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Molecular Biology Sabah Linjawi ١
Lecture 6
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Molecular Biology Sabah Linjawi ٢
Eukaryotic chromosome
The eukaryotic cells show a varied number of chromosomes.
The eukaryotic chromosomes occur inside the nucleus of a cell, separated from the cytoplasm by a distinct nuclear membrane
The eukaryotic chromosomes become clearly visible when the cell enters the phase of
division
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Molecular Biology Sabah Linjawi ٣
Chromosome Number
The number of chromosomes is fixed for a given species.
It varies from a minimum of two to several hundreds in different species of plants and animals.
The chromosome number of a given species is generally represented as the diploid
number (2n) since chromosomes occur in
pairs
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Molecular Biology Sabah Linjawi ٤
The mitotic chromosome
The classic picture of paired sister
chromatids at mitosis represents the most highly condensed state of chromatin
The linear DNA traces a single path from one tip of the chromosome to the other
In succcessive loops of up to 100 kb of 30 nm fiber anchored to the nuclear matrix in the
core
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Molecular Biology Sabah Linjawi ٥
Chromatin
The major structures in DNA compaction; DNA, the
nucleosome, the 10nm "beads-on-a-string" fibre, the
30nm fibre and the metaphase chromosome
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Molecular Biology Sabah Linjawi ٦
The centromere
A centromere is a region of DNA typically found near the middle of a chromosome
where two identical sister chromatids come in contact.
It is involved in cell division as the point of mitotic spindle.
The centromeres are, together with telomeres and origins of replication, one of the essential parts of any eukaryotic
chromosome.
The centromere usually contains specific types of DNA sequences
which are in higher eukaryotes typically tandem repetitive sequences, often called "satellite DNA".
Kinetochores and the spindle apparatus are responsible for the movement of the two sister chromatids to opposite poles of
dividing cell nucleus during anaphase.
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Molecular Biology Sabah Linjawi ٧
Telomeres
A telomere is a region of repetitive DNA at the ends of chromosomes
which protects the end of the chromosome from destruction
Telmeres are short repeating sequences
synthesized by a specific enzyme, telomerase,
independently of normal
DNA replication
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Molecular Biology Sabah Linjawi ٨
Interphase chromatin
During interphase (the period of the cell cycle where the cell is not dividing), two types of chromatin can be distinguished:
Euchromatin, which consists of DNA that is active, e.g., being expressed as protein.
Heterochromatin, which consists of mostly inactive DNA. It seems to serve structural purposes during the chromosomal stages. Heterochromatin can be further distinguished into two types:
Constitutive heterochromatin, which is never expressed. It is located around the centromere and usually contains repetitive sequences.
Facultative heterochromatin, which is sometimes expressed.
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Molecular Biology Sabah Linjawi ٩
The genome
The genome of a diploid organism including eukarya refers to a full set of chromosomes in a gamete
Thereby, a regular somatic cell contains two full sets, or two genomes.
In haploid organisms ،including bacteria ،viruses ،and mitochondria ،a cell similarly contains genes
but usually in a single circular or contiguous linear chain of DNA (or RNA for retroviruses ،(likewise referred to as a genome
In modern molecular biology the genome of an organism also can refer to all of its hereditary information encoded in DNA (or, for retroviruses, RNA).
The genome includes both the genes and the non-coding sequences of the DNA
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Molecular Biology Sabah Linjawi ١٠
Noncoding DNA
In genetics, non-coding DNA describes DNA which does not contain instructions for making proteins.
In eukaryotes, a large percentage of many
organisms' total genome sizes is noncoding DNA.
Some noncoding DNA is involved in regulating the activity of coding regions.
However, much of this DNA has no known function and is sometimes referred to as "junk DNA“.They also known as introns.
Some non-coding DNA are genetic "switches" that
do not encode proteins, but do regulate when and
where genes are expressed.
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Molecular Biology Sabah Linjawi ١١
Intron
An intron is a DNA region within a gene that is not translated into protein.
These non-coding sections are transcribed to
precursor mRNA (pre-mRNA) and some other RNAs (such as long noncoding RNAs)
And subsequently removed by a process called splicing during the processing to mature RNA.
After intron splicing (ie. removal), the mRNA
consists only of exon derived sequences, which are
translated into a protein.
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Molecular Biology Sabah Linjawi ١٢
Intron
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Molecular Biology Sabah Linjawi ١٣
Satellite DNA
Satellite DNA occurs mostly near the centromeres of chromosomes
It involved in attachment of the mitotic spindle
Consists of huge numbers of tandem repeats of short (up to 30 bp) sequences
Hypervariability in satellite DNA is the basis
of the DNA fingerprinting technique
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Molecular Biology Sabah Linjawi ١٤
Genetic polymorphism
(1) The existence together of many forms of DNA sequences at a locus within the population.
Base changes (mutations) in a gene or a chromosomal locus can create multiple forms (polymorphs)
Also can describe different alleles of a single copy gene in a single individual
As well as the different sequences present in different individuals in a population
Common types are single nucleotide polymorphisms (SNPs) and simple sequence length polymorphisms (SSLPs)
The SNPs sequence is recognized by a restriction enzyme to produce restriction fragment length polymorphism (RELP)
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Molecular Biology Sabah Linjawi ١٥
References
users.rcn.com/jkimball.ma.ultranet/.../G/GenomeSiz es.html
en.wikipedia.org/wiki/Chromatin
www.tutorvista.com/.../chromosomes/eukaryotic- chromosome.php
en.wikipedia.org/wiki/Genome
Molecular Biology. P.C. Turner, A.G. Mclennan, A.D.
Bates & M.R.H. White.School of Biological Sciences, University of Liverpool, Liverpool, UK. Second edition.
BIOS Scientific Publishers, 2000.
