34

Chromosomes were first known about from karyograms (that's the word for chromosome pictures like these) sort of like this one (1)(2): The scientists looking at these chromosomes didn't know much about them at first. They were discovered before anything was understood about genes, but by 1922 it was thought they were the carriers of genes. Without much ...


33

It is true that the Y chromosome is shorter than the X chromosome and that there are more genes on the X chromosome. Do men have fewer genes? One could (mis)understand three things in the expression "number of genes". Number of gene copies (see Copy Number Variation) Number of genes Number of alleles Thanks to @GerardoFurtado for correcting my ...


30

Great question, and one about which there has historically been a lot of speculation, and there is currently a lot of misinformation. I will first address the two answers given by other users, which are both incorrect but have been historically suggested by scientists. Then I will try to explain the current understanding (which is not simple or complete). My ...


28

Chart of C-values (the mass of DNA in a single haploid cell); there is no logical order to the groups: [source] Base pairs in haploid genome (some examples): Escherichia coli (bacterium): ~4.5 million Caenorhabditis elegans (nematode worm): ~100 million Homo sapiens (we all know what these are): ~3 billion Pinus taeda (coniferous tree): ~22 billion ...


25

Are there examples of cells with more than one nucleus? Yes, they are called Multinucleate cells. There are two types of multinucleated cells Syncytia Coenocytes I highly recommend having a look at this answer for the definitions. Examples of Syncytia include Osteoclasts Skeletal muscle fibers (thanks @kmm) Examples of Coenocytes include Codium (...


16

I think the OP is asking why we have one less chromosome pair than chimpanzees (for example) [23 pairs instead of 24]. The is an abundance of evidence, as alluded to above by shigeta, that human chromosome 2 is the result of a telomere-to-telomere fusion of two ancestral chromosomes (IJdo et al., 1991). This event did not occur in our closest ancestors, ...


14

Each chromosome is a pair of distinct, separate DNA molecules. A chromosome of an eukaryotic cell nucleus is a (long) helix of two linear molecules and so has two ends, which are called telomeres. DNA naturally forms a double helix with its complementary DNA molecule, and the double helix can further curl in what are called supercoils. In humans, the ...


13

You nailed it! Nomenclature Autosomal chromosomes Chromosomes are numbered based on their size (in base pairs). Sexual chromosomes Sexual chromosomes (when they exist) are defined as the largest number regardless of their length. Note that the definition of a sex chromosome can sometimes be a little bit troublesome (see this book). Exceptions It ...


12

The majority of normal autosomal genes have biallelic expression, i.e. the alleles on both chromosomes are used to make protein. This is why mutations in a gene which cause non-function of the product can be silent if the mutation is heterozygous (only found on one of the alleles), because the effect is masked by the second, fully-functional, copy of the ...


11

Note that anyone with a Y chromosome is considered a genetic male. Also, aneuploidy is usually a result of nondisjunction (but can have other causes as well) and is usually not inherited. 47,XXX is called Triple X syndrome and occurs in approximately 1 in 1,000 female births. These individuals usually appear normal, but may have tall stature, a small head, ...


11

There is both an upper and lower limit which are species specific. The upper limit is caused by incomplete segregation of sister chromatids and subsequent trimming of the long arm(s). The reason for the lower limit is, as far as I can tell, unknown. Upper Limit Schubert I, Oud JL. 1997. There is an Upper Limit of Chromosome Size for Normal Development of ...


11

This Nature paper from 2004, by Jane Grimwood et al. goes at least a long way towards giving an answer to the question of the OP. In short: there were inordinately many duplications, especially during an event 30-40 million years ago, as well as during a much more recent event. These duplications are, uncharacteristically, predominantly intra-chromosomal ...


11

There is a branch of life called the Diplomonads, most of which have two nuclei. They are single cell organisms and an early offshoot of the eukaryotic linage. A good example is Giardia lamblia. https://en.wikipedia.org/wiki/Diplomonad Giardia lamblia


10

I think it is the wrong question. You assume that eukaryotes developed from a single-cell organism with circular DNA. Then, clearly, there must have been an advantage of (newly) developing a linear genome. But eukaryotes could have developed from an organism with linear DNA, too. There are still a few bacterial species with linear chromosomes, so this is not ...


10

During the generation of gametes (sperm, eggs), chromosomes can cross-over - this swaps paternal and maternally-derived genetic material. So none of the descendant's chromosomes would be a direct copy of Einstein's, and furthermore, each offspring receives half of its genetic complement from each parent. This means that if you pool the DNA data from small ...


10

The authors of this 2012 review article summarize the problem well in their introduction: In contrast to the tremendous advances in throughput, assembling sequencing reads remains a substantial endeavor, much greater than the sequencing efforts alone would suggest [22-24]. Large complex plant genomes remain a particularly difficult challenge for de novo ...


9

The reason is that X-inactivation is not complete (Carrell & Willard, 2005; Ahn & Lee, 2008), and as many as 15-25% of X-linked genes escape silencing (Carrell & Willard, 2005; Cheng et al, 2005). This means that some genes on the Barr body are expressed in XX-females, although often at lower levels compared to the active X-chromosome, and this ...


9

Almost all mammals (including mice and humans) have two sexes where the males have a Y chromosome and an X chromosome (whereas females have two X and no Y chromosomes). This is not the only way organisms can determine sexes, but it's the way mammals evolved to do it. So how does having a Y cause an individual to be male? The Y has only a few genes (less ...


8

@nico is right. the number of chromosomes is the result of an evolutionary timeline, puncutated by sometimes spontaneous events which shape the DNA. These events occur in the course of evolution: 1) Chromosomal rearrangements. Large sections of the genome can flip around or become integrated in other chromosomes. By homologous recombination, regions of ...


8

I think the picture will help you. In fact, to determine which kind of rearrangement for the chromosomal, we need to define an "orientation". However, this orientation is not fixed. It is a definition compared with the two chromosomes before and after the rearrangement. As the picture shows, the duplicated sequence after rearrangement should be in the same ...


8

The picture can be a bit misleading because it represents 22 autosomes (autosome = non-sexual chromosome) while there are 22 pairs of autosomes (so the homologous chromosome is not represented). And it represents the whole pair of sexual chromosomes (the individual is a male). homologous chromosome For each autosome represented, there is another one which ...


8

Since I have used more than 1 image in my answer; with numbers starting from 1; I'll call your provided figure as figure-0 What is shown in following picture? Though the image showing many things; in overall it is an image of a set of chromosomes; seemingly almost certainly from human. The chromosomes has been stained with a banding method (though I'm ...


8

According to this article The hairy beast with seven fuzzy sexes Tetrahymena thermophila has two: a large macronucleus and a small micronucleus. The macronucleus controls the everyday functions of the cell, while the micronucleus deals with its complicated sex life. In fact this is true for all ciliates.


7

Here is a paper you might want to take a look at: Phylogenetic Origin of Human Chromosomes 7, 16, and 19 and their Homologs in Placental Mammals From the abstract: From their origin, these chromosomes underwent the following rearrangements to give rise to current human chromosomes: centromeric fission of the two submetacentrics in ancestors of all ...


7

No, an external physical examination would be inconclusive. The reason is the TDF gene. To be more specific, if a person has XY and the gene is not active then the subject would have a female appearance. Also we cannot conclude that a person has a Y chromosome even if it has penis because that gene could be transferred on the X chromosome. Here is a ...


7

Regarding the Papers referenced above: If the authors maintain that the evidence at the site of fusion is not clear and that explanations as to how this happened are flawed that is possibly concedable. They would need to do more homework than these papers. The discussion of vestigial centromere sequences and telomere motifs being inverted is interesting,...


7

Mycoplasma genitalium was one of the first full bacterium genomes sequenced and since its a symbiotic organism that lives on the moist and warm genital skin surface it doesn't need as many genes as many bacteria. It has a 582 kbp genome sequence with only 521 genes. But that is so 1995. The 159 kbp genome of Candidatus carsonella was published in 2006. ...


7

I got $26$% as the answer. To get a recombination between C and E, there are two possible mechanisms:- C and D produce a recombinant, but D and E remain linked, therefore the final genotype will be a recombinant considering C and E(Chiasmata between C and D). Here $P_1=P_{CD}\times P'_{DE}$ where $P$ is the probability of recombination and $P'=1-P$ is ...


7

It is already mentioned by canadianer that genome size differs between organisms. But what about complexity? First we should define what complexity is: complexity can be defined as number of different cell types that a multicellular organism can produce, with the same genome. Yes complexity does not correlate with genome size. However, it seems to correlate ...


6

First of all, they are not caused by the same mechanism. They are both aneuploidies, but the mechanism is different. Turner syndrome happens when one of the gametes (most commonly father's) lacks a copy of X chromosome, or the X chromosome is heavily damaged. This means that while normal women have two X chromosomes, Turner women have only one, and nothing ...


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