skibidi bio quiz

Chargaff’s rule

Equal ratio of A to T and C to G

Purines and Pyrimidines. How do they bond?

Purines: Double ring structure (A and G)

Pyrimidines: Single ring structure (C and T (and U)).

DNA uses hydrogen bonding, Purines have 2 hydrogen bonds (both form amine) and pyrimidines have 3 (2 amine, one carbonyl)

DNA Structure

Double helix structure, phosphate sugar backbone, inner made of nucleotides

Antiparallel strands: One runs 5’ to 3’ (5 → open phosphate group, 3’ → open hydroxyl) One runs 3’ to 5’

Chromosome Structure

DNA is wrapped around histones, forming nucleosomes. Nucleosomes form chromatin, which in its condensed state forms chromosomes. Remember: DNA is slightly negatively charged, and histones r positively charged

Eukaryote and Prokaryote DNA. What are plasmids, and why r they important?

Eukaryotes: DNA in nucleus, mostly linear

Prokaryotes: DNA in nucleoid region, circular

Plasmids: small circular DNA molecules that replicate independently of chromosomes and often contain useful genes. Can be inserted or removed easily or modified. Great for gene editing. Bacteria can exchange plasmids by conjunction

Diff between DNA and RNA

DNA: sugar backbone is deoxyribose (no extra oxygen on 2’ carbon), double stranded, thymine

RNA: ribose backbone, single strand, uses uracil

DNA Replication models

Conservative: Parent strands directly synthesize an entirely new double stranded DNA molecule: Kinda cloning, the og is still there but there is another one

Semi conservative: Parental strands each make copy, so each strand is half parental

Dispersive: Material in parent strand is randomly dispersed: each new dna molecule has random mix of parent strand

Meselson Stahl experiment

First gen tagged with one isotope, then bacteria transferred to other isotope, so they can measure DNA ratios across generations. Ended up proving semi conservative model

Origin of replication and Replication fork

Replication starts at origin of replication at a specific sequence of the genome, Replication fork is where the proteins are splitting the DNA

Helicase and Topoisomerase

Helicase unwinds DNA strands at the replication fork. SSBP (single strand binding proteins) keep the DNA open by binding to each strand

Topoisomerase stays ahead of helicase and relaxes DNA to keep it from supercoiling

Primase

Initiates replication by adding short segments of RNA called primers

DNAP III

DNAP 3 (dna polymerase 3) attaches to parent DNA at a primer and move 3’ to 5’ on the PARENT STRAND, so dna is added 5’ to 3 on the NEW STRAND. Leading strand: DNAP III is following helicase on that strand. Lagging strand: DNAP III is moving away from helicase and requires many primers

What are Okazaki Fragments?

chunks of the lagging strands created by primers being placed and DNAP 3 using them

What do DNAP 1 and Ligase do?

DNAP 1: replaces RNA primers with DNA, DNA ligase joins okazaki fragments to form continuous strand

Steps of DNA replication:

1: Replication begins at origin of replication

2: Helicase unwinds DNA strand at each replication fork

3. Primase places primers

4. Antiparallel elongation (DNAP 3 creates new strand)

Problem with Lagging strand; why are telomeres needed?

DNAP can only add nucleotides to a 3’ end, finishing the 5’ end of the lagging strand is impossible, and primers can’t be placed at a 5’ end either. This results in DNA shrinkage. Telomerase adds telomeres (junk dna) to the end so that no genetic info is lost to shrinkage.

How are errors avoided in replication?

DNAPIII self checks the bases added, and any errors still made are fixed with mismatch repair. Large errors are removed by nuclease and DNAP and ligase