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 ...
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
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.
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
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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