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Samenvatting hoofdstuk 5 Humane levenscycles
Samenvatting hoofdstuk 5
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Hoofdstuk 5: DNA and ChromosomesThe structure of DNA
DNA -> Double-stranded polynucleotide formed from two separate chains of covalently linked
deoxyribonucleotide units. It serves as the cell's store of genetic information that is transmitted from
generation to generation. The typical structure of a DNA molecule is a double helix in which the two
complementary polynucleotide strands are wound around each other with base-pairing between the
strands. All the bases are inside, and the sugar-phosphate backbones are on the outside of the helix.
DNA is simply a long polymer composed of only four types of nucleotide subunits, (chemically very
similar). Nucleotides are composed of a nitrogen-containing base and a five-carbon sugar, to which a
phosphate group is attached. For the nucleotides in DNA, the sugar is deoxyribose. Two
complementary nucleotides in an RNA or a DNA molecule that are held together by hydrogen bonds
are G with C, and A with T or U. The nucleotide subunits within a DNA strand are held together by
phosphodiester bonds that link the 5ʹ end of one sugar with the 3ʹ end of the next. Complementary
describes two molecular surfaces that fit together closely and form noncovalent bonds with each
other.
Each strand DNA has a chemical polarity, this polarity in a DNA strand is indicated by referring to one
end as the 3ʹ end and the other as the 5ʹ end. The 3’ end is an OH- bond, this is a polar bond. The 5’
ends with a phosphor bond, this is a nonpolar bond.
In each case, a bulkier two-ring base (a purine) is paired with a single-ring base (a pyrimidine). 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. For the members of each base pair to fit
together within the double helix, the two strands of the helix must run antiparallel to each other—
that is, be oriented with opposite polarities. The antiparallel sugar–phosphate strands then twist
around each other to form a double helix containing 10 base pairs per helical turn.
Information is encoded in the order, or sequence, of the nucleotides along each DNA strand. Each
base—A, C, T, or G—can be considered a letter in a four-letter alphabet that is used to spell out
biological messages. Organisms differ from one another because their respective DNA molecules
have different nucleotide sequences and, consequently, carry different biological messages. The
genetic code: a set of rules by which the information contained in the nucleotide sequence of a gene
and its corresponding RNA molecule is translated into the amino acid sequence of a protein. Gene
expression: the process by the nucleotide sequence of a gene is transcribed into the nucleotide
sequence of an RNA molecule and then, in most cases, translated into the amino acid sequence of a
protein or in other words the process by which a gene makes a product that is useful to the cell or
organism by directing the synthesis of a protein or an RNA molecule with a characteristic activity.
The structure of eukaryotic chromosomes
Chromosome: long, threadlike structure composed of DNA and proteins that carries the genetic
information of an organism; becomes visible as a distinct entity when a plant or animal cell prepares
to divide. The complex task of packaging 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. Amazingly, this
DNA is folded in a way that allows it to remain accessible to all the enzymes and other proteins that
replicate and repair it, and that cause the expression of its genes.
, In eukaryotes, nuclear DNA is distributed among a set of different chromosomes. The DNA in a
human nucleus is parceled out for men into 22 + X and Y chromosome and for female into 22 with
and X chromosome. Each of these chromosomes consists of a single, enormously long, linear DNA
molecule associated with proteins that fold and pack the fine thread of DNA into a more compact
structure. This complex of DNA and protein is called chromatin. Chromosomes also associate with
many other proteins involved in DNA replication, DNA repair, and gene expression. Except for the
gametes (sperm and eggs) and highly specialized cells that lack DNA entirely (such as mature red
blood cells), human cells each contain two copies of every chromosome, one from the mother and
one from the father. The maternal and paternal versions are called homologous chromosomes
(homologs). The only nonhomologous chromosome pairs are the sex chromosomes. The total
genetic information carried by all the chromosomes of a cell or organism (3.2 × 109 nucleotide pairs)
is called a genome; in humans, the total number of nucleotide pairs in the 22 autosomes plus the X
and Y chromosomes. In addition to being different sizes, the different human chromosomes can be
distinguished from one another by a variety of techniques. A karyotype -> An ordered display of the
full set of chromosomes of a cell, arranged with respect to size, shape, and number.
A gene is often defined as a unit of heredity containing the instructions that dictate the
characteristics or phenotype of an organism; in molecular terms, a segment of DNA that contains the
instructions for making a particular protein or RNA molecule.
Chromosomes from many eukaryotes also contain a large excess of interspersed DNA. This extra
DNA is called “junk DNA,” -> its usefulness to the cell has not yet been demonstrated. It does not
code for protein or has a biological function. Comparisons of the genome sequences from many
different species reveal that small portions of this extra DNA are highly conserved among related
species, suggesting their importance for these organisms.
The process to form a functional chromosome by replicating a DNA molecule and separating the
replicated DNA into equally parts into the two daughter cells at each cell division occur through an
ordered series of events, known collectively as the cell cycle. During interphase of the cell cycle,
chromosomes are extended as long, thin, tangled threads of DNA in the nucleus and cannot be easily
distinguished in the light microscope. During interphase DNA replication takes place. One type of
nucleotide sequence, called a replication origin, is where DNA replication begins; eukaryotic
chromosomes contain many replication origins to allow the long DNA molecules to be replicated
rapidly. Another DNA sequence forms the telomeres that mark the ends of each chromosome.
Telomeres contain repeated nucleotide sequences that are required for the ends of chromosomes to
be fully replicated. They also serve as a protective cap that keeps the chromosome tips from being
mistaken by the cell as broken DNA in need of repair. Eukaryotic chromosomes also contain a third
type of specialized DNA sequence, called the centromere, that allows duplicated chromosomes to be
separated during M phase; be the constricted region of a mitotic chromosome.
Interphase chromosomes are much longer and finer than mitotic chromosomes. They are
nevertheless organized within the nucleus in several ways. First, each chromosome tends to occupy
a particular region, or territory, of the interphase nucleus. This loose organization prevents
interphase chromosomes from becoming extensively entangled. In addition, some chromosomal
regions are physically attached to sites on the nuclear envelope or to the underlying nuclear lamina.
These attachments help interphase chromosomes remain within their distinct territories.
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