Chromosome - Wikipedia
DNA, chromosomes, and genomes. Homologous chromosomes, sister chromatids, and haploid/diploid. But what exactly is this genetic material, and how does it behave over the course of a cell division?. Each gene has a special place within a chromosome, which is called its locus. Most cells Occasionally, however, problems arise wherein this is not the case. Chromosome condensation is caused by condensins difficulties of replicating the ends of linear DNA. .. What is the relationship between the.
Mitochondria, organelles that harvest energy for the cell, contain their own mitochondrial DNA, and chloroplasts, organelles that carry out photosynthesis in plant cells, also have chloroplast DNA. The amounts of DNA found in mitochondria and chloroplasts are much smaller than the amount found in the nucleus. In bacteria, most of the DNA is found in a central region of the cell called the nucleoid, which functions similarly to a nucleus but is not surrounded by a membrane.
Since all of the cells in an organism with a few exceptions contain the same DNA, you can also say that an organism has its own genome, and since the members of a species typically have similar genomes, you can also describe the genome of a species.
Chromosomes and Chromatin
In general, when people refer to the human genome, or any other eukaryotic genome, they mean the set of DNA found in the nucleus.
Mitochondria and chloroplasts are considered to have their own separate genomes. Chromatin In a cell, DNA does not usually exist by itself, but instead associates with specialized proteins that organize it and give it structure. In addition to organizing DNA and making it more compact, histones play an important role in determining which genes are active. The complex of DNA plus histones and other structural proteins is called chromatin.
- Chromosomes, chromatids and chromatin
Image of a long, double-stranded DNA polymer, which wraps around clusters of histone proteins. The DNA wrapped around histones is further organized into higher-order structures that give a chromosome its shape. For most of the life of the cell, chromatin is decondensed, meaning that it exists in long, thin strings that look like squiggles under the microscope.
In this state, the DNA can be accessed relatively easily by cellular machinery such as proteins that read and copy DNAwhich is important in allowing the cell to grow and function. Condensation takes place when the cell is about to divide.
When chromatin condenses, you can see that eukaryotic DNA is not just one long string. Bacteria also have chromosomes, but their chromosomes are typically circular. Chromosomes Each species has its own characteristic number of chromosomes. Like many species of animals and plants, humans are diploid 2nmeaning that most of their chromosomes come in matched sets known as homologous pairs.
The 46 chromosomes of a human cell are organized into 23 pairs, and the two members of each pair are said to be homologues of one another with the slight exception of the X and Y chromosomes; see below. Human sperm and eggs, which have only one homologous chromosome from each pair, are said to be haploid 1n. When a sperm and egg fuse, their genetic material combines to form one complete, diploid set of chromosomes. So, for each homologous pair of chromosomes in your genome, one of the homologues comes from your mom and the other from your dad.
Image of the karyotype of a human male, with chromosomes from the mother and father false-colored purple and green, respectively. Image modified from " Karyotype ," by the National Institutes of Health public domain.
The two chromosomes in a homologous pair are very similar to one another and have the same size and shape. Most importantly, they carry the same type of genetic information: However, they don't necessarily have the same versions of genes.
That's because you may have inherited two different gene versions from your mom and your dad. It's possible for a person to have two identical copies of this gene, one on each homologous chromosome—for example, you may have a double dose of the gene version for type A. On the other hand, you may have two different gene versions on your two homologous chromosomes, such as one for type A and one for type B giving AB blood. The sex chromosomes, X and Y, determine a person's biological sex: XX specifies female and XY specifies male.
These chromosomes are not true homologues and are an exception to the rule of the same genes in the same places. Aside from small regions of similarity needed during meiosis, or sex cell production, the X and Y chromosomes are different and carry different genes. The 44 non-sex chromosomes in humans are called autosomes. Chromosomes and cell division Image of a cell undergoing DNA replication all the chromosomes in the nucleus are copied and chromosome condensation all the chromosomes become compact.
In the first image, there are four decondensed, stringy chromosomes in the nucleus of the cell. After DNA replication, each chromosome now consists of two physically attached sister chromatids.
After chromosome condensation, the chromosomes condense to form compact structures still made up of two chromatids. As a cell prepares to divide, it must make a copy of each of its chromosomes.
After mitosis occurs within the daughter cells, they have the correct number of genes which are a mix of the two parents' genes.
In diploid 2n organisms, the genome is composed of one set of each homologous chromosome pair, as compared to tetraploid organisms which may have two sets of each homologous chromosome pair.
The alleles on the homologous chromosomes may be different, resulting in different phenotypes of the same genes. This mixing of maternal and paternal traits is enhanced by crossing over during meiosis, wherein lengths of chromosomal arms and the DNA they contain within a homologous chromosome pair are exchanged with one another.
That data and information was further explored by Thomas Morgan. Using test cross experiments, he revealed that, for a single parent, the alleles of genes near to one another along the length of the chromosome move together. Using this logic he concluded that the two genes he was studying were located on homologous chromosomes. Later on during the s Harriet Creighton and Barbara McClintock were studying meiosis in corn cells and examining gene loci on corn chromosomes. There are two main properties of homologous chromosomes: Centromere placement can be characterized by four main arrangements, consisting of being either metacentric, submetacentric, telocentric, or acrocentric.
Both of these properties are the main factors for creating structural homology between chromosomes.
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Therefore, when two chromosomes of the exact structure exist, they are able to pair together to form homologous chromosomes. Sister chromatids result after DNA replication has occurred, and thus are identical, side-by-side duplicates of each other.
The additional 23rd pair is the sex chromosomes, X and Y. If this pair is made up of an X and Y chromosome, then the pair of chromosomes is not homologous because their size and gene content differ greatly.
The 22 pairs of homologous chromosomes contain the same genes but code for different traits in their allelic forms since one was inherited from the mother and one from the father.
They allow for the recombination and random segregation of genetic material from the mother and father into new cells. Sorting of homologous chromosomes during meiosis. Meiosis is a round of two cell divisions that results in four haploid daughter cells that each contain half the number of chromosomes as the parent cell.
The process of meiosis I is generally longer than meiosis II because it takes more time for the chromatin to replicate and for the homologous chromosomes to be properly oriented and segregated by the processes of pairing and synapsis in meiosis I. In prophase I, the DNA has already undergone replication so each chromosome consists of two identical chromatids connected by a common centromere.
SDSA recombination involves information exchange between paired homologous chromatidsbut not physical exchange. SDSA recombination does not cause crossing-over. Chiasmata physically link the homologous chromosomes once crossing over occurs and throughout the process of chromosomal segregation during meiosis. At the diplotene stage of prophase I the synaptonemal complex disassembles before which will allow the homologous chromosomes to separate, while the sister chromatids stay associated by their centromeres.
Meiotic spindles emanating from opposite spindle poles attach to each of the homologs each pair of sister chromatids at the kinetochore. The homologs are cleaved by the enzyme separase to release the cohesin that held the homologous chromosome arms together. The two haploid because the chromosome no.