Mitochondrial DNA

The human mitochondrial genome. (Schon et al. 1997) 

As we know, mitochondria are cytoplasmic organelles exclusive for eukaryotes. They have the central role of oxidative energy metabolism to the viability of the cell. Mitochondria contain a small specialized genome and a complete process of gene expression for their particular activities, distinct from that of the cell nucleus and cytosol. In humans, the mitochondrial genome is constituted by a circle of double-stranded DNA with 16, 569 base pairs. This circle DNA is highly compact and contains only 37 genes: 2 genes encode ribosomal RNAs, 22 encode transfer RNAs, and 13 encode polypeptides. The 13 polypeptides are components of the respiratory chain, the oxidative phosphorylation system (Schon et al. 1997).

           The female egg contains a cytoplasm with many mitochondria and other organelles. Besides the nuclear DNA, it has the entire mitochondrial DNA, which has the same genetic information for all mitochondria. The sperm has mitochondria placed where they can most efficiently control the flagellum, providing energy for their movement. In the fertilization, the only genetic material that the embryo inherits from the male is from the nuclear genetic material contained in the head’s sperm. Therefore, the individual inherits all the genetic mitochondrial material from the female, contained in the egg. In resume, we have the same genetic mitochondrial material and mitochondrial phenotype as our mother.

               Studies in evolutionary biology analyze mitochondrial inheritance to identify the degree of proximity between populations and species. Allen and de Paula (2013) also suggest that the contributions of male/female for the mitochondria arose with the evolutionary origin of separate sexes.  This knowledge is also useful in cases whereby a child mother is not known, and then tests with mitochondrial DNA may be done to prove if a woman is the birth mother. Another importance is observed in studies about human disorders. Pathogenic mutations in the mitochondrial genome can have devastating consequences; one of these is respiratory chain deficiency. 

REFERENCES

Allen J. F. & de Paula W. B. M. (2013). Mitochondrial genome function and maternal inheritance. Biochemical Society Transactions 41(5) 1298-1304.

Schon, E. A.; Bonilla, E; DiMauro S. (1997). Mitochondrial DNA Mutations and Pathogenesis. Journal of Bioenergetics and Biomembranes,  29(2) 131-149.

Chromosomal alterations and human disorders

                On the last post I talked about the relevance of chromosomal alterations for evolution. Therefore, I am going to explain a little more about the effects of these events on the phenotype of humans. As I said before, modifications on chromosome structure, duplication or deletion of entire or parts of chromosomes, can alter the genome, changing the genetic sequence or/and the normal way of gene expression. Chromosomal abnormalities in humans usually lead to patterns known as syndromes, combinations of signs and symptoms (Haydon, 2008).

            As I have mentioned, there are numerical and structural chromosomal alterations. The numerical ones are known as aneuploidy in syndromes. They occur as a result from meiosis error that originates gametes without any chromosome of a chromosome pair and gametes with the both 2 chromosomes. When these gametes fertilize they provide individuals with monosomy (1 chromosome) or trisomy (3 chromosomes) for that pair (Haydon, 2008).

            The most recognized autosomal aneuploidy is the Down syndrome, or trisomy 21 (Some symptoms at the figure above). Despite individuals with this syndrome have developmental delay and heart defect, they usually live well into adult life (Haydon, 2008). Klinefelter syndrome (47, XXY) is the most common sexual trisomy, whereby men have tall stature, small testes, scant body hair and infertility (Haydon, 2008). The only monosomy compatible to life is sexual: the Turner syndrome (45, X). The symptoms are short stature and infertility. Despite, women 45, X usually have a normal life, I believe this is due to the natural activation of only one X in women, which I have explained in previous posts.

            Structural chromosomal alterations are result of breakage and subsequent reunion of chromosome regions. We can have deletions of regions, losing material, or duplication by gaining a copy of a segment at the original location on the chromosome (Theisen and Shaffer, 2010). Also, it is possible to have rearrangements such as inversions of segments after two-break events; translocations by exchange of segments between chromosomes; insertions by translocation and insertion of segments into a new region of the same or other chromosome. These events are pathogenic when they disrupt important genes (Theisen and Shaffer, 2010). An example of deletion is found at the cri du chat syndrome, in chromosome 5. Symptoms include microcephaly, low-set ears, round face, high-pitched cat-like cry, mental retardation and some health problems(Theisen and Shaffer, 2010).

I would like to talk about more syndromes but there are so many for a single post.

Thanks for reading!

REFERENCE LIST:

Theisen, A & Shaffer L G (2010). Disorders caused by chromosome abnormalities. The Application of Clinical Genetics. 2010: 3; 159–174

Haydon, J (2008). Chromosome disorders. In: Genetics in Practice: A Clinical Approach for Healthcare Practitioners (ed. Wiley). pp. 85-100. Hoboken, NJ, USA.

Image source (19/04/2014): http://www.rayur.com/chromosomal-disorder-down-syndrome-trisomy-21.html

Chromosomes Alterations and Evolution

As we know, chromosomes alterations in human being have usually negative effects, causing diseases, sterile individuals or even death.  However, many species not only survive with these alterations but they can evolve from them. These alterations are a type of mutation, which is source of changes in the genome, leading to genome evolution (Reece et al., 2011). I going to explain how some types of chromosome alterations contribute to the evolution: Polyploidy (widely common in plants and rare in animals) and alterations of chromosome structure.

             Duplication of entire chromosome sets is result of accidents in meiosis that produce diploid gametes. According to Freeman and Herron (2004), if an individual self-fertilize its produced male and female diploid gametes, it will originate a polyploid organism. If this organism can self-fertilize or mate with another poplyploid, it is established a tetraploid population.  This event is important in evolution because if the tetraploid population mates with their parental diploid population, they will originate triploid organisms, which are sterile due to the odd number of chromosome set. Therefore, this leads to branching off of a new species. Also, the extra sets can accumulate mutations that promote new functions of the extra genes, maintained by natural selection (Freeman and Herron, 2004). These events change the individual phenotype and the new population diverges further from their parental population.

          Another contribution to evolution is observed in alterations of chromosome structure. Researchers have noticed chromosome rearrangements comparing a species to another. For example, ancestral chimp chromosomes 12 and 13 seem to have fused end to end, forming the chromosome 2 of a human ancestor. Comparing some species, scientists find many duplications and inversions of large portions of chromosomes, as result of DNA breaks and incorrect rejoining during meiotic recombination. These events might have accelerated about 100 million years ago, contributing to the generation of new species (Reece et al., 2011).

REFERENCE LIST:

Freeman, S. & Herron, J.C. (2004) Evolutionary Analysis. 3th ed. Pearson Prentice Hall. United States of America. p 124.

Reece, J. B.; Urry, L. A.; Cain, M. L.; Wasserman, S. A.; Minorsky, P. V.; Jackson, R. N.(2011) Campbell Biology. 9^th ed. Pearson Australia Group. p 446.