chapter 16 Flashcards
What are Chargaff’s rules?
1) The base composition of DNA varies between species
2) In any species the number of A and T bases are equal and the number of G and C bases are equal
How did Chargaff’s second rule contribute to the construction of a model for the DNA molecule?
The Watson-Crick model explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = C, from this Watson and Crick determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C)
What is a chromosome?
a piece of genetic material composed of chromatin (DNA and proteins)
Where are chromosomes found in eukaryotes?
nucleus
Where are chromosomes found in prokaryotes?
nucleoid
What type of bond forms between the nucleotides of complementary strands of DNA?
Hydrogen bond
nucleotide
molecule that consists of a pentose sugar, a nitrogenous base, and one phosphate
nucleoside triphosphate
molecule that consists of a pentose sugar, a nitrogenous base, and three phosphates
deoxyribose
a pentose sugar with one fewer oxygens than ribose
purine
a nitrogenous base made of a six-member ring fused to a five-member ring
pyrimidine
a nitrogenous base made of a six-member ring
Describe the sugar-phosphate backbone of a nucleic acid
The backbone of a nucleic acid is made up of alternating pentose sugars and phosphates. The phosphates on the 5 carbon of one nucleotide is bonded to the 3 carbon of another nucleotide.
Describe the antiparallel, complementary, double-helix nature of DNA
Watson and Crick built models of a double helix in which the backbones were antiparallel (their subunits run in opposite directions); The two strands of the double helix are complementary: adenine (A) always with thymine (T), and guanine (G) always with cytosine (C); Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication
To which end of the DNA strand can additional nucleotides be attached? Why?
The 3’ (3 carbon) end because DNA polymerases can only attach nucleotides to this end.
For both the leading and lagging strand, helicase…
binds to the origin of replication and separates the strands, generating a replication bubble.
What is at each end of the replication bubble
replication forks where replication is occurring.
What happens as the replication bubble grows?
single-strand binding proteins stabilize the newly separated strands and keep them from reattaching.
What is the initial nucleotide strand?
a short RNA primer that is attached to the DNA strand by an enzyme called primase.
What can primase start?
an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template.
How long is the primer?
The primer is short (5-10 nucleotides long).
What does the 3’ end serve as?
the starting point for the new DNA strand.
During DNA synthesis, enzymes called ___ ________ catalyze the elongation of new DNA at a replication fork.
DNA polymerases
What can’t DNA polymerases do?
initiate synthesis of a polynucleotide.
What can DNA polymerases do?
add nucleotides only to the free 3’ end of a growing strand; therefore, a new DNA strand can elongate only in the 5’ to 3’ direction.
What does DNA polymerase do along one template strand of DNA?
DNA polymerase synthesizes a leading strand continuously, moving away from the origin of replication and toward the replication fork.
What must DNA polymerase do to elongate the other new strand (lagging strand)?
DNA polymerase must work in the direction away from the replication fork and toward the origin of replication.
Okazaki fragments
The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase.
Helicase
enzyme that untwists the double helix at the replication forks
Topoisomerase
corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands
DNA polymerase III
enzyme that elongates the DNA strands
DNA polymerase I
enzyme that removes the RNA primer and replaces it with DNA
Primase
an enzyme that can start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template
DNA ligase
enzyme that binds together fragments of DNA
single strand binding proteins
proteins that bind to and stabilize single-stranded DNA
Adenine
Purine, nitrogenous base represented by the letter A
Guanine
Purine, nitrogenous base represented by the letter G
Cytosine
Pyrimidine, nitrogenous base represented by the letter C
Thymine
Pyrimidine, nitrogenous base represented by the letter T
Purine
a nitrogenous base made of a six-member ring fused to a five-member ring; includes adenine and guanine
Pyrimidines
a nitrogenous base made of a six-member ring; includes cytosine and thymine
Elongation
lengthening of the DNA molecule through the addition of nucleotides to the 3’ end
Histone
proteins that, together with DNA, make up chromatin; responsible for protection and condensation of the DNA
