What Is Chromosome In Biology

How Are Chromosomes Inherited

In humans and most other complex organisms, one copy of each chromosome is inherited from the female parent and the other from the male parent. This explains why children inherit some of their traits from their mother and others from their father.

The pattern of inheritance is different for the small circular chromosome found in mitochondria. Only egg cells – and not sperm cells – keep their mitochondria during fertilization. So, mitochondrial DNA is always inherited from the female parent. In humans, a few conditions, including some forms of hearing impairment and diabetes, have been associated with DNA found in the mitochondria.

• How are chromosomes inherited?

  In humans and most other complex organisms, one copy of each chromosome is inherited from the female parent and the other from the male parent. This explains why children inherit some of their traits from their mother and others from their father.

  The pattern of inheritance is different for the small circular chromosome found in mitochondria. Only egg cells – and not sperm cells – keep their mitochondria during fertilization. So, mitochondrial DNA is always inherited from the female parent. In humans, a few conditions, including some forms of hearing impairment and diabetes, have been associated with DNA found in the mitochondria.

How Much Would You Know If Two Chromosomes Were Homologous

If they synapse during prophase of meiosis I, they are homologous otherwise, they are not. If you do not have the luxury of seeing them in prophase I for example, you are looking at metaphase chromosomes in a karyotype where staining was not used then you could look at their length and centromere positions.

Chromosome Morphology And Classification

There are two kinds of cell division: mitosis and meiosis. Mitosis is a somatic cell division responsible for the growth, proliferation, and tissue differentiation of the body, whereas meiosis is responsible for the production of gametes. Because mitotic cells are easy to obtain, morphological studies are generally based on mitotic metaphase chromosomes. Chromosomes are not visible under a light microscope in non-dividing cells. As the cell begins to divide, the thread-like chromatin material in the nucleus begins to condense in the metaphase stage, the chromosomes are best recognizable.

A chromosome consists of two arms separated by a primary constriction called a centromere. The short chromosome arm is designated as p and the long arm as q . A centromere consists of several hundred kilobases of repetitive DNA and is responsible for the separation of chromosomes during cell division.

Each chromosome consists of two identical strands known as chromatids or sister chromatids, which are visible after the S phase of the cell cycle. Each of the two sister-chromatids contains a highly coiled double helix of DNA and is joined at the centromere.

Figure 24.1. Diagrammatic representation showing chromosome classification according to centromere position and size.

Figure 24.2. Karyotype showing a normal male chromosomal constitution.

Table 24.1. Grouping of chromosomes based on descending order of size and position of the centromere.

N. Urraca, L.T. Reiter, in, 2013

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Whole Chromosome Dna Sequencing Reveals Organizational Order

What have we learned from whole genome sequencing? Annotations of complete genome sequences have revealed essentially all the genes and their surrounding sequences, their position in each chromosome, the precise variation in all the repeat families, and the distribution of repeat members throughout the genome. The sequence of one or a few genomes per species provides only a static snapshot of the chromosomes of the species. It requires sequences from many carefully selected accessions to provide estimates of the diversity within a species, as was gained from the 1,001 Arabidopsis genome project and the 3,000 rice genotypes . Herein lie some challenges for the future. The many monocot and dicot genomes now sequenced make it impossible in this perspective to cover all their salient features. However, below are a few of the general points on chromosome organization that have come from whole chromosome DNA sequencing and molecular biology. All teach us the outcomes of the forces that, on the one hand, generate variation during evolution and on the other preserve and exploit variation for species survival.

The Example Of Ribosomal Dna Silencing By Rna

Control of condensation of multigene arrays of rDNA into heterochromatin. A, Map of rDNA repeat unit from wheat showing 12 135 bp repeats upstream of the major transcription start site and the intergenic transcripts initiated from different positions including readthrough from the 25S RNA sequence. Dotted lines indicate ends of the RNA transcripts not determined. DNA elements labeled BD are other duplicated sequences. Results from . Copyright held by ASPB. B, The organization of an rDNA repeat unit in Arabidopsis showing transcripts from the intergenic promoters upstream of the major transcript . The intergenic transcripts are processed into 24 nt RNAs that then direct do novo methylation of spacer sequences by domains rearranged methyltransferase 2. Binding of methylcytosine binding protein BD6 and MBD10 in conjunction with deacetylation facilitates chromatin compaction into heterochromatin. Reprinted from with permission from Elsevier. C, In situ hybridization of labeled rRNA to wheat cells, showing the clustering of the silenced genes in heterochromatin and the active rDNA dispersed through the nucleolus .

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Chromosomes And Protein Production

Protein production is a vital cell process that is dependent upon chromosomes and DNA. Proteins are important molecules that are necessary for almost all cell functions. Chromosomal DNA contains segments called genes that code for proteins. During protein production, the DNA unwinds and its coding segments are transcribed into an RNA transcript. This copy of the DNA message is exported from the nucleus and then translated to form a protein. Ribosomes and another RNA molecule, called transfer RNA, work together to bind to the RNA transcript and convert the coded message into a protein.

History And Analysis Techniques

The term karyotype was coined by German anatomist Heinrich von Waldeyer in 1888. Primarily, WBCs are used in this technique. However, cells from bone marrow, amniotic fluid, and placenta can also be used as a sample. In this technique, the chromosomes are isolated, followed by staining and examination under the microscope. The cells are stained with the dye like Gimesa stain after arresting the cell division at the metaphase stage with the help of colchicine. A trained cytogeneticist then arrange the chromosome labeled as 1-23 according to their size from largest to the smallest. In humans, chromosome number 1 is the largest, while chromosome number 18 is the smallest.

Try to answer the quiz below to check what you have learned so far about chromosomes.

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Chromatin And Chromosome Biology

The genome is the blueprint of life. It is a set of DNA molecules that contain all of the instructions for an organisms development and the ability to respond to the environment. Every cell contains the same blueprint but depending on how these instructions are executed will lead to a variety of cell types and ultimately a complex organism. This is a tremendous amount of information and in fact if each molecule of DNA was aligned end to end it would span 2 meters . Yet amazingly, these molecules all fit into a cell nucleus that is only 0.000006 meters in diameter.

The packaging of the genome into the cell nucleus is accomplished through chromatin. Chromatin is the complex of genomic DNA with proteins called histones, where each histone-bound DNA molecule is referred to as a chromosome. However, chromatin not only compacts the genome into the nucleus, but is also the mechanism controlling how the genome is read out from cell to cell. Thus, chromatin is often referred to as the epigenome . Chromatin is incredibly dynamic, reorganizing during development to establish cell-type, as well as in response to an array of environmental stimuli. Importantly, dysregulation of chromatin underlies a number of diseases including developmental disorders, cancer, heart disease, neurological disorders, and more.

Lets Understand All The Parts Of The Above Chromosome Structure One By One:

Centromere: It is also known as Kinetochore and is the primary constriction at the center where the chromatids or spindle fibers are attached. It functions in the movement of the chromosome during a stage called anaphase during cell division.

Chromatid: When a chromosome is divided into two identical strands during cell division, a chromatid is formed as half of the chromosome. Each half strand is joined by a centromere, both are known as sister chromatids and it contains DNA and separates at Anaphase to form a separate chromosome.

Chromatin: Chromatin is a complex of DNA consisting of DNA, RNA, and protein and it forms chromosomes within the nucleus of eukaryotic cells. It doesnt exist freely as linear stands, rather it is highly condensed and wrapped around nuclear proteins.

Telomere: The terminal region of each side of the chromosome is called a telomere.

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Modifying Chromosome Information And Plant Traits By Inserting Novel Genes And Gene Editing

Molecular geneticists and plant breeders have long dreamed of being able to modify existing alleles or insert new genes, specially designed, into the chromosomes of their favorite plant species to evaluate gene function, to provide new highly desired traits, for example disease resistances, and also to avoid backcrossing new alleles/genes into a given genotype. The value of this was obvious to me from the time I entered the plant breeding world after my postdoctoral fellowship. I therefore took special note of the most powerful and remarkable of discoveries around the late 1970s that crown gall disease of plants was the outcome of the transfer of a piece of DNA from a plasmid of Agrobacterium tumefaciens into dicot cells and incorporation of the DNA into plant chromosomes . This outstanding and consequential piece of both bacterial genetics and plant biology changed the course of plant science. It opened up in 1982/3 the means of moving genes from bacterial cloning vectors into plants via infecting plant tissue cultures with the modified bacteria, selecting in culture the cells receiving the DNA followed by regenerating the selected cells into whole plants using regimes involving switches of hormones. The initial achievements of transformation and selection of genetically transformed cells were achieved essentially simultaneously by four groups, including our own , partly in collaboration and partly in competition with one another.

How Is Dna Packaged In The Nucleus

Chromosomal DNA is packaged inside microscopic nuclei with the help of histones. These are positively-charged proteins that strongly adhere to negatively-charged DNA and form complexes called nucleosomes. … Nucleosomes fold up to form a 30-nanometer chromatin fiber, which forms loops averaging 300 nanometers in length.

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What Do Chromosomes Do

The unique structure of chromosomes keeps DNA tightly wrapped around spool-like proteins, called histones. Without such packaging, DNA molecules would be too long to fit inside cells. For example, if all of the DNA molecules in a single human cell were unwound from their histones and placed end-to-end, they would stretch 6 feet.

For an organism to grow and function properly, cells must constantly divide to produce new cells to replace old, worn-out cells. During cell division, it is essential that DNA remains intact and evenly distributed among cells. Chromosomes are a key part of the process that ensures DNA is accurately copied and distributed in the vast majority of cell divisions. Still, mistakes do occur on rare occasions.

Changes in the number or structure of chromosomes in new cells may lead to serious problems. For example, in humans, one type of leukemia and some other cancers are caused by defective chromosomes made up of joined pieces of broken chromosomes.

It is also crucial that reproductive cells, such as eggs and sperm, contain the right number of chromosomes and that those chromosomes have the correct structure. If not, the resulting offspring may fail to develop properly. For example, people with Down syndrome have three copies of chromosome 21, instead of the two copies found in other people.

The Folding Of The Dna

• The first step is the assembly of the DNA with a newly synthesized tetramer , are specifically modified ), to form a sub-nucleosomal particle, which is followed by the addition of two H2A-H2B dimers.
• This produces a nucleosomal core particle consisting of 146 base pairs of DNA bind around the histone octamer. This core particle and the linker DNA together form the nucleosome.
• The next step is the maturation step that requires ATP to establish regular spacing of the nucleosome cores to form the nucleo-filament.
• During this step the newly incorporated histones are de-acetylated.
• Next, the incorporation of linker histones is accompanied by folding of the nucleo-filament into the 30 nm fiber, the structure of which remains to be elucidated.
• Two principal models exist- the solenoid model and the zig-zag.
• Finally, further successive folding events lead to a high level of organization and specific domains in the nucleus.

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How To Make Notes On Chromosomes

• Go through Chromosome – Definition, Structure, Function and Important FAQs on Vedantu

• Read the page carefully and then highlight all the important portions

• Try to understand whatever has been explained instead of just mugging it up

• Write down in your own words and make mini notes

• Follow the sequence thats on the page so as to learn in an organized manner

• Go through what youve written and then revise everything before the tests

• Do not copy-paste whats on the page

• Re-read those areas that require more understanding

• Draw some pictures if you need to memorize certain things as pictures tend to get retained in the memory more

• Write brief sentences

Adoption Of Model Species Revolutionized Plant Science

While Arabidopsis did and does still today attract more researchers than any other species, especially in Europe and in the USA, rice became a model for discovery and application and complemented Arabidopsis nicely with its small genome and facile regeneration from cell culture and because it is a monocot. Today the literature describing the knowledge and tools for rice research is immense . The need for another monocot model gave rise, especially in the USA, to the adoption of Brachypodium distachyon, a wild grass species, because of its small genome, rapid life cycle, and now, easy tissue culture and rapid plant regeneration. Much useful genetics and genomics has been achieved using it . The arguments for adopting model plant species have been completely justified. They have dominated the discoveries. It is hard to imagine what would have happened to plant science if researchers and funders had not adopted model species. Much hard work had to be put into working out easy procedures and generating genomic resources for these model species. Tremendous thanks are due to the pioneers. They changed plant science for ever.

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Techniques For Spreading And Staining Chromosomes

To make chromosome preparations suitable for observation by light microscopy, cells are usually fixed in a freshly prepared solution of one part acetic acid and three parts ethanol or methanol. The chromosomes are then spread and flattened onto a glass microscope slide, either by squashing cells under a cover glass or by bursting cells that have been hypotonically swollen and permitting the chromosomes to dry on the slide. Chromosomes can then be stained for examination by light microscopy.

Many stains have been used for chromosomes. Without special treatments, dyes such as crystal violet, Giemsa stain, orcein, and carmine stain chromosomes homogeneously throughout their length. Historically, the Feulgen technique has been important because the intensity of staining is a direct indicator of the amount of DNA present. Some techniques result in the formation of bands on chromosomes due to differential staining based on the location of euchromatin and heterochromatin or on differences in base composition of DNA along the length of chromosomes. The latter type of banding is observed with fluorescent dyes such as Hoechst 33258, daunomycin, chromomycin A, mithramycin, DAPI, and quinacrine dihydrochloride, which either bind better or fluoresce more strongly in AT-rich or GC-rich DNA .

Fluorescence in-situ hybridization

Immunofluorescence

Why Is Complex Packing Critical For Eukaryotic Chromosomes

The multiple levels of packing that exist within eukaryotic chromosomes not only permit a large amount of DNA to occupy a very small space, but they also serve several functional roles. For example, the looping of nucleosome-containing fibers brings specific regions of chromatin together, thereby influencing gene expression. In fact, the organized packing of DNA is malleable and appears to be highly regulated in cells.

Chromatin packing also offers an additional mechanism for controlling gene expression. Specifically, cells can control access to their DNA by modifying the structure of their chromatin. Highly compacted chromatin simply isn’t accessible to the enzymes involved in DNA transcription, replication, or repair. Thus, regions of chromatin where active transcription is taking place are less condensed than regions where transcription is inactive or is being actively inhibited or repressed .

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Control Of Gene Expression

There are many types of cells in a persons body, such as heart cells, liver cells, and muscle cells. These cells look and act differently and produce very different chemical substances. However, every cell is the descendant of a single fertilized egg cell and as such contains essentially the same DNA. Cells acquire their very different appearances and functions because different genes are expressed in different cells . The information about when a gene should be expressed is also coded in the DNA. Gene expression depends on the type of tissue, the age of the person, the presence of specific chemical signals, and numerous other factors and mechanisms. Knowledge of these other factors and mechanisms that control gene expression is growing rapidly, but many of these factors and mechanisms are still poorly understood.

The mechanisms by which genes control each other are very complicated. Genes have chemical markers to indicate where transcription should begin and end. Various chemical substances in and around the DNA block or permit transcription. Also, a strand of RNA called antisense RNA can pair with a complementary strand of mRNA and block translation.