Gauvrit section post midterm Flashcards
should you go back and do the first lecture in this section?
yes
What is DNA supercoiling?
DNA that is more compact than its relaxed counterpart is supercoiled
- Underwound DNA is negatively supercoiled
- Overwound DNA is positively supercoiled
- negative supercoiling is important to allow chromosomes to fit within nucleus
What are the two kinds of topoisomerases?
- Change the level of DNA supercoiling
- Type I - change the supercoiled state by creating a transient break in one strand of the duplex
- Type II - make a transient break in both strands of DNA duplex - can tie or untie knots - can interlink or separate circles
What is DNA denaturation?
Ability to sperate into components
- thermal denaturation
- more GC means higher melting point
What is nucleic acid hybridization?
complementary strands of nucleic acids from different sources can form hybrid molecules - important for sequencing and cloning
What are the three broad classes of DNA sequences?
- classes by how often their nucleotide sequence is repeated
- highly repeated fraction
- moderately repeated fraction
-nonrepeated fraction
What are three common highly repeated DNA sequences? How much of our total DNA are they?
1-10% of total DNA
- Satellite DNA - short sequences that evolve rapidly
- Minisatellite DNA - unstable and tend to be variable in the population - DNA fingerprinting
- Microsatellite DNA - shortest sequences and typically found in small clusters - found in genetic disorders
What is FISH?
Fluorescence in situ hybridization
- used to determine location of a DNA sequence within a genome
- can be used to visualize repetitive sequences like satellite DNA found in the centromeric regions of the chromosome - or to determine the position of single copy genes
What are moderately repeated DNA sequences? How much of our total DNA do they make up?
Varies greatly - between 20 and 80% of DNA
- some sequences code for abundant gene products like RNA but most lack a coding function
- noncoding elements are scattered throughout the genome and can be grouped into SINES or LINES
What are SINES and LINES?
Short interspersed elements and long interspersed elements
What are nonrepeated DNA sequences?
single copy DNA sequences invlude genes that exhibit mendelian patterns of inheritance and localize to a particular site on a particular chromosome
- includes coding sequences for virtually all proteins other than histones - less than 1.5% of genome
- even though sequences are only in one copy - genes that code for polypeptides are usually members of a family of related genes like globins actins etc
What is polyploidization?
Whole genome duplication - offspring have four chromosome homologues rather than 2
- often done to produce bigger fruits
-
What is the globin family and how did these genes evolve?
Hemoglobin, myoglobin, plant leghemoglobin
- ancestral forms have given rise to modern forms by duplication fusion and divergence
- some sequences called pseudogenes resemble globin genes but are nonfunctional
What do “jumping genes” refer to?
genetic rearrangement of genetic elements called transposition - via transposable elements
- only certain sequences can act on transposons
- requires enzyme transposase
- retrotransposons use RNA which produces complementary DNA vs reverse transcriptase
How are jumping genes important in adaptive genome evolution?
- transposable elements can carry parts of the host genome with them as they move from one site to another
- transposable elements themselves appear to have given rise to genes
How many genes does the human genome have?
20 000
What is alternative splicing?
when a single gene can encode a number of related proteins
is a significant proportion of functional DNA conserved?
no they are constantly evolving
The majority of the genome lies between ____ and its termed ____
lies between protein coding genes and is termed intergenic
- much of the 21000 protein coding regions consist of non coding portions (intronic DNA)
What are three genes that are relevant in humans and chimps
FOXP2 - similar in humans and chimps -speech gene
HAR1 - similar between humans and chimps - function unknown
AMY1 - amylase enzyme - frequency is very different
What is the exome?
portion of the genome that codes for proteins
What are the three parts of a chromosome?
- chromatin - fibers composed of DNA and proteins
- histones - highly conserved proteins
- nucleosomes - the subunits that DNA and histones are organized into
Explain the structure of a nucleosome complex;
Histone H1 serves as a linker
- DNAis wrapped around the core complex
- the core complex is 2 H2A, H2B, H3, H4 - forming an octomer
-
What groove faces the histone core?
minor groove
What is important about the histone tail?
- each core histone has a flexible N term tail that projects beyond the DNA helix
- histone tails can be modified
What are chromatin loops?
Higher level chromatin structure
- takes 30nm chromatin fibers and gathers them into large 80-100nm supercoiled loops
- DNA loops are tethered at their bases to proteins
-
What does cohesin do?
-holds replicated DNA molecules together during mitosis
- maintains chromatin loops
What is euchromatin?
Chromatin that returns to a dispersed state after mitosis in interphase
What is constitutive and facultative heterochromatin?
Constitutive - remains condensed all the time - found mostly around centromeres and telomeres and consists of highly repeated sequences
Facultative - inactivated during certain phases of the organisms life - one of the X chromosomes as a Barr Body through X inactivation
What is a Barr body?
condensed inactive X chromosome in females - to preserve ratios
What is epigenetics?
study of how your behaviours and environment can cause changes that affect the way your genes work - not changes in genes itself but how body reads the genes and expresses them
What is the histone code hypothesis?
The histone code hypothesis states that the activity of a chromatin region depends on the degree of chemical
modification of histone tails
What are the two things histone tail modifications can do?
Can serve as docking sites to recruit nonhistone proteins
Can alter the way histones of neighboring nucleosomes interact
Where is the majority of modified amino acids found in histones?
N terminus of H3 and H4
What inactivates H3 histones?
methylation - stability
removal of acetyl groups from H3 and H4 convert to heterochromatin
- Inactive heterochromatic X chromosome has deacylated histones
What is a telomere?
repeated sequence on DNA tips that with a group of specialized proteins form a cap called a telomere
Why are telomeres used?
to get around the end replication problem - RNA primer causes the removal of some DNA at the 5’ end
- telomerase is a reverse transcriptase that makes DNA from RNA that the enzyme itself has - once end has lengthenes the DNA polymerase can return complementary strand to previous length
What is the significance of telomeres?
- required for complete chromosome replication
- form caps that protect chromosomes from nucleases
- prevent ends of chromosomes from fusing with one another
What is the centromere?
indented site on chromosome that is repeated heterochromatin
- has centromeric DNA where microtubules attach during mitosis
What is RNA folding driven by?
formation of regions having complementary base pairs
What are transcription enzymes also known as?
RNA polymerases
What direction does RNA polymerase move in? How fast does it assemble the complementary strand?
Moves in the ????? (3’ to 5’) direction - roughly 20-50 nucleotides a second.
What is different about bacterial transcription and translation?
They occur at the same time - there is no physical separation of DNA/RNA/ribosomes
How are genes organized in bacteria?
organized in clusters that are transcribed together as a single mRNA molecule encoding for multiple proteins - an operon
What do transcription factors do?
assist in beginning transcription and making sure rna polymerase is working
How many RNA polymerases do bacteria have?
one - 5 subunits
Where does transcription and RNA processing occur in eukaryotes?
in the nucleus
What is a holoenzyme?
complete enzyme with core enzyme, sigma unit and promoter region
Where does translation occur in eukaryotic cells?
cytoplasm
what binds the promoter
sigma subunit (?)
What are the three types of RNA polymerase in eukaryotes and what do they transcribe?
Most rRNAs are transcribed by RNA polymerase I
-mRNAs are transcribed by RNA polymerase II
- tRNA are transcribed by RNA polymerase III
What is the most common type of RNA?
mRNA
What direction does RNA polymerase incorporate nucleotides?
5-3
In transcription a 9nt what is formed?
DNA RNA transient hybrid
Transcription generates ___ DNA ahead and ___ behind
overwound (positively supercoiled) and underwound (negatively supercoiled)
What are transcription factors?
proteins that bind DNA, each recognizes different DNA sequences - controlling the rate of transcription.
RNA polymerase II is assisted by _________ to form the ________
general transcription factors (GTFs) to form the preinitiation complex (PIC)
What does BRE mean?
B recognition element
What does DPE mean?
downstream promoter element
What is on either end of an mRNA molecule?
3’ poly A tail, 5’ cap
eukaryotic genes contain ___ which are missing from mature mRNAs
intervening sequences
The presence of genes with intervening sequences are called ??
split genes
What are exons and introns?
exons are coding sequences that are present in the final mRNA, introns are removed during mRNA processing
What are RNA transcripts associated with after they are synthesized?
ribonucleoproteins
Pre mRNA transcripts are processed ___
cotranscriptionally
What is the point of introns?
thought to increase number of proteins made per gene - able to remove specific sections to encode for different proteins.
Transcription factors bind to (2)
- bind at core promoter sites in association with RNA pool
- bind to various regulatory sites of particular genes (can act as transcription activators or inhibitors)
What two domains do transcription factors contain?
- DNA binding domain
- activation domain
what kind of cells are capable of indefinite self renewal?
embryonic stem cells
More than 80% of the RNA in most cells consists of ____ RNA
ribosomal (rRNA)
What is the size of the bacterial ribosome and subunits
70s, 50s large, 30s small
What is the size of the eukaryotic ribosome and subunits?
80s,, 60s large and 40s small
What are ribosomal subunits made of?
rRNA and proteins
How is mammalian ribosomal RNA processed (complicated)
- pre-rRNA generation
- the 120 nt 5S rRNA is synthesized by RNAPolIII
-the 45S precursor is synthesized by RNAPolI - cleaved to 28S, 18S and 5.8S rRNA
-5 “cut” locations - first cut at site 1 and 5, then 2 or 3
What is the site of ribosome biogenesis?
Nucleoli
How is the 45S pre rRNA processed?
posttranscriptionally
What carries out the processing of pre rRNA?
snoRNPs (small nucleolar ribonucleoprotein particles) that begin to associate with the primary transcripts before transcription is completed.
What is present in all snoRNAs that guide ribose methylation?
BoxD, 5-CUGA-3
How are tRNAs made? Which rna polymerase
- synthesized from genes found in small clusters scattered around the genome
- transcribed by RNA polymerase III and the primary transcript is trimmed on its 5’ and 3’ sides
What endonuclease is involved in tRNA processing?
ribonuclease P
What is the start codon?
AUG - codes for Met/M
What are the stop codons?
UAA, UGA and UAG
What are missense mutations?
an amino acid specifying codon is replaced with a codon for a different amino acid
What are nonsense mutations?
an amino acid specifying codon is replaced by a premature stop codon
What decodes the information in an mRNA?
tRNA
What sequence do all mature tRNAs have?
triplet CCA sequence at 3’ end
What is the wobble base pair?
pairing between 2 nucleotides that does not follow base pair rules
- U of the anticodon can pair with A or G of the mRNA
-nucleotide of the 5’ end of the tRNA anticodon is capable of pairing with more than one nucleotide at the 3’ end of the mRNA codon
is energy required to add amino acids to each tRNA?
yes af
What do the small and large subunit do in translation?
- small subunit decodes the genetic message
- large subunit catalyzes peptide bond formation
What are the three stages in translation?
initiation, elongation, termination
How is translation initiated in eukaryotes? (complicated)
- initiator tRNA-Met complex is loaded into small subunit
- requires at least 12 initiation factors
- once the 43S preinitiation complex is formed it is ready to find the 5’mRNA
- small subunit moves 5-3 along mRNA to find start codon
- initiation factor dissociates
What is the 43S preinitiation complex?
40S subunit and initiator complexes together (i think)
What are the three places on the ribosome where tRNAs can bind?
A - Aminoacyl site binds to the incoming aminoacyl site carrying the new AA
P - Peptidyl site holds the tRNA with the protein
E- exit site holds the tRNA without AA which is then released by the ribosome
What happens in the first step of elongation during translation?
Step 1 - aminoacyl tRNA selection
- binding of the second aminoacyl tRNA to the A site requires the GTPase and elongation factor EF-Tu (bacteria)
- once bound to the mRNA codon - GTP is hydrolyzed and the EF-Tu-GDP complex released leaving the new AA-tRNA situated at ribosomes A site
What happens in the second step of elongation in translation?
Step 2 - peptide bond formation
- occurs spontaneously
- catalyzed by large subunit
- tRNA in P site has no AA while tRNA in A site has 2 AA
-What happens in the third step of elongation?
Step 3 - translocation
- binding of elongation factor and hydrolysis of its GTP results in the translocation of the ribosome
- as result of this motion, ribosome moves 3 nt along the mRNA in 5-3 direction