DNA, Chromosomes and
DNA Replication
Dr.Aida Fadhel Biawi
DNA REPLICATION
DNA replication is a biological process that occurs in
all living organisms and copies their DNA ;it is the
basis for biological inheritance .
The process starts when one double-stranded DNA
molecule produces two identical copies of the
molecule .
How does DNA replicate?
DNA Replication is a semiconservative process that results in a doublestranded molecule that synthesizes to produce two new double stranded
molecules such that each original single strand is paired with one newly made
single strand .
Semiconservative replication would produce two
copies that each contained one of the original
strands and one new strand .
"replication begins at specific sites on DNA molecule called "origins of replication,
origins are specific sequence of bases
mammalian DNA have many origins
The replication fork is a structure that forms within the
nucleus during DNA replication. It is created by helicases,
which break the hydrogen bonds holding the two DNA
strands together.
Replication fork is where the parental DNA strands hasn't untwist. Replication
bubbles allow DNA replication to speed up therefore the untwisted DNA would not
be attacked by enzymes while replicating .. ( Which enzymes can attack DNA?? )
Specific enzymes & proteins recognize origins & bind DNA :
1- primase and DNA polymerase will find these specific
portions and will bind to the template DNA at the
correct location .
( DNA replication requires a RNA primer , primer
synthesized by the enzyme primase , primer is a short
strand RNA about 5 bases and RNA primer is
complementary to DNA )
- new DNA synthesized by DNA polymerase , DNA
polymerase binds to parent DNA strand with primer.
- DNA polymerase sequentially adds
deoxyribonucleotides to RNA primer ,
deoxyribonucleotides added have bases
complementary to parent strand DNA .
- The rate nucleotide additions in bacteria add about
500 bases/second while in mammels add about 50
bases/second ??!!
2-replication requires strand separation
a. strand separation begins at origin of
replication (Helicase)
b. specific proteins prevent the two separated
DNA strands from coming back together
(single strand binding protein)
At origin of replication ,one strand of DNA is made in a continuous
manner (the leading strand) and the other in a discontinuous manner (the
lagging strand(.
DNA is made in only the 5-prime to 3-prime direction and the replication
bubble opens the original double stranded DNA to expose both a 3-prime
to 5-prime template (Leading strand template) and it complement .
The lagging strand must be synthesized as a series of discontinuous
segments of DNA .??
These small fragments are called Okazaki fragments and they are joined
together by an enzyme known as DNA ligase .
Enzyme
Function in DNA replication
DNA Helicase
Also known as helix destabilizing enzyme. Unwinds the DNA double helix at the
Replication Fork.
DNA
Polymerase
Builds a new duplex DNA strand by adding nucleotides in the 5' to 3' direction.
Also performs proof-reading and error correction.
Single-Strand
Binding (SSB)
Proteins
Bind to ssDNA and prevent the DNA double helix from re-annealing after DNA
helicase unwinds it thus maintaining the strand separation.
Topoisomeras
e
Relaxes the DNA from its super-coiled nature.
DNA Ligase
Primase
Re-anneals the semi-conservative strands and joins Okazaki Fragments of the
lagging strand.
Provides a starting point of RNA (or DNA) for DNA polymerase to begin synthesis
of the new DNA strand.
DNA CONDENSATION
A cell's genetic information, in the form of DNA, is
stored in the nucleus. The space inside the nucleus is
limited and has to contain billions of nucleotides that
compose the cell's DNA. Therefore, the DNA has to be
highly organized or condensed. There are several
levels to the DNA packaging .
At the finest level, the nucleotides are organized in the form
of linear strands of double helices. The DNA strand is
wrapped around histones, a form of DNA binding proteins.
Each unit of DNA wrapped around a histone molecule is
called a nucleosome . The nucleosomes are linked together
by the long strand of DNA .
Characteristics of Histone Proteins
- Octamere structure
- Responsible for packaging DNA into nucleosomes
- 4 different types: H2A, H2B, H3, H4
Nucleosome Structures
Histone octamer
2 H2A
2 H2B
2 H3
2 H4
Beads on a String—10 nm Fiber
protein
purification
histones
(= 1g per g DNA)
09/01/12
H1 •Basic (arg, lys);
•+ charges bind
H3 to - phosphates
H2A on DNA
H2B
H4
SBL201
DNA
22
To further condense the DNA material,
nucleosomes are compacted together to form
chromatin fibers .The chromatin fibers then fold
together into large looped domain .During the
mitotic cycle, the looped domains are organized
into distinct structures called the chromosomes .
Packing of DNA into Chromatin
- histone-DNA complexes, referred to
as nucleosomes, which are further
folded into higher-order chromatin
structures .
Nucleofilament
or solenoid
30 nm fiber
Fig. 9
Orders of chromatin
structure from
naked DNA to
chromatin to fully
condensed
chromosomes...
- Chromosomes are also used as a way of referring
to the genetic basis of an organism as either diploid
or haploid .Many eukaryotic cells have two sets of
the chromosomes and are called diploid. Other cells
that only contain one set of the chromosomes are
called haploid .
• A diploid cell has two sets of each of its chromosomes
• A human has 46 chromosomes (2n = 46)
• In a cell in which DNA synthesis has occurred all the chromosomes are
duplicated and thus each consists of two identical sister chromatids
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Chromosome Duplication
•
In preparation for cell division, DNA is replicated and the chromosomes condense
•
Each duplicated chromosome has two sister chromatids, which separate during cell
division
An eukaryotic cell has multiple
chromosomes, one of which is
represented here. Before
duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosome
consists of two sister chromatids
connected at the centromere. Each
chromatid contains a copy of the
DNA molecule.
Mechanical processes separate
the sister chromatids into two
chromosomes and distribute
them to two daughter cells.
0.5 µm
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Centrometers
Sister
chromatids
Sister chromatids
Chromosome Duplication
• Because of duplication, each condensed chromosome
consists of 2 identical chromatids joined by a
centromere.
• Each duplicated chromosome contains 2 identical DNA
molecules (unless a mutation occurred), one in each
chromatid:
Non-sister
chromatids
Centromere
Duplication
Sister
chromatids
Two unduplicated
chromosomes
Sister
chromatids
Two duplicated chromosomes
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Structure of Chromosomes
• The centromere is a constricted region of the chromosome containing a
specific DNA sequence, to which is bound 2 discs of protein called
kinetochores.
• Kinetochores serve as points of attachment for microtubules that move
the chromosomes during cell division.???!!!
Metaphase chromosome
Centromere
region of
chromosome
Kinetochore
Kinetochore
microtubules
Sister Chromatids
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chromosome structure
p: short arm
q: long arm
C: constriction point or centromere , the location of
centromere give the chromosome its shape and can be
used to help describe the location of the genes.
A typical mitotic chromosome at
metaphase
– Diploid - A cell possessing two copies of each chromosome (human
body cells).
• Homologous chromosomes are made up of sister chromatids joined at the
centromere.
– Haploid - A cell possessing a single copy of each chromosome (human
sex cells).
Phases of the Cell Cycle
• Interphase
– G1 - primary growth
– S - genome replicated
– G2 - secondary growth
• M - mitosis
• C - cytokinesis
Cell cycle begins with the formation of two cells from the division of a parent cell
and ends when the daughter cell does so as well.
Observable under the microscope, M phase consists of two events, mitosis
(division of the nucleus) and cytokinesis (division of the cytoplasm).
As replication of the DNA occurs during S-phase, when condensation of the
chromatin occurs two copies of each chromosome remain attached at the
centromere to form sister chromatids.
After the nuclear envelope fragments, the microtubules of the mitotic spindle
separate the sister chromatids and move them to opposite ends of the cell.
Cytokinesis and reformation of the nuclear membranes occur to complete the
cell division.
-Most of the time, cells are in interphase, where growth occurs and cellular
components are made. DNA is manufactured during S phase.
-To prepare the cell for S phase (DNA synthesis), G1 phase occurs (the
preparation of DNA synthesis machinery, production of histones).
-In an analogous manner, the cell prepares for mitosis in the G2 phase by
producing the machinery required for cell division.
-The length of time spent in G1 is variable. In growing mammalian cells often
spend ??? hours in G1 phase. G2 is usually shorter than G1 and is usually ???
hours. And S phase ???
Interphase
• G1 - Cells undergo majority of growth
• S - Each chromosome replicates (Synthesizes)
to produce sister chromatids
– Attached at centromere
– Contains attachment site (kinetochore)
• G2 - Chromosomes condense - Assemble
machinery for division such as centrioles
Mitosis
 Some haploid & diploid cells divide by mitosis.
 Each new cell receives one copy of every
chromosome that was present in the original cell.
 Produces 2 new cells that are both genetically
identical to the original cell.
DNA duplication
during interphase
Mitosis
Diploid Cell