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Monozygotic (identical) twins develop from a single egg fertilized by a single sperm that divides and gives rise to two zygotes. Thus, monozygotic twins are genetically identical, in the sense that they possess identical DNA sequences, but they often differ somewhat in appearance, health, and behavior. The nature of these differences in the phenotypes of identical twins is not well understood, but recent evidence suggests that at least some of these differences may be due to epigenetic changes. In one study, Mario Fraga, at the Spanish National Cancer Center, and his colleagues examined 80 pairs of identical twins and compared the degree and location of their DNA methylation and histone acetylation. They found that DNA methylation and histone acetylation in identical twin pairs were similar early in life, but that older twin pairs had remarkable differences in their overall content and distribution of DNA methylation and histone acetylation. Furthermore, these differences affected gene expression in the twins. This research suggests that identical twins do differ epigenetically and that phenotypic differences between them may be caused by differential gene expression.

Early in the development of female mammals, one X chromosome in each cell is randomly inactivated to provide an equal expression of X-linked genes in males and females. Through this process, termed X inactivation, many genes on the inactivated X chromosome are permanently silenced and are not transcribed. Once a particular X chromosome is inactivated in a cell, that same X chromosome remains inactivated when the DNA is replicated, and the inactivation mark is passed on to daughter cells through mitosis. This phenomenon is responsible for the patchy distribution of black and orange pigment seen in tortoiseshell cats. X inactivation is a type of epigenetic effect because it results in a stable change in gene expression that is passed on to other cells.

In plants, DNA methylation also occurs at CpNpG trinucleotides, where N represents a nucleotide with any base. Some DNA regions have many CpG dinucleotides and are referred to as CpG islands. In mammalian cells, CpG islands are often located in or near the promoters of genes. These CpG islands are usually not methylated when genes are being actively transcribed. However, methylation of CpG islands near a gene leads to repression of transcription. Cells repress and activate genes by methylating and demethylating cytosine bases. Enzymes called DNA methyltransferases to methylate DNA by adding methyl groups to cytosine bases to create 5-methylcytosine. Other enzymes, called demethylases, remove methyl groups, converting 5-methylcytosine back to cytosine.

In 2003, researchers declared that essentially the entire human genome had been sequenced. This monumental achievement provided a wealth of information about how genetic information is encoded within the genome. Yet the DNA base sequence is only a partial record of heritable information. As we have discussed, additional epigenetic information is contained within the chromatin structure—information that is heritable and affects how the DNA base sequence is expressed. The overall pattern of chromatin modifications in a genome has been termed the epigenome. Over the past few years, techniques have become available for detecting and describing epigenetic modifications across the genome.

The term epigenetics was first used by Conrad Waddington in 1942 to describe how, through the process of development, a genotype produces a phenotype. In coining the term, Waddington combined the words epigenesis, the development of an embryo, with genetics, the study of genes and heredity. Waddington’s goal was to encourage the merging of genetics and development. However, his use of the term preceded our modern understanding of DNA and chromosome structure, and today, epigenetics has taken on a narrower meaning.

The amount of a protein that is synthesized depends on the amount of the corresponding mRNA that is available for translation. The amount of available mRNA, in turn, depends on both the rate of mRNA synthesis and the rate of mRNA degradation. Eukaryotic mRNAs are generally more stable than bacterial mRNAs, which typically last only a few minutes before being degraded. Nonetheless, there is great variation in the stability of eukaryotic mRNAs: some persist for only a few minutes, whereas others last for hours, days, or even months. These variations can produce large differences in the amount of protein that is synthesized. Cellular RNA is degraded by ribonucleases, enzymes that specifically break down RNA. Most eukaryotic cells contain 10 or more types of ribonucleases, and there are several different pathways of mRNA degradation. In one pathway, the 5′ cap is first removed, followed by 5′→3′ removal of nucleotides. A second pathway begins at the 3′ end of the mRNA and removes nucleotides in the 3′→5′ direction. In a third pathway, the mRNA is cleaved at internal sites.

In bacteria, transcription, and translation take place simultaneously. In eukaryotes, transcription takes place in the nucleus, and the pre-mRNAs then undergo processing before being moved to the cytoplasm for translation, which allows opportunities for gene control after transcription. Consequently, posttranscriptional gene regulation assumes an important role in eukaryotic cells. RNA processing and degradation is a common level of gene regulation in eukaryotes.

Birds have a special organ called the syrinx, that allow them to sing the way they sound. It is located on the trachea to produce sound. It is similar to the mammalian larynx but the way air is used is different. In the syrinx, 100% of the air is converted to sound where the larynx only uses 2%. The vibrated air passes through the syringeal passageway to project on the tympaniform membrane. This vibration is the result of how birds can vocalize. The syrinx is a complex organ itself but the muscle attached around it is also complex. There are layers of muscle structures to create fine adjustment of vibration. The sound produced by the syrinx can be filtered to change the loudness and the pitch. The experiment was done to prove that sound travels faster than helium atmosphere showed sound produced in such atmosphere had different pitch and frequency than the sound produced in our normal atmosphere. The understanding of syrinx and physics of sound helped scientists to learn more about vocalization.

The songs produced by each bird are diverse. Some songs can be soft or high, and others can have short or long notes. Frequency and amplitudes can be studied by creating a graph to visually study the sound of the birds. The higher the amplitude, the louder the song produced by the bird.  The difference between a song and a call is that songs have patterns, syllables, and phrases whereas call is short and simple. Calls are used for multiple situations for defense, conversation, also to attract mates. Most songs are similar to human songs. Each bird has its unique type of calls and vocalization to attract mates. The tempo and frequency are what attract neighbors and predators. Sound travels much further than distance, by using this physical property, birds can communicate with a larger range to other birds. Not only the vocalization produced by the bird itself but also the environment can affect how the bird is heard by other individuals. Some birds like Great Tits that live in the urban area adapted by a vocalization that will not be erased from the traffic noise caused by human activities and other loudness.

The amino acids in proteins are joined together by peptide bonds to form polypeptide chains; a protein consists of one or more polypeptide chains. Like nucleic acids, polypeptides have polarity under physiological conditions: one end (often called the amino end) has a free amino group (NH3+) and the other end (the carboxyl end) has a free carboxyl group (COO−). Proteins consist of 50 or more amino acids; some have as many as several thousand.