cycle 3 Flashcards
n value
number of unique nuclear chromosomes present in an organism
coefficient of n (ploidy)
number of unique sets of chromosomes that are present in an organism (e.g. humans are 2n)
C value
amount of DNA in one set of an organism’s nuclear chromosomes, genome size (count # of chromatids)
coefficient of C
how many times the entire genome is present in a cell (changes throughout the cell cycle)
are n and C related?
no
n and C in mitosis
n stays the same throughout, C doubles after S phase until cytokinesis
n and C in meiosis
n divides by 2 after the first division, C divides by 2 after the first division and then divides by 2 again after cytokinesis II
DNA structure
2 strands run antiparallel to confer polarity (5’ phosphate end lines up with 3’ hydroxyl end), backbone is composed of alternating sugars and phosphate groups
complementary base pairing
A and T (double bonds), C and G (triple bonds)- Chargaff’s rule
pyrimidines
C, T, U
purines
A, G
how does DNA replicate?
semi-conservatively (each strand is used as a template to make a new strand)
replisomes
carry out DNA replication, composed of many proteins (all enzymes involved)
DNA helicase
cuts hydrogen bonds between DNA bases to produce replication forks
DNA polymerase 3
goes from 5’ to 3’ end adding nitrogenous bases
leading strand
replicated continuously
lagging strand
replicated in fragments (Okazaki fragments)
Wilkins and Franklin
discovered double helix structure
Watson and Crick
came up with nitrogenous base pairing rules, finalized structure of DNA
Meselson and Stahl
discovered the semi-conservative manner of replication
initiation of DNA replication
begins with unwinding of DNA at origin of replication (ori) by DNA helicase (uses ATP as energy source), unwound area is called a replication bubble, primase synthesized RNA primers so DNA polymerase can work, SSBs keep the bubble unwound
elongation of DNA replication
DNA polymerase III adds complementary base pairs in 5’ to 3’ direction (sliding clamp protein tethers enzyme to DNA), bases are linked through H-bonds in a process called polymerization, DNA polymerase I removes RNA primers and ligase seals gaps between
lagging strand issue with removing RNA primers
removes primer at end of chromosome and gap can’t be filled in by new bases because there is no available 3’ end, loses part of DNA sequence
cell senescence
irreversible cell cycle arrest (no more cell division, telomeres are too short)
