what is the relationship between a chromosome and a chromatid? | Yahoo Answers
Cut and pasted verbatim from posavski-obzor.info,page Difference Between Chromosome and Chromatid: > Chromosome vs Chromatid Chromosome. The paternal bonds refer to the relationship between a child and a father. On the Paternal chromosomes are heterogametic while maternal. A chromosome is made up of two chromatids which are joined by the centromere. The chromatids separate from each other during mitosis to.
Higher variations in genome size among plants were discussed in several previous studies [ 5 - 7 ]. Chromosome number showed less variations among living groups compared to genome size because chromosome number is not a continuous variable and is represented in even numbers. Generally, it showed 62 folds variation which represent the maximum range in living groups and variations within any group will be at less magnitude.
Genome size and chromosome number The lack of correlation between GS and CN as well as the location of human genome among other genomes provide evidence against the Darwinian evolution theory.
Results indicate that human which is considered the most developed and complicated species does not have the largest genome or chromosome number among living organisms.
What is the difference between Chromosomes/Chromatids etc? - The Student Room
The genomes smaller than human genome and the genomes larger than human genome have a mix of plant and animal genomes. In addition, some genomes have the same genome size, but form and reproduce completely different organisms Supplementary Material 3. Variations in genome size were mentioned to be independent of changes in chromosome numbers. This makes the process of genome evolution more complicated composed of many different factors including genome size changes, chromosome number shifts, and other factors that contribute to converting the impact of these genome evolution mechanisms into function turning their effect as organism evolution [ 6 ].
Some early theories explained variation in genome size by large amounts of non-coding DNA [ 29 ], but it was criticized by the fact that evolution does not possess such foresight and the non-coding DNA in eukaryotic genomes mostly consists of repetitive elements of various lengths and does not contribute to the structure of functional genes [ 6 ].
This confirms the lack of genome size evolution trend of living groups and that plants and animal genomes appeared simultaneously not in a specific sequence as it has been claimed by the Darwinian evolution theory. All three relationships did not reveal any type of correlation between CNS and the three GS estimates, yet they showed independent relationships.
This also support the idea that beside the independence of absolute GS and CN data in the genomes, the lack of correlation also was the case at the higher level of CNS and the aforementioned genome size estimates. Location of human genome contradicts evolution from common ancestor It is certain that a genome controls the organism structure and development therefore; the genome is expected to evolve before the evolution of the organism.
So, based on Darwinian evolution from common ancestor, we expect gradual change increase in genome size from the assumed common ancestor smallest detected genome in this study, Buchnera sp. Based on this assumption, human is expected to have the largest genome because it is the most recent and the most developed species on earth [ 30 - 32 ] and consequently is expected to lie at the end of genome size evolution curve. In addition, according to the Darwinian evolution from common ancestor, the gradual increase in genome size must be correlated with gradual increase or decrease in chromosome number chromosome number evolution as well as with organism evolution.
The location of human genome among other genomes based on genome size and chromosome number Figure 2 confirms that there is no correlation between genome size of species and their emergence on earth genome evolution. This rolls out the idea that human genome evolved from smaller pre-existing genome. A chromosome is simply the product of the DNA and the proteins that are attached to it. There are 23 pairs of the chromosomes in every human being.
One set is inherited from the father, and one set is inherited from the mother. A DNA is a sort of a bio molecule. The entire DNA in cells can be found in individual pieces that are called chromosomes.
The main difference between DNA and chromosome is regarding the role of genes. DNA stands for deoxyribonucleic acid. The DNA is basically made up of cytosine, adenine, thymine and guanine. When you arrange these four bases to create a particular segment, it is called a gene. When these segments are coiled in a form that can be easily duplicated, they are known as chromosomes. Try to remember it like this- a gene is composed of tiny chromosomes, each of which determines a particular characteristic in a human.
Meiosis is initiated in the human foetal ovary at 11—12 weeks of gestation [ 2 ], but becomes arrested after completion of homologous chromosome pairing and recombination.
This meiotic-halt lasts for several years until the elevated level of LH and FSH resume the process at the onset of puberty. It completes the meiosis II after the sperm enter its cytoplasm following fertilization. Thus, the oocyte, whose ovulation marks the menarche, remains in pause for shortest period and that ovulate just preceding menopause experiences longest period of arrest. This long tenure of oocyte development makes it vulnerable to acquire environmental hazards within its microenvironment which inevitably increases the risk of chromosomal NDJ.
The first documented risk factor for maternal NDJ is advancing maternal age of conception [ 3 ]. The first molecular correlate found to associate nondisjoined chromosome 21 is altered recombination along 21q. Chiasma formation and subsequent recombination between homologous chromosome pair are essential for proper segregation at anaphase. Chiasma holds the two chromatids in physical contract and counterbalances the pull by spindle fibers from the poles up to a certain time.
Following chiasma formation recombination is initiated by double stranded break on DNA molecules. The resultant single stranded DNA then invades homologous duplex and the recombining pair is then resolved in either in exchanged crossover or non-exchanged noncrossover products [ 4 ].
Deviation from optimal chiasma positioning at the middle of arm of chromosome pair and reduction in crossover frequency impose risk of nondisjunction on the chromosome pair Figure 1.
Double strand break model of recombination explains production of cross-over and non-cross over gametes. Further examination has shown that in addition to the absence of an exchange along the nondisjoined Ch21, the suboptimal placement of an exchange is an important susceptibility factor for NDJ.
In their study on US population Oliver et al. Here we should mention that Ch21 without exchange may nondisjoin at MII, but owing to technical definition this group may be misidentified as post zygotic mitotic error.
Difference Between DNA and Chromosome
Study of Ghosh et al. In their study on US sample Oliver et al. This finding suggests that absence of detectable exchange is maternal age independent risk factor as the trend of decrease in proportion was not associated linearly with increasing maternal age of conception. Almost similar trend was observed in the study conducted by Ghosh et al. In US sample there was a less proportion of non-exchange event in middle age group than expected and in Indian sample middle age group exhibited highest proportion of nonexchange events.
These two sets of results provide primary evidence for a secondary backup mechanism that helps to distribute non-exchange bivalents which is age dependent. Human Proteins with similar function as those in yeast that are involved in the proper segregation of non-exchange homologues have been shown to be down regulated with increasing ovarian age [ 1112 ].
Difference Between DNA and Chromosome | Difference Between | DNA vs Chromosome
Thus, the age-dependent down-regulation of essential proteins may lead to the decreased ability to segregate non-exchange chromosomes properly in aging oocytes. Very recently, Ghosh et al. The genetic etiology for absence of recombination of nondisjoined Ch21 is not clear. But studies on MLH3 and MLH1 knock out mouse model have suggested some intuitive link with mutant alleles of certain genes that are involved in regulation of meiotic recombination.
In a different study mutant allele of chtf 18 causing impairment in spermatogenesis and defective meiotic recombination with premature chromatid separation in prophase I in male mouse has been documented. Additionally reduction in MLH1 foci has been found in this mouse strain [ 15 ]. These findings are suggestive of probable existence of similar etiology in Ch 21 nondisjunction in human, though such study is yet to be conducted.
Usually a single chiasma at the middle of chromosome arm ensures proper segregation of chromosome at meiosis Figure 2. The US study [ 5 ] revealed the majority of single exchange occurred in the distal 6. In Indian study [ 7 ] the single exchange was scored at terminal 5. These observations suggest that the single telomeric chiasma formation is the risk of NDJ of Ch 21 even in younger women who otherwise do not suffer from deterioration related to the aging.
Two important inferences have been drawn from this finding. The first one is that the single telomeric chiasma formation is maternal age independent risk of Ch21 NDJ.