Biology

The continuity of life is based on the reproduction of cells

A cell's endowment of DNA, it's genetic information

The replication and distribution of so much DNA is manageable because the DNA molecules are packaged into

Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus; the nuclei of human somatic cells each contain 46 chromosomes made up of two sets of 23, one set is inherited from the each parent.

complex of DNA and associated protein molecules

When a cell is not dividing, and even as it duplicates its DNA in preparation for cell division, each chromosome is in the form of a long, thin chromatin fiber.

A specialized region where the two chromatids are most closely attached

This phase includes prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis

During all three subphases, the cell grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum.

Mitosis is usually broken into 5 subphases

In this phase, the chromosomes become visible and are tightly coiled, with sister chromatids joined together at centromeres, the nucleoli disappear and the mitotic spindle begins to form.

In this phase, the chromosomes have been duplicated but are not condensed. A nuclear membrane bounds the nucleus, which contains one or more nucleoli. The centrosome has been replicated to form two centromeres.
The single centrosome replicates to form two centrosomes.

In this phase, the nuclear envelope fragments, and microtubules from the spindle interact with the condensed chromosomes. Each of the two chromatids of a chromosome has a kinetochore, a specialized protein structure located at the centromere. Kinetochore microtubules from each pole attach to one of two kinetochores

In this phase, the centromeres divide, separating the sister chromatids. Each s now pulled toward the pole to which it is attached by spindle fibers. By the end, the two poles have equivalent collections of chromosomes.

In this phase, daughter nuclei begin to form at the two poles. Nuclear envelope arise from the fragments of the parent cell's nuclear envelope and other portions of the endomembrane system.

In animal cells, cytokinesis occurs by a process called cleavage. The first sign of cleavage is the appearance of a cleavage furrow in the cell surface near the old metaphase plate.

In plant cells: in plants, which have cell walls, cytokinesis involves a completely different mechanism. During telophase, vesicles from the Golgi coalesce at the metaphase plate, forming a cell plate.

Bacterial chromosomes lack visible mitotic spindles or even microtubules.

Fusion of an S phase cell and a G1 phase cell induces the G1 nucleus to start S phase. Fusion of a cell in mitosis (M phase) with one in interphase (even G1 phase) induces the second cell to enter mitosis.

For many cells, the G1 checkpoint, the "restriction point" in mammalian cells, is the most important. If the cell recieves a go-ahead signal at the G1 checkpoint, it usually completes the cell cycle and divides. If it does not recieve a go-ahead signal, the cell exits the cycle and switches to a nondividing state, the G0 phase. Most cells in the human body are in this phase.

Every living species has a characteristic number of chromosomes. Humans have 46 chromosomes in almost all of their cells each somatic cells are two sets of 23, a maternal set and a paternal set. Distinguished by size, position of the centromere, and pattern of staining with certain dyes. Images of the 46 chromosomes can be arranged in pairs in order of size to produce karyotype display.

The two chromosomes comprising a pair have the same length, centromere position, and staining pattern. The pairs carry genes that control the same inherited characters. Karyotypes are generally prepared at Metaphase. Two distinct sex chromosomes, the X and Y, are an exception to the general pattern of ? in human somatic cells.

The number of chromosomes in a single set is represented by n. Any cell with two sets of chromosomes is called a ? and has a ? number of chromosomes..2n.

Sperm cells or ova have only one set of chromosomes--22 autosomes and an X (in an ovum) and 22 autosomes and an X or a Y(in a sperm cell). For humans, the haploid number of chromosomes is 23(n=23) and the dipolid numnber is 46(2n=46).

The multicellular diploid stage. Meiosis in this stage produces haploid spores that develop by mitosis into the haploid gametophyte state. Gametes produced via mitosis by the gametophyte fuse to form the zygote, which grows into the sporophyte by mitosis.

Meiosis reduces the number of chromosome sets from diploid to haploid

The four daughter cells have only half as many chromosomes as the parent cell and 1/4 the amount of DNA.

group of four chromatids

Each tetrad has one or more ?, sites where the chromatids of homologous chromosomes have crossed and segments of the chromatids have been traded.

In this phase, movement of homologous chromosomes continues until there is a haploid set at each pole. Each chromosome consists of two sister chromatids.

The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number of the organism. If n=3, there are 2^3=8 possible combinations. For humans with n=23, there are 223, or more than 8 million possible combinations of chromosomes.

Crossing over produces recombinant chromosomes, which combine genes inherited from each parent. Crossing over begins very early in prophase I as homologous chromosomes pair up gene by gene. In crossing over, homologous portions of two nonsister chromatids trade places.

When true breeding plants self-pollinate, all their offspring have the same traits.

Chromosomes and genes are both present in pairs in diploid cells. Homologous chromosomes separate and alleles segregare during meiosis. Fertilization restores the paired condition for both chromosomes and genes.

Morgan deduced that the gene with the white-eyed mutation is on the X chromosome, with no corresponding allele present on the Y chromosome.

Genes located on the same chromosome that tend to be inherited together are called

In humans and other mammals, there are two varieties of sex chromosomes, X and Y. An individual who inherits two X chromosomes usually develops as a female. An individual who inherits and X and Y chromosome usually develops as a male.

The X-0 system is found in some insects. Females are XX, males are X. In birds, some fishes, and some insects , females are ZW and males are ZZ. In bees and ants, females are diploid and males are haploid.

In the X-Y system, the Y chromosome is much smaller than the X chromosome. Only relatively short segments at either end of the Y chromosome are homologous with the corresponding regions of the X chromosome. The X and Y rarely cross over.

Fathers pass sex-linked alleles to all their daughters but none of their sons. Mothers pass sex-linked alleles to both sons and daughters.

Because males have only one X chromosome (hemizygous), any male receiving the recessive allele from his mother will express the recessive trait. Therefore, males are far more likely to exhibit sex-linked recessive disorders than are females. For example, color blindness is a mild disorder inherited as a sex-linked trait.

Duchenne muscular dystrophy affects one in 3,500 males born in the US. This disorder is due to the the absence of an X-linked gene for a key muscle protein called dystrophin. The disease involves a progressive weakening of the muscles and a loss of coordination.

During female development, one X chromosome per cell condenses into a compact object called a ?. Most of the genes on the Barr-body chromosome are not expressed.

In humans, this mosaic pattern is evident in women who are heterozygous for an X-linked mutation that prevents the development of sweat glands.

Physical and chemical disturbances can damage chromosomes in major ways. Errors during meiosis can alter chromosome number in a cell.

..occurs when problems with the meiotic spindle cause errors in daughter cells.

Offspring resulting from fertilization of a normal gamete with one produced by nondisjunction will have an abnormal chromosome number, a condition known as

These cells have three copies of a particular chromosome type and have 2n+1 total chromosomes.

These cells have only one copy of a particular chromosome type and have 2n-1 chromosomes.

Organisms with more than two complete sets of chromosomes are called

Tetraploid plants are unable to breed with diploid plants.

Occurs when a chromosome fragment lacking a centromere is lost during cell division. This chromosome will be missing certain genes.

Occurs when a fragment becomes attached as an extra segment to a sister chromatid

Occurs when a chromosomal fragment reattaches to the original chromosome, but in the reverse orientation.

when a chromosomal fragment joins a nonhomologous chromosome.

Genetic disorders caused by aneuploidy can be diagnosed before birth by fetal testing.

One anueploid condition, Down Syndrome, is due to three copies of chromosome 21 or trisomy 21. The frequency of Down syndrome increases with the age of the mother. This may be linked to some age-dependant abnormality in the spindle checkpoint during Meiosis 1, leading to nondisjunction.

James Watson and Francis Crick. Your genetic endowment is the DNA you inherited from your parents. Nucleic acids are unique in thier ability to direct thier own replication. The resemblance of offspring to their parents depends on the precise replication of DNA and its transmission from one generation to the next. It is this DNA program that directs the development of your biochemical, physiological, and (to some extent) behavioral traits.

The nitrogenous bases are paired in specific combinations: Adenine with thymine and guanine with cytosine. Pairing like nucleotides did not fit the uniform diameter indicated by the X-ray data. A purine-purine pair is too wide, and a pyrimidine-pyrimidine pairing is too short. Only a pyrimidine-purine pairing produces the 2-nm diameter indicated by the X-ray data. Purine with 2 rings and pyrimidine with one ring.

Predicts that when a double helix replicates, each of the daughter molecules will have one old strand and one newly made strand.

It takes E Coli 25 minutes to copy each of the 5 million base pairs in its single chromosome and divide to form two identical daughter cells. A human cell can copy its 6 billion base pairs and divide into daughter cells in only a few hours. This process is remarkably accurate, with only one error per ten billion nucleotides.

The replication of a DNA molecule begins at special sites, origins of replication. In bacteria, this is a specific sequence of nucleotides that is recognized by the replication enzymes. These enzymes seperate the strands, forming a replication "bubble".

DNA polymerases catalyse the elongation of new DNA at a replication fork. As nucleotides align with complimentary bases along the template strand, they are added to the growing end of the new strand by the polymerase

The strands in the double helix are antiparallel. The sugar-phosphate backbones run in opposite directions. Each DNA strand has a 3' end with a free hydroxyl group attached to deoxyribose and a 5' end with a free phosphate group attatched to deoxyribose.

The 5'--->3' direction of one strand runs counter to the 3'---->5' direction of the other strand. DNA polymerases can only add nucleotides to the free 3' end of a growing DNA strand. A new DNA strand can only elongate in the 5'--->3' direction.

Helicase untwists the double helix and seperates the template DNA strands at the replication fork. This untwisting causes tighter twisting ahead of the replication fork, and topoisomerase helps relieve this strain.

At the replication fork, the leading strand is copied continuously into the fork from a single primer. The lagging strand is copied away from the fork in short segments, each requiring a new primer.

In human telomeres, this sequence is typically TTAGGG, repeated between 100 and 1000 times. Telomeres protect genes from being eroded through multiple rounds of DNA replication. It is possible that the shortening of telomeres is somehow connected with the aging process of certain tissues and perhaps to aging in general. Telomeric DNA and specific proteins associated with it also prevents the staggered ends of the daughter molecule from activating the cell's system for monitoring DNA damage.

An enzyme called ? catalyzes the lenghtening of telomeres in eukaryotic germ cells, restoring their original length. uses a short molecule of RNA as a template to extend the 3' end of the telomere.