Chromosome 222 Cell Biolgy Diagram of a duplicated

Chromosome 222 Cell Biolgy • Diagram of a duplicated and condensed metaphase eukaryotic chromosome. (1) Chromatid – one of the two identical parts of the chromosome after S phase. (2) Centromere – the point where the two chromatids touch, and • kinetochores where the microtubules attach. • (3) Short arm. (4) Long arm. 1 1

§ A chromosome is an organized structure of DNA and protein that is found in cells and contains thousand hundred of genes. § A cell is not dividing, the chromosomes are present in an extended form known as chromatin. During cell division, the chromain fibers condense and chromosomes become visible. § Chromosomes may exist as unduplicated (2 chromatids) each chromatid has a constricted region called centromere or duplicated (copied during S phase) consist of a pair of units termed sister chromatids joined by their centromeres. § Attached to each centromere a kinetochore which is a structure formed of protein to which microtubules can 2 bind.

NUCLEOSOMES § Nucleosomes form the fundamental repeating units of eukaryotic chromatin which is used to pack the large eukaryotic genomes into the nucleus. § In mammalian cells approximately 2 m of linear DNA have to be packed into a nucleus of roughly 10 µm diameter § Nucleosomes are folded through a series of successively higher order structures to eventually form a chromosome. § This both compacts DNA and creates an added layer of regulatory control which ensures correct gene expression. 3

DNA REPLICATION (Fig. 3) – (Fig 5) DNA replication is a semi-conservative process. One strand serves as the template for the second strand. DNA replication is initiated at a region on a chromosome called an origin of replication. An enzyme called DNA Helicase binds to the origin and unwinds the DNA in both directions from the origin. Unwinding of DNA at the origin, and synthesis of new strands, forms a replication fork. As the DNA is unwound, specific single stranded DNA binding proteins (helix –destabilizing proteins) bind to single DNA strands preventing the re-forming of double helix until the strands are copied. Because DNA polymerase can only synthesize a new DNA strand in a 5' to 3' manner, the process of replication goes differently for the two strands comprising the DNA. Nucleic acids can only be synthesized in a 5'- to 3'-direction, as the polymerase used to assemble new strands it must attach the 5’ phosphate group of the new nucleotide to the 3'-hydroxyl (-OH) group at the end of preexisting strand via a 5 phosphodiester bond § § § §

DNA synthesis requires an RNA primer and is discontinuous in one strand continuous in the other: (Fig. 3) 1) RNA primer § RNA primers are produced by primase, which bind to the DNA and start a new strand of RNA opposite a DNA strand. § After a few nucleotides have been added the primase is displaced by DNA polymerase which can add subunits to the 3’ end of short RNA primer. § RNA primer is degraded by specific enzymes and space filled with DNA. 2) DNA is discontinuous in 1 strand continuous in the other § DNA strands are antiparallel. (Fig. 4) § DNA synthesis proceeds only in the 5’ 3’ direction it means that the strand to be copied is being read in 3’ 5’ direction 6 § So, 2 DNA polymerase molecules are responsible for replication.

• One (polymerase III) adds nucleotide to the 3’ end of the new strand that grows ‘’toward’’ the replication fork called the ‘’leading strand’’. (Fig. 5) • The other strand, (lagging strand) is synthesized discontinuously, primase synthesize RNA primers which is then extended by DNA polymerase III to form Okazaki fragments (series of segments). • After Okazaki fragment has been elongated by DNA polymerase III, RNA primer is degraded, the gaps are filled with DNA by DNA polymerase I and the adjacent fragments are linked together by DNA ligase. 7

DNA REPLICATION TYPES § § i) Conservative mode This hypothesis states that during replication both the strands of the DNA remains with the parent cell and the newly synthesized two strands of DNA is retained by the daughter cell. However this hypothesis is not accepted anymore. (ii) Semi conservative mode Here the replication process of a parent DNA double helix results in two hybrid DNAs. Here each of the daughter DNA double helices retains a parent strand a newly synthesized strand. 9

3 end 5 end P 4 3 P P P 5 2 1 2 3 1 4 5 P P P The opposite orientations Of DNA strands Fig. 4 ﺍﻻﺗﺠﺎﻫﻴﻦ ﺍﻟﻤﺘﻌﺎﻛﺴﻴﻦ ﻟﺨﻴﻄﻲ ﺍﻟﺪﻧﺎ P 3 end 5 end 10

Fig 5 11 11