CHROMOSOMES
and KARYOTYPES
Chromosomes of higher organisms (eukaryotes) contain DNA and protein.
One long DNA double helix is present per chromatid
DNA is highly organized in association with histones (protein)
¥ 5 histones are highly conserved from 1 species to another
¥ 4 histones (2 copies of each) make a ball
¥ 1 histone links adjacent balls together
¥ 146 base pairs of DNA wrap around each ball (2 loops/ball)
¥ each ball is a nucleosome
Nucleosomes are coiled into solenoids (6 nucleosomes/solenoid)
¥ solenoids also coil, forming a fiber
¥ fibers coil to give a visible (microscopic) chromosome
Chromosomes can
best be seen when they are highly condensed at metaphase
Stains such as Giemsa that bind to DNA make condensed chromosomes visible
Adding colchicine to
actively dividing cells destroys spindle fibers and traps the chromosomes at
metaphase
Individual chromosomes can be cut from a picture and aligned by pairs, as shown from the work of Dr. T. C Shu
By convention, the largest
chromosome pair is #1, the next # 2 etc.
The resulting picture shows the karyotype (chromosome
composition) of the individual.
Humans have 23 "pairs" of chromosomes; 22 pairs of autosomes and one pair (XX 0r XY) of sex chromosomes
Special techniques produce banding patterns that permit chromosomes of similar size and shape to be distinguished
Especially
useful are fluorescent probes that identify individual chromosomes based on DNA
hybridization (FISH or fluorescent in situ hybridization)
Dyes that result in different colors help in karyotyping:
Chromosome vary in relative length of the arms and position of the centromere;
¥ telocentric: the centromere is at the end (telomere) of the chromosome
¥ metacentric: the centromere is near the middle
¥ submetacentric: slightly off-center
¥ acrocentric: very off center
Centromeres provide the point of attachment (kinetochore) for pulling daughter chromosomes to opposite poles in mitosis and meiosis.
Telomeres are very special; they cannot replicate using DNA polymerase
. Telomeres:
¥ are short highly repeated sequences that protect the ends of chromosomes from exonucleases
¥ grow shorter with age in most cells
¥ are maintained by telomerase enzyme in germline cells, and in some cancer cells (required for "immortality")
-telomerase is a ribozyme, ie it has an RNA component
-the RNA overlaps the repeat and serves as a template to re-exetend
The number of
chromosomes varies between species:
Humans |
46 (23 pairs) per diploid cell |
Dogs |
78 (39 pairs) |
Cats |
38 (19 pairs) |
Horses |
64 (32 pairs) |
Ducks |
80 |
Alligator |
32 |
Drosophila (fruitflies) |
8 |
Horsetail |
216 |
Sequoia |
22 |
A genome is one complete
set of chromosomes
"ploidy" refers to the number of sets of chromosomes per cell
Gametes
each contain one genome so are haploid
Most body cells contain 2 sets so are diploid
The amount of DNA (or RNA in some viruses) per genome varies among species
Some viruses have only around a thousand bases, enough for 3 genes
Humans have over 3 billion base pairs per genome
Amphibians and some primitive plants have 10-100 times as much DNA as mammals.
Crickets have 10 times as much DNA/cell as Fruitflies
Yeast, a eukaryotic fungus, and E. coli, a bacterium, both have about 6,000 genes even though yeast has more DNA/genome