Humane Levenscyclus I Gezondheid & Leven jaar 1 Ezri van Kraanen
5 DNA AND CHROMOSOMES
The structure of DNA
● Each chain, also called strand, is
composed of four types of nucleotide
subunits, and the two strands are held
together by hydrogen bonds between
the base portions of the nucleotides.
This base can either be adenine (A),
cytosine (C), guanine (G) or thymine (T).
Hereby A always pairs with T, and C
always pairs with G.
● The nucleotide subunits within a DNA
strand are held together by
phosphodiester bonds that link the 5’
end to the 3’ end. A strand is always
formed from 5’ to 3’. The polarity in a
DNA strand is indicated by referring to
one end as the 3’ end and the other as
the 5’ end.
● Each purine-pyrimidine pair is called a base pair, and this
complementary base-pairing enables the base pairs to
be packed in the energetically most favorable
arrangement along the interior of the double helix. In this
arrangement, each base pair has the same width, thus
holding the sugar-phosphate backbones an equal
distance apart along the DNA molecule.
● The two strands of the helix must run antiparallel to each
other.
● The complete human DNA sequence would fill more
than 1000 books the size of Essential Cell Biology. How
can all this information then neatly be packed into the cell nucleus?
The structure of eukaryotic chromosomes
● In eukaryotic cells, very long, double stranded DNA is packaged into chromosomes.
These chromosomes not only fit handily inside the nucleus, but, after they are
duplicated, they can be accurately apportioned between the two daughter cells at
each cell division. The complex task of packaging the DNA is accomplished by
specialized proteins that bind to and fold the DNA, generating a series of coils and
loops that provide increasingly higher levels of organization and prevent the DNA
from becoming a tangled, unmanageable mess.
, Humane Levenscyclus I Gezondheid & Leven jaar 1 Ezri van Kraanen
● The complex of DNA and protein is called chromatin.
● Human cells each contain two copies of every chromosome, one inherited from the
mother and one from the father. The maternal and paternal versions of each
chromosome are called homologous chromosomes. The only nonhomologous
chromosome pairs in humans are the sex chromosomes in males, where a Y
chromosome is inherited from the father and the X chromosome from the mother.
● An ordered display of the full set of 46 human chromosomes is called the human
karyotype.
● The most important function of chromosomes is to carry the genes. A gene is often
described as a segment of DNA that contains the instructions for making a particular
protein or RNA molecule.
● To form a functional chromosome, a DNA molecule must do more than simply carry
genes: it must be able to be replicated, and the replicated copies must be separated
and partitioned equally and reliably into the two daughter cells at each cell division.
● During the interphase, chromosomes are extended as long, thin, tangled threads of
DNA in the nucleus. The chromosomes in this state are now called interphase
chromosomes.
● The proteins that bind to DNA to form eukaryotic chromosomes are traditionally
divided into two classes: the histones and the nonhistone chromosomal proteins.
Histones are present in enormous quantities, and their total mass in chromosomes is
about equal to that of the DNA itself. Nonhistone chromosomal proteins are also
present in large numbers; they include hundreds of different chromatin-associated
proteins.
● Histones are responsible for the first and most
fundamental level of chromatin packaging: the
formation of the nucleosome. Nucleosomes
convert the DNA molecules in an interphase
nucleus into a chromatin fiber that is
approximately one-third the length of the initial
DNA.
5 DNA AND CHROMOSOMES
The structure of DNA
● Each chain, also called strand, is
composed of four types of nucleotide
subunits, and the two strands are held
together by hydrogen bonds between
the base portions of the nucleotides.
This base can either be adenine (A),
cytosine (C), guanine (G) or thymine (T).
Hereby A always pairs with T, and C
always pairs with G.
● The nucleotide subunits within a DNA
strand are held together by
phosphodiester bonds that link the 5’
end to the 3’ end. A strand is always
formed from 5’ to 3’. The polarity in a
DNA strand is indicated by referring to
one end as the 3’ end and the other as
the 5’ end.
● Each purine-pyrimidine pair is called a base pair, and this
complementary base-pairing enables the base pairs to
be packed in the energetically most favorable
arrangement along the interior of the double helix. In this
arrangement, each base pair has the same width, thus
holding the sugar-phosphate backbones an equal
distance apart along the DNA molecule.
● The two strands of the helix must run antiparallel to each
other.
● The complete human DNA sequence would fill more
than 1000 books the size of Essential Cell Biology. How
can all this information then neatly be packed into the cell nucleus?
The structure of eukaryotic chromosomes
● In eukaryotic cells, very long, double stranded DNA is packaged into chromosomes.
These chromosomes not only fit handily inside the nucleus, but, after they are
duplicated, they can be accurately apportioned between the two daughter cells at
each cell division. The complex task of packaging the DNA is accomplished by
specialized proteins that bind to and fold the DNA, generating a series of coils and
loops that provide increasingly higher levels of organization and prevent the DNA
from becoming a tangled, unmanageable mess.
, Humane Levenscyclus I Gezondheid & Leven jaar 1 Ezri van Kraanen
● The complex of DNA and protein is called chromatin.
● Human cells each contain two copies of every chromosome, one inherited from the
mother and one from the father. The maternal and paternal versions of each
chromosome are called homologous chromosomes. The only nonhomologous
chromosome pairs in humans are the sex chromosomes in males, where a Y
chromosome is inherited from the father and the X chromosome from the mother.
● An ordered display of the full set of 46 human chromosomes is called the human
karyotype.
● The most important function of chromosomes is to carry the genes. A gene is often
described as a segment of DNA that contains the instructions for making a particular
protein or RNA molecule.
● To form a functional chromosome, a DNA molecule must do more than simply carry
genes: it must be able to be replicated, and the replicated copies must be separated
and partitioned equally and reliably into the two daughter cells at each cell division.
● During the interphase, chromosomes are extended as long, thin, tangled threads of
DNA in the nucleus. The chromosomes in this state are now called interphase
chromosomes.
● The proteins that bind to DNA to form eukaryotic chromosomes are traditionally
divided into two classes: the histones and the nonhistone chromosomal proteins.
Histones are present in enormous quantities, and their total mass in chromosomes is
about equal to that of the DNA itself. Nonhistone chromosomal proteins are also
present in large numbers; they include hundreds of different chromatin-associated
proteins.
● Histones are responsible for the first and most
fundamental level of chromatin packaging: the
formation of the nucleosome. Nucleosomes
convert the DNA molecules in an interphase
nucleus into a chromatin fiber that is
approximately one-third the length of the initial
DNA.
