Why gametes have 23 chromosomes

The letter n is used to represent a single set of chromosomes; therefore a diploid organism is designated 2 n. Human cells that contain one set of 23 chromosomes are called gametes , or sex cells; these eggs and sperm are designated n , or haploid. The matched pairs of chromosomes in a diploid organism are called homologous chromosomes. Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus.

Genes, the functional units of chromosomes, determine specific characteristics by coding for specific proteins. Traits are the different forms of a characteristic. For example, the shape of earlobes is a characteristic with traits of free or attached. Each copy of the homologous pair of chromosomes originates from a different parent; therefore, the copies of each of the genes themselves may not be identical. In the third level of packing, a variety of fibrous proteins is used to pack the chromatin.

These fibrous proteins also ensure that each chromosome in a non-dividing cell occupies a particular area of the nucleus that does not overlap with that of any other chromosome see the top image in Figure 3. DNA replicates in the S phase of interphase.

After replication, the chromosomes are composed of two linked sister chromatids. When fully compact, the pairs of identically packed chromosomes are bound to each other by cohesin proteins. The connection between the sister chromatids is closest in a region called the centromere.

The centromeric region is highly condensed and thus will appear as a constricted area. Prokaryotes have a single circular chromosome composed of double-stranded DNA, whereas eukaryotes have multiple, linear chromosomes composed of chromatin surrounded by a nuclear membrane. The 46 chromosomes of human somatic cells are composed of 22 pairs of autosomes matched pairs and a pair of sex chromosomes, which may or may not be matched.

This is the 2 n or diploid state. Human gametes have 23 chromosomes or one complete set of chromosomes; a set of chromosomes is complete with either one of the sex chromosomes. This is the n or haploid state. Genes are segments of DNA that code for a specific protein.

Duplicated chromosomes are composed of two sister chromatids. Chromosomes are compacted using a variety of mechanisms during certain stages of the cell cycle. Several classes of protein are involved in the organization and packing of the chromosomal DNA into a highly condensed structure. The condensing complex compacts chromosomes, and the resulting condensed structure is necessary for chromosomal segregation during mitosis. Eukaryotic chromosomes are thousands of times longer than a typical cell.

Explain how chromosomes can fit inside a eukaryotic nucleus. Human somatic cells have 46 chromosomes: 22 pairs and 2 sex chromosomes that may or may not form a pair.

He started with "true breeding plants", for example he used purple flowered plants that, when self pollinated, gave rise to only purple flowered plants. He learned most of the important stuff by looking not at the offspring of the first generation but at their offspring. Crossing purple flowered plants with white flowered plants was done by taking the pollen from the purple plants and putting it on the flowers of the white plants after first removing the pollen making structures of the white flowers 9.

The seeds from this cross pollination were planted and the types of offspring were noted. They were all purple. The white did not disappear, however. When this first generation F1 was mated some white flowered plants appeared in their offspring 9.

Imagine trying to figure this out without the knowledge that we have today. Mendel reasoned that each plant got something from each parent during fertilization. So during the first cross each offspring plant got some white and some purple but the purple was what he called the "dominant" trait. We call the different versions of a gene in this case the flower color gene alleles.

In his first crosses he got plants with white flower alleles and purple flower alleles. We know that each of those alleles was on one half of a homologous pair of chromosomes Mendel didn't. When the purple flowers of the first generation were crossed with each other they yielded one white flowering plant for every three purple flowering plants 9. From this evidence he developed the following four ideas:. Alternative versions of genes account for the variations in inherited characters.

We call those alternative versions alleles. We now know that DNA at the same locus on each of a homologous pair of chromosomes can have different information. For each character, an organism inherits two genes, one from each parent.

Mendel didn't even know what you know about meiosis. You know that diploid organisms get one of each chromosome from the parents and that's how we get two alleles for each character.

If the two alleles differ, then one, the d ominant allele , is expressed in the organisms appearance. The recessive allele does not show up. The two alleles for each character segregate during gamete production. So if an individual has a dominant allele and a recessive allele, the gametes may get either one; they will separate. The gametes could have either the dominant or the recessive allele. This is called Mendel's law of segregation.

Some more terminology: By convention, we use an upper case letter to represent the dominant allele and a lower case letter to represent the recessive allele.

An individual with two of the same alleles is called homozygous for that character. If an individual possesses two different alleles we say it is heterozygous for that character.

Mendel had purple flowering plants that were "true breeders", that is, when self pollinated they always produced purple flowered plants. The plants were homozygous for the purple allele. The purple flowering plants in the first generation were heterozygous.

They had white recessive alleles. The phenotype of both purple flowered plants was the same; they're genotypes differed.

Phenotype can be determined by observation, it is the appearance of an individual. The genotype is the underlying genetic makeup of an individual. Not all alleles are completely dominant or recessive. An example from the text is the color of snapdragons. When homozygous red flowers RR are crossed with homozygous white flowers rr the F1 generation is all pink.

The colors appear to have "blended", but the genetic material, the genes, have not blended. This is an example of incomplete dominance. Evidence that the genes haven't blended can be found in the f2 generation 9.

Dominant alleles don't subdue the recessive alleles. Consider Tay-Sachs disease. People with the disease can't metabolize a lipid that accumulates in the brain. These people are homozygous recessives tt. Heterozygotes and homozygous dominant individuals Tt, TT appear normal. So we say T is dominant. What's really going on is that the T allele has the genetic information to produce the enzyme necessary to metabolize the deadly lipids.

Gametes contain half the chromosomes contained in normal diploid cells of the body, which are also known as somatic cells. Haploid gametes are produced during meiosis, which is a type of cell division that reduces the number of chromosomes in a parent diploid cell by half.

Some organisms, like algae, have haploid portions of their life cycle. Other organisms, like male ants, live as haploid organisms throughout their life cycle.