4C ¼ 2n. This does not make intuitive sense to most students – if the DNA
molecules have doubled, wouldn’t the number of chromosomes double? The word
chromosome (literally meaning colored body) is a cytological term coined long
before DNA was known to be a constituent of chromosomes. Whether a chromosome has 1, 2, 4, or thousands of DNA molecules in it, if it appears under the
microscope as a single, cohesive/connected unit, it is considered a single chromosome. The term that is now used for each DNA molecule (and its associated proteins)
in a chromosome is chromatid. Thus, after S phase each chromosome is composed of
two sister chromatids. It is critical that the two sister chromatids remain attached
together until anaphase of mitosis to ensure their proper segregation into different
daughter cells. Once the sister chromatids are no longer physically connected, they
are called sister chromosomes.
As mentioned above, a few plant cells do not go through the standard cell cycle
but rather enter meiosis. At the beginning of meiosis, a cell is 2n and 4C. In prophase
I of meiosis, homologous chromosomes pair, synapse, and undergo crossing over.
When two homologs synapse, they are collectively called a bivalent and are considered a single chromosome as they are now physically connected. Consequently,
when synapsis occurs, the DNA content remains 4C, but the chromosome number is
halved (i.e., it becomes 1n). At prometaphase I, bivalents are moved to the metaphase plate, and at anaphase I, homologs, which were held together by their
crossovers, separate and are moved to opposite cell ends. The second meiotic
division results in four cells, each which possesses the 1C DNA content and the
1n chromosome number. In higher plants, the 1C products of meiosis give rise to a
1C gametophyte and, via mitosis, eventually give rise to 1C gametes. Of particular
importance to this conversation is the fact that homologs must pair, synapse, undergo
crossing over, and stay closely connected until they are pulled into different daughter
cells at anaphase I. If these things do not happen, balanced gametes will not be
produced.
2.4 Genome Size
In any discussion of genomes and sequencing genomes, someone will invariably ask
for the genome size of the species of interest. The larger the genome size, the more
DNA sequencing that must be done to ensure adequate coverage of any given
genetic locus. The genome size of a 1n ¼ 4C cell is twice that of a 2n ¼ 2C cell
and four times that of a 1n ¼ 1C cell. Which value should be used?
The general convention is to provide genome size values in terms of 1C DNA
content (i.e., the amount of nuclear DNA in an unreplicated gamete). The units in
which 1C content are typically provided are megabases (Mb; millions of bases),
gigabases (Gb; billions of bases), or picograms (pg; 10
À12 g) (Bennett and Leitch
2005). The logic behind using the 1C value is that it is the minimum amount of DNA
to contain one copy of every locus. If genome size is presented as 2C or 4C DNA
amount, this must be indicated.
Sequencing Plant Genomes
117
molecules have doubled, wouldn’t the number of chromosomes double? The word
chromosome (literally meaning colored body) is a cytological term coined long
before DNA was known to be a constituent of chromosomes. Whether a chromosome has 1, 2, 4, or thousands of DNA molecules in it, if it appears under the
microscope as a single, cohesive/connected unit, it is considered a single chromosome. The term that is now used for each DNA molecule (and its associated proteins)
in a chromosome is chromatid. Thus, after S phase each chromosome is composed of
two sister chromatids. It is critical that the two sister chromatids remain attached
together until anaphase of mitosis to ensure their proper segregation into different
daughter cells. Once the sister chromatids are no longer physically connected, they
are called sister chromosomes.
As mentioned above, a few plant cells do not go through the standard cell cycle
but rather enter meiosis. At the beginning of meiosis, a cell is 2n and 4C. In prophase
I of meiosis, homologous chromosomes pair, synapse, and undergo crossing over.
When two homologs synapse, they are collectively called a bivalent and are considered a single chromosome as they are now physically connected. Consequently,
when synapsis occurs, the DNA content remains 4C, but the chromosome number is
halved (i.e., it becomes 1n). At prometaphase I, bivalents are moved to the metaphase plate, and at anaphase I, homologs, which were held together by their
crossovers, separate and are moved to opposite cell ends. The second meiotic
division results in four cells, each which possesses the 1C DNA content and the
1n chromosome number. In higher plants, the 1C products of meiosis give rise to a
1C gametophyte and, via mitosis, eventually give rise to 1C gametes. Of particular
importance to this conversation is the fact that homologs must pair, synapse, undergo
crossing over, and stay closely connected until they are pulled into different daughter
cells at anaphase I. If these things do not happen, balanced gametes will not be
produced.
2.4 Genome Size
In any discussion of genomes and sequencing genomes, someone will invariably ask
for the genome size of the species of interest. The larger the genome size, the more
DNA sequencing that must be done to ensure adequate coverage of any given
genetic locus. The genome size of a 1n ¼ 4C cell is twice that of a 2n ¼ 2C cell
and four times that of a 1n ¼ 1C cell. Which value should be used?
The general convention is to provide genome size values in terms of 1C DNA
content (i.e., the amount of nuclear DNA in an unreplicated gamete). The units in
which 1C content are typically provided are megabases (Mb; millions of bases),
gigabases (Gb; billions of bases), or picograms (pg; 10
À12 g) (Bennett and Leitch
2005). The logic behind using the 1C value is that it is the minimum amount of DNA
to contain one copy of every locus. If genome size is presented as 2C or 4C DNA
amount, this must be indicated.
Sequencing Plant Genomes
117