DNA Structure And Replication Flashcards
Polymer composed of nucleotide building blocks
Chemical basis of hereditary and is grouped into genes, which are the fundamental units of genetic information
Doxynucleotides covalently linked by 3’5’-phosphodiester bonds:
- DeoxyAdenylate
- DeoxyGuanylate
- DeoxyCytidylate
- Thymidylate
Double helix structure with major and minor grooves
Contained in the cytoplasm of prokaryotes and in the nucleas of eukaryotes
DNA (deoxyribonucleic acid)
DNA structure
3’5’ Phosphodiester bonds
- 5’-OH group attached to 3’-OH group
- strands have directionality
- bonds are cleaved hydrolytically by chemicals or hydrolyzed enzymatically by exonucleasesmor endonucleases
Strands run into opposite directions
Antiparallel strands
Held together by hydrogen bonds and hydrophobic interactions
Adenine to thymine
Guanine to cytosine
Complementary base pairing
Chargall’s Rules
In any sample of dsDNA, the amount of adenine equals the amount of THYMINE, the amount of guanine equals the amount of CYTOSINE
the total amount of PURINES equals the total amount of pyrimidines
Temperature at which one half of the helical structure is lost (denaturation)
Under appropriate conditions, renaturation (annealing) may occur
Melting temperature
Structural forms of the double helix
B-DNA
A-DNA
Z-DNA
Right-handed helix with ID residues per 360* turn of the helix
B-DNA
Moderately dehydrated B form, also right-handed with about 11 base pairs per turn
A-DNA
Left-handed helix that contains about 12 base pairs per turn, naturally in regions of alternating purines and pyrimidines
Z-DNA
Five classes of small, positively charged proteins that form ionic bonds with negatively charged DNA
2 each of histones H2A, H2B, H3 and H4 form a structural core around which DNA is wrapped creating a nucleosome
The DNA connecting the nucleosome is called LINKER DNA, and is bound to histone H1
High in ARGININE and LYSINE
HISTONES
Further packing of DNA due to hydrophobic interactions and in association with other non-histone proteins compacts it into chromatin
HETEOCHROMATIN
EUCHROMATIN
Densely packed and transcriptionally inactive chromatin during interphase, observed by electron microscopy
Heterochromatin
Transcriptionally active chromatin that stains less densely
Euchromatin
Also called nucleofilament
Nucleosomes that are packed more tightly
Organized into loops that are anchored by a nuclear scaffold containing several proteins
Polynucleosome
Properties of DNA
- Coding regions are often interrupted by intervening sequences
- More than half of the DNA in eukaryotic organisms is in unique or nonrepetitive sequences
- At least 30% of the genome consists of repetitive sequences 1% of cellular DNA is in mitochondria
- each strand serves as a template for complementary daughter strand
- each strand becomes part of the daughter strand
Prokaryotic DNA synthesis
Begins at the origin of replication (one in prokaryotes, multiple in eukaryotes), a short sequence composed almost exclusively of AT base pairs
Strands are separated locally, forming two replication forks
Replication of double stranded DNA is bidirectional
STEP 1: DNA A PROTEIN
- group of proteins that recognize the origin of replication
STEP 2: HELICASE
- unwind the double helix ahead of the advancing replication fork
STEP 3: SINGLE-STRANDED DNA-BINDING PROTEINS
- maintain the separation of the parental strands
STEP 4: DNA TOPOISOMERASES
- remove supercoils that interfere with the further unwinding of the double helix
- two types: TYPE I - SWIVELASE (cleaves one strand); TYPE II - GYRASE (cleaves both strands, target of quinolone antibiotics)
STEP 5: PRIMASE
- synthesize short stretches of RNA called primers, needed by DNA polymerase to begin in DNA chain elongation
STEP 6: DNA POLYMERASE II
- catalyzes chain elongation, using 5’-deoxyribonucleic triphosphate substrates
- proofreads the newly synthezied DNA using its 3’–> 5’ exonuclease activity
- DNA polymerases are only able to read the template in the 3’ –> 5’ direction and synthesize in the 5’–> 3’ direction
- thus, DNA is synthezised in the opposite direction in both strands: leading strand, lagging strand (with formation of Okazaki fragments)
STEP 7: DNA POLYMERASE I
- removes RNA primers using its 5’–> 3’ exonuclease activity, and fills in the resulting gaps
STEP 8: LIGASE
- seals the nicks between Okazaki fragments and catalyzes the final phospholipid ester linkage
Central dogma
Replication
Transcription
Translation
