Principles of Genetics (BIOL2250)

Department of Biology
Memorial University of Newfoundland

Cytogenetics

Chromosomes can be identified by microscopy.
The chromosome set is the characteristic complete of chromosomes that carries the genetic information of a particular species.
In addition, alterations to the standard chromosome complement can reveal the genetic basis of many genetic abnormalities.

Chromosome number is a characteristic of each species and range from two to hundreds.
Homo sapiens has 46 chromosomes (22 pairs of autosomes and two sex chromosomes).
The characteristics can be summarized as in a diagram of corn chromosome set.

The features of individual chromosomes can be used to identify the individual chromosomes within the set that characterizes a species such as ...
1) size
Chromosome size can vary drastically from largest to smallest.
Human chromosome 1 (the largest one) is 3-4 times the size of the (smallest) chromosome 21.

2) arm ratio
Centromeres is the region of the eukarotic chromosome where the kinetochore binds and the position of the centromere along the chromosome length is characteristic of a chromosome.
    1) metacentric chromosomes have a central centromere.
    2) acrocentric chromosomes have a subterminal centromere.
    3) telocentric chromosomes have a terminal centromere.

3) number and position of chromomeres
Chromomeres, thickenings of the chromosome, are small bead-like structures found within the chromosome during prophase.

4) heterochromatin patterns,
Heterochromatin is densely staining condensed chromosomal material that is mostly genetically inert.
Euchromatin is made up of normally staining chromosomal material that contains most normal genes.
With the use of a number of staining techniques such as Giemsa Staining, banding patterns of G dark and G light bands that are characteristic and reproducible.
Giemsa staining of human chromosomes reveals  ~850 dark G bands at mitotic metaphase.
The G light bands seem to be correlated with active genes.

5) number and location of nucleolar organizers
Nucleolus is an organelle within the nucleus that containing multiple tandem copies of the rRNA genes and transcribed ribosomal RNA.
Nucleolar organizer is the region(s) of the chromosome that contains the rRNA genes and are physically associated with nucleolus.

6) other banding patterns

Polytene chromosomes

Some insects, such as Drosophila melanogaster have highly replicated polytene chromosomes in some tissues such as the salivary glands.
Polytene chromosomes are a giant chromosome containing non-separated multiple copies of replicated DNA.
Chromocentre is the point where polytene chromosomes appear to be attached.
Along the length of the  polytene chromomes several features such as bands, puffs and Balbiani rings are present.
 

DNA packaging

Mitotic pre-DNA synthesis chromosomes have but one DNA molecule per chromosome stretching from one end to the other.
The diffuse chromosomal material or chromatin must be organized and physically restrained (or "packaged") in such a way that the extremely long DNA molecule is available to direct cellular function.

Role of histone proteins in packaging DNA

Under low salt, electron micrographs of chromatin appear to resemble 10 nm wide "beads on a string".
As DNAse can be used to remove the "string", it is DNA.
The beads are nucleosomes, which are each made up of an octomer of histone proteins  (two H2A, two H2B, two H3 and two H4).
The DNA wraps around the histone core much like string on a spool.
In turn the nucleosomes coil into a 30 nm solenoid structure and are stabilized by H1, another histone protein.
To achieve higher order of coiling, the 30 nm solenoid loops around a cellular protein scaffolding.comprised mostly of topoisomease II.
The enzyme topoisomease II allows one DNA strand to pass through another by cutting one strand while pushing another through and rejoining the cut strand.
The chromosome binds to the scaffold protiens at special regions called the scaffold attachment regions (SARs).

In summary,
Each eukaryotic chromosome contains a single, long, folded DNA molecule.
In the progressive levels of chromosome packing,
1. DNA winds onto nucleosome spools.
2. The nucleosome chain coils into a solenoid.
3. The solenoid forms loops, and the loops attach to a central scaffold.
4. The scaffold plus loops arrange themselves into a giant supercoil.
 

Eucaryotic DNA

Chromatin can be divided into heavily staining heterochromatin and lightly staining euchromatin.
Euchromatin contains most of the active genes.
Heterochromatin is more condensed and densely staining.
One major diffference between the two types of chromatin is that heterochromatin contains much simple repetitive DNA.
In heterochromatin single-copy genes are embedded in a complex array of tandem and dispersed types of repetitive DNA, most of which have no known function.

Sequence organization was revealing by heating DNA to a single-stranded state then allowing the DNA to reanneal by cooling.
This revealed several types of repetitive DNA.
Multiple "copies" of similar functional repetitive sequences can be described as dispersed gene families (globin genes, actins, tubulins).
Non-functional copies of genes are known as pseudogenes.
Tandem gene family arrays are made up of multiple copies of the same gene all next to each other (such as histones).
The nucleolar organizer, which is cytologically distinct, is a tandem array of genes that encode ribosomal RNA.
Noncoding functional sequences, such as the short tandem repeats that act to maintain the telomeres at the ends of a linear chromosome.

There are a number of sequences with no known function include
1) Highly repetitive centromeric DNA including satellite DNA.
2) Variable number tandem repeats (VNTRs) or minisatellite DNA which provide the differences in DNA used in DNA fingerprinting.
3) Microsatellites, regions of dinucleotide repeats

Transposed sequences are "jumping genes" that are dispersed throughtout the genome.
Transposons move as DNA elements and retrotransposons move via an RNA intermediate which is reverse transcribed and reinerted into the genome.
Examples of a retrotransposons include the 1-to 5 kilobase Long interspersed elements (LINES) and the much smaller (>200 basepairs) short interspersed elements (SINES)
Such as the human Alu sequences.
The presence of these various elements provides a great deal of variety to the spacing and locations of genes in the genomes of organisms.

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