have a definitive number of replication cycles, dictated by the length of telomeres
The central dogma
DNA, RNA, protein
Ribozymes
RNA can catalyze biological reaction like proteins
- join amino acids to make proteins
- splicing pieces of DNA together
- tRNA processing
Messenger RNA (mRNA)
Used to encode the sequence of amino acids in a polypeptide. May be polycistronic (encoding two or more polypeptides) in bacteria and archaea. Encodes single polypeptides in nearly all eukaryotes
How many bonds are there between A and T?
2 hydrogen bonds
how many bonds are between G and C?
3 hydrogen bonds
histone charge
positively charged, DNA is negative –> interaction is favorable
histone structure
globular domain around which the DNA is attached, have tails that are accessible for things to bind to them, allows for modification of the interaction between nucleosomes and DNA
DNA polymerase III
adds new nucleotides in the 5’ –> 3’ direction
falls off of DNA - need the DNA polymerase III holoenzyme (catalytic core + accessory proteins), turns DNA pol III into a processing enzyme
Beta clamp
protein made of many dimers, latches DNA polymerase onto DNA by pushing it along
End replication problem
solution - add a DNA sequence that doesn’t encode for genetic information
telomeres
made up of repetitive “non-coding” DNA of unique sequences for every species, 500-3000 repeats, G-rich overhang, function as a protective cap by sheltering the 3’ overhang of the DAN into a T-loop
Sheltering complex
hides the 3’ overhang
stem cells
essentially immortal unlike somatic cells
RNA vs DNA
RNA has an extra OH at the 2’ carbon
What was the genetic info of the first organisms made of?
RNA: Acetyl-coA, Vit B12, dNTPs, primers
tRNA
decoders of DNA information into amino acids for proteins
rRNA
molecular machines that catalyze the assembly of proteins by using mRNAs and tRNAs
snRNA
process mRNAs
transcription direction
template: 3 –>5
RNA: 5 –> 3
+1 nucleotide
1st nucleotide of the coding region, not the first nucleotide that encodes information, upstream of the part that encodes info
transcription initiation prokaryotes
specific sequence mark for the start of the gene: -35, -10
- RNA pol holoenzyme scans DNA for the right start, opens the helix and binds it to start transcription
- core enzyme: synthesizes RNA
- sigma subunit: recognizes -35 and -10
bacteria RNA pol subunits
- sigma: bind to -35/-10 regions and unwinds the -10 to allow the polymerase to bind to the promoter
- beta’ - binds DNA
- alpha - binds regulatory proteins
- beta - catalytic
- w - enzyme assembly and gene expression