20.2 - the structure of DNA & RNA Flashcards
A single strand of DNA is a polymer of nucleotides bonded to one another by
3′–5′ phosphodiester bonds.
The backbone of the polymer is called the
sugar-phosphate backbone
why is it called sugar-phosphate backbone
because it is composed of alternating units of the five-carbon sugar 2′-deoxyribose and phosphoryl groups in phosphodiester linkage.
A nitrogenous base is bonded to each sugar by an
N-glycosidic linkage
DNA is a _____- of two strands of DNA wound around one another.
double helix
The structure of the double helix is often compared to a
spiral staircase
The sugar-phosphate backbones of the two strands of DNA spiral around the ______- like the handrails on a spiral staircase.
outside of the helix
the nitrogenous bases extend into the ____ at ___ angles to the ___of the helix.
center
right
axis
One noncovalent attraction that helps maintain the double helix structure is
hydrogen bonding between the nitrogenous bases in the center of the helix.
_____-forms two hydrogen bonds with ___, and _____forms three hydrogen bonds with ____
adenine, thymine
cytosine, guanine
adenine thymine
cytosine guanine
are called
base pairs
The two strands of DNA are
complementary strands
why are they called complementary strands
because the sequence of bases on one automatically determines the sequence of bases on the other.
When there is an adenine on one strand, there will always be a thymine in the ____location on the ____ strand.
same
opposite
The diameter of the double helix is
2 nm
what dictates the diameter of the double helix
by the dimensions of the purine-pyrimidine base pairs.
The helix completes one turn every
10 base pairs
One complete turn is
3.4 nm
each base pair advances the helix by
0.34 nm
the DNA double helix is that the two strands are
antiparallel strands
antiparallel strands meaning
the two strands of the helix run in opposite directions
only when the two strands are _____ can the base pairs form the hydrogen bonds that hold the two strands together.
antiparallel
Chromosomes
are pieces of DNA that carry the genetic instructions, or genes, of an organism
Organisms such as the prokaryotes have only a ____ chromosome, and its structure is relatively simple. Others, the eukaryotes, have ____ chromosomes, each of which has many different levels of structure.
single
many
The complete set of genetic information in all the chromosomes of an organism is called the
genome
are organisms with a simple cellular structure in which there is no true nucleus surrounded by a nuclear membrane and there are no true membrane-bound organelles.
prokaryotes
bacteria are
prokaryotes
in prokaryotic organisms the chromosome is a ___________ that is ______, which means that the helix is coiled on itself.
circular DNA molecule
supercoiled
The supercoiled DNA molecule is attached to a complex of proteins at roughly ____ sites along its length, forming a series of loops. This structure, called the ______
40
nucleoid
are organisms that have cells containing a true nucleus enclosed by a nuclear membrane
Eukaryotes
all animals, plants, and fungi are
eukaryotes
humans have ____ pairs of chromosomes, while the adder’s tongue fern has ____ pairs of chromosomes.
23
631
what is the first level structure of eukaryotic chromosome structure
nucleosome
what does a nucleosome consist of
a strand of DNA wrapped around a small disk made up of histone proteins.
what does the DNA look like at the nucleosome level
DNA looks like beads along a string.
nucleosome coils into a larger structure called
condensed fiber
This complex of DNA and protein is termed (condensed fiber)
chromatin
chromatin makes up what
the eukaryotic chromosome
Some human genetic disorders are characterized by
unusual chromosome #s
Down syndrome is characterized by an
extra copy of chromosome number 21
The presence of an additional chromosome 18 causes
Edward syndrome
extra chromosome 13
Patau syndrome
The presence of extra copies of the sex chromosomes, X or Y, is
not lethal
Males with two X chromosomes and one Y suffer from
Klinefelter syndrome and show sexual immaturity and breast development
Males with an extra Y chromosome are
unusually tall
women with three X chromosomes
unusually tall
A woman with only a single X chromosome experiences
turner syndrome
The sugar-phosphate backbone of RNA consists of
also linked by 3′–5′ phosphodiester bonds.
RNA molecules differ from DNA molecules in three basic properties.
RNA molecules are usually single-stranded.
The sugar-phosphate backbone of RNA consists of ribonucleotides linked by 3′–5′ phosphodiester bonds. Thus, the sugar ribose is found in place of 2′-deoxyribose.
The nitrogenous base uracil (U) replaces thymine (T).
Although RNA molecules are single-stranded, base-pairing between uracil and adenine and between guanine and cytosine
can still occur
