DNA is the molecule that carries genetic information,
which turns out to be in the form of codes that can be
translated into protein sequences, but of course its much
more complicated than that. As mentioned in a previous section,
much of the DNA that organisms carry is not protein coding sequences. The coding
sequences, the genes, are not all separate, but are strung between
non-coding stretches on chromosomes. When cells
divide to form daughter cells, all of the genes are copied
and the copies distributed. If genes were all separate
pieces of DNA, that would involve getting thousands of copies
properly separated, but with genes linked together on
chromosomes, the numbers that have to be moved is manageable.
Chromosome fact sheet.
In organisms that reproduce sexually,
chromosomes are "mixed" (recombination) during sex -
all of the genes are remixed in pairs (when versions of the gene are different,
allowing mixing of different code "recipes," they're
called alleles). These code pairs are carried on "matching" chromosomes,
homologous (Latin for "same information") chromosomes.
Each member of a homologous set has a 50% chance of being passed along to each
offspring, producing offspring with a very different chromosome complement
than the parent and providing that all-important-for-evolution variety in the
group of offspring. Homologous chromosomes are found in most eukaryotes, so it turns out that,
technically, when we discuss gene expression, the reading (transcription)
and conversion-to-protein (translation), it is to be understood that
this happens twice each time for most genes being expressed
in a cell. If your cell is going to make a protein, it is accessing
two codes from two homologous chromosomes.
Clickable site with
info on all of the human chromosomes.
Image of human chromosomes "paired up."
In preparation of cell reproduction, a copy
of the existing DNA must be made, a process called replication,
which will be covered in detail a bit later. Once copies have
been made, they have to be distributed properly to the new cells - this is
when it matters how many bits of DNA there are. If every one of the
hundreds to tens of thousands of genes were copied and separated
individually, the task would be extremely complicated, which is probably
why many genes are linked, stuck together on chromosomes. The fewer separate
molecules there are, the easier it will be to distribute copies
properly. However, if the number of pieces is quite low, then
sexual recombination will produce very limited variety.
How chromosome numbers might change over time (blog post).
Chromosome number is the typical number a
particular species of organism carries in a typical cell - in humans the
number is 46, made up of 23 pairs of chromosomes. Because of
homologous pairs, most chromosome numbers are even numbers. High
numbers present difficulties during cell division - more copies to keep
track of while sets are separated out. Daughter cells with extra
or missing chromosomes are more common if the number is high. Numbers in some species may be
low, as few as 4, which would produce less variation in
offspring. Since evolution depends upon variation, one might think
that organisms with low numbers might live in very stable
microenvironments, or have high natural adaptability. However, in
organisms (simultaneously male and female) that can self-fertilize, low
numbers allow a quasi-asexual mode, since a decent proportion of offspring
would get the same chromosome mix as the parent. Variation plus
copying ability would be very useful in unstable or challenging
environments. Particular species have not only particular chromosome
numbers, but that number is made of of unique combinations of chromosome types
- long, short, fat, thin, connecting in different spots, etc. - that
altogether is called a karyotype.
Chromosome numbers in a few different animal species.
Oddly enough, a lot of the literature on karyotypes uses descriptive
words rather than pictures.
A chromosome is more than just DNA. To fit
very long molecules into very small places, (it is sometimes estimated that the DNA
is any given human cell reaches 2 meters in length!), the molecules are
tightly wound up and packaged around spool-like proteins called histones.
There are several levels of DNA packaging, from small individual histones
which DNA loops around, to complexes that are like tight spool clusters. This tightly-bundled mass of DNA and histones is called
Any kind of processing, whether it involves replication or gene
expression, must involve unwrapping at least some of the packaging to get
at the actual DNA.
Chromosome structure (animation).
Chromatin packing levels.
Lengths of DNA at this level are spooled in histone complexes called
nucleosomes, which are arranged in spirals
that are themselves arranged in loops. Not all areas of a chromosome
are wrapped equally tightly - some parts are easier to open for processing
than others, and mutations that move DNA around on a chromosome (called transposition)
can produce a position effect merely by making a gene easier or harder to get
at. A current theory on how human brains and chimpanzee brains can
be produced by the same genes but seem to function at different levels of
complexity is that the genes involved are expressed to
different extents in the two species - transposition could have had a role
is used to explain why genes in some spots on a chromosome seem to
be harder or easier to access - and transposition will sometimes change a gene's accessibility. Recent work has suggested that these
sorts of mutational changes strongly affect the expression of genes
and may be a powerful driver of evolution - in these cases, one doesn't
need a random allele change to prove useful so much as a change in protein
availability and expression to be useful, and that seems an event that might happen more
often. Additionally, evidence is accumulating that
methylation of histones is an important part of altering when,
how, and whether genes are expressed.
Example of position effect.
How modification might affect your smarts.