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| Chromosomes |
Chromosomes contain the genetic information of cells. Replication of chromosomes assures that genetic information is correctly maintained as cells divide.
The genome of an organism is the sum total of all the genetic information of that organism. In eukaryotic cells, this information is contained in the cell’s nucleus and organelles, such as mitochondria and plastids. In prokaryotic organisms (bacteria and archaea), which have no nucleus, the genomic information resides in a region of the cell called the nucleoid.
A chromosome is a discrete unit of the genome that carries many genes, or sets of instructions for inherited traits. Genes, the blueprints of cells, are specific sequences of deoxyribonucleic acid (DNA) that code for messenger ribonucleic acids (monas), which in turn direct the synthesis of proteins.
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Each eukaryotic chromosome contains a single long DNA molecule that is coiled, folded, and compacted by its interaction with chromosomal proteins called histone. This complex of DNA with chromosomal proteins and chromosomal RNAs is chromatin.
DNA of higher eukaryotes is organized into loops of chromatin by attachment to a nuclear scaffold. The loops function in the structural organization of DNA and may increase transcription of certain genes by making the chromatin more accessible.
To maintain the genetic information of a cell, it is essential that chromosomes correctly replicate and divide as a cell divides. After DNA replication, chromosomes separate in a process called mitosis.
During this process, the nuclear envelope breaks down and chromosomes condense into compact structures. A cellular structure known as the mitotic spindle forms, pulling pairs of replicated chromosomes apart so that the two cells receive identical sets of chromosomes.
Chromosomes are readily visualized when they condense during cell division. All the chromosomes of a cell visualized during mitosis constitute that cell’s karyotype.
Each chromosome has a centromere—a constricted area of the condensed chromosome where the mitotic or meiotic spindle attaches to assure correct distribution of chromosomes during cell division—and a telomere, the end or tip of a chromosome, which contains tandem repeats of a short DNA sequence.
The number of chromosomes in a gamete (either egg or sperm) is the haploid number, n. The haploid number of chromosomes in humans is 23; in corn, 10; in peas, 7; in Arabidopsis (the model organism used in much botanical research), 4.
Some carp and some ferns have more than 50 chromosomes in the haploid genome. Pollen grains of some plants, such as pear, contain three haploid cells: One directs the growth of the pollen tube down the style to the ovary; the other two are sperm.
In flowering plants (angiosperms), there is a unique double fertilization where by one sperm nucleus fuses with the egg nucleus to form the diploid (2n) zygote, and the other sperm nucleus fuses with two polar nuclei to form the triploid nutritive tissue, or endosperm, which will nourish the embryo in the seed.
The zygote then increases in cell number by mitosis, a type of cell division during which chromosomes in a nucleus are replicated and then separated to form two genetically identical daughter nuclei.
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| Schematic of a chromosome |
This is followed by cytokinesis, the process of cytoplasmic division, which results in two daughter cells, each having the same number of chromosomes and genetic composition as the parent cell. The mature 2n plant forms the haploid (n) gametes by meiosis, a type of cell division that reduces the number of chromosomes to the haploid number.
A distinctive feature of plant cell division is the plant cell has three genomes (the nuclear, mitochondrial, and plastid genome) to replicate and divide. The chromosomes of eukaryotes consist of unique genes among a complex pattern of repeated DNA sequences. Arabidopsis has only 4 chromosomes containing about 120 million base pairs.
There are typically between twenty and one hundred copies of the mitochondrial genome per mitochondrion, ranging in size from two hundred to twenty-four hundred kilobase pairs (or kb; one kilobase pair equals one thousand base pairs).
Plant mitochondrial genomes are much larger than the mitochondrial genomes of yeast or animals. Chloroplast genomes range in size from 130 to 150 kb, with 50 to 150 copies of that genome per plastid.
In cell division in plant cells, the two daughter nuclei are partitioned to form two separate cells by a cell plate that grows at the equator of the mother cell. In animal cells, this separation involves the constriction of the cell at a central contractile ring. DNA replication is strictly controlled during the cell cycle.
DNA synthesis occurs in the synthesis (S) phase, beginning at origins of replication distributed around the genome, occurring on average every 66 kb in dicotyledonous plants and on average every 47 kb in monocotyledonous plants.
Heterochromatin is the term for regions of chromosomes that are permanently in a highly condensed state, are not transcribed, and are late-replicating.
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| chromosomes made out of |
Heterochromatin contains highly repeated DNA sequences. Euchromatin is the rest of the chromosomes that is extended, accessible to RNA polymerase, and at least partially transcribed.
Some plants and animals have extra chromosomes that do not seem to be essential. These are called accessory or supernumerary chromosomes. They have been most studied in corn where these extra chromosomes are called B-chromosomes. B-chromosomes are usually highly condensed heterochromatin that may or may not be present in an individual of that species.
An increase in the copy number of the genome is common in plants and animals, occurring during the development of individuals. Polyploids have three or more complete sets of chromosomes in their nuclei instead of the two sets found in diploids. For example, in Arabidopsis, tissues of increasing age have an increase in polyploidy, reaching up to sixteen duplications.
