The importance of noncoding DNA sequences
Since the completion of Human Genome Project in 2003, it was found that more than 98% of human genome is occupied by noncoding DNA sequences (Genome.gov, 2003), existing between genes and as introns within genes (Figure-1). In genomic, noncoding DNA regions are defined as the sequences of an organism’s DNA, which do not code for proteins. Although it has been known for decades that some of these sequences code for noncoding RNAs that involve in controlling many cellular processes, recently study suggested that many of them are in fact vital in regulating how our genome works (The Encode Project, 2004), while some research proposed that others may have no known biological function for …show more content…
According to the observation made during the replication of Drosophila melanogaster chromosomes in 1973, the opening of Drosophila’s DNA double helix, known as replication forks (formerly named as eye forms) at a number of different sites were noted (Kriegstein and Hogness, 1974). The creation of replication forks is the result of the formation of replication bubbles. As shown in Figure-4, replication origins are recognized by the origin recognition complex (ORC) to create a replication bubble in late mitosis and early G1 (Leman and Noguchi, 2013, pp. 5-6). Situated at the ends of the replication bubble are replication forks where initiation of DNA replication occurs allowing the replication to proceed in both directions (Figure-4). Similarly, DNA replication in human cells is also initiated at a number of different replication origins on multiple chromosomes simultaneously so that the duration of S phase of the interphase – about 8 hours (Tortora and Derrickson, 2011, pp.96) - is not limited by the total amount of DNA. The form of DNA replication in human is referred to as semi-conservative replication as each of daughter double helix inherited one of the polynucleotide chains of the parent double (Waston and Crick, 1953; Meselson and Stahl, …show more content…
An American geneticist, Hermann J. Muller, was the first to discover the existence of telomere in 1938. He was irradiating Drosophila melanogaster with X-ray to produce mutation at the Drosophila’s chromosomal ends and he observed no mutants with deletions or inversions during the experiment (Muller, 1938). He suggested the presence of a protective cap, known as telomere, at the chromosomal end. It wasn’t a year later; Barbara McClintock - a cytogeneticist from the University of Cornell - proposed another main function of telomeres. While investigating the behaviour of broken chromosomes in maize plants by studying the cytology of x-rayed plants, she observed that when the breakage occurs on one end of a chromosome, fusion of the broken meiotic chromatids does occur. Such consequence result in anaphase bridge configuration termed as the breakage-fusion-bridge (BFB) cycle to ensure cell death (McClintock, 1939).
Non-functional sequences of noncoding DNA include: 1) transposable elements: Class I (retrotransposons) and Class II (DNA transposons and endogenous retroviruses), 2) highly repetitive DNA, 3) introns, 4) pseudogenes, and 5) conserved sequences (McClintock,
Since the completion of Human Genome Project in 2003, it was found that more than 98% of human genome is occupied by noncoding DNA sequences (Genome.gov, 2003), existing between genes and as introns within genes (Figure-1). In genomic, noncoding DNA regions are defined as the sequences of an organism’s DNA, which do not code for proteins. Although it has been known for decades that some of these sequences code for noncoding RNAs that involve in controlling many cellular processes, recently study suggested that many of them are in fact vital in regulating how our genome works (The Encode Project, 2004), while some research proposed that others may have no known biological function for …show more content…
According to the observation made during the replication of Drosophila melanogaster chromosomes in 1973, the opening of Drosophila’s DNA double helix, known as replication forks (formerly named as eye forms) at a number of different sites were noted (Kriegstein and Hogness, 1974). The creation of replication forks is the result of the formation of replication bubbles. As shown in Figure-4, replication origins are recognized by the origin recognition complex (ORC) to create a replication bubble in late mitosis and early G1 (Leman and Noguchi, 2013, pp. 5-6). Situated at the ends of the replication bubble are replication forks where initiation of DNA replication occurs allowing the replication to proceed in both directions (Figure-4). Similarly, DNA replication in human cells is also initiated at a number of different replication origins on multiple chromosomes simultaneously so that the duration of S phase of the interphase – about 8 hours (Tortora and Derrickson, 2011, pp.96) - is not limited by the total amount of DNA. The form of DNA replication in human is referred to as semi-conservative replication as each of daughter double helix inherited one of the polynucleotide chains of the parent double (Waston and Crick, 1953; Meselson and Stahl, …show more content…
An American geneticist, Hermann J. Muller, was the first to discover the existence of telomere in 1938. He was irradiating Drosophila melanogaster with X-ray to produce mutation at the Drosophila’s chromosomal ends and he observed no mutants with deletions or inversions during the experiment (Muller, 1938). He suggested the presence of a protective cap, known as telomere, at the chromosomal end. It wasn’t a year later; Barbara McClintock - a cytogeneticist from the University of Cornell - proposed another main function of telomeres. While investigating the behaviour of broken chromosomes in maize plants by studying the cytology of x-rayed plants, she observed that when the breakage occurs on one end of a chromosome, fusion of the broken meiotic chromatids does occur. Such consequence result in anaphase bridge configuration termed as the breakage-fusion-bridge (BFB) cycle to ensure cell death (McClintock, 1939).
Non-functional sequences of noncoding DNA include: 1) transposable elements: Class I (retrotransposons) and Class II (DNA transposons and endogenous retroviruses), 2) highly repetitive DNA, 3) introns, 4) pseudogenes, and 5) conserved sequences (McClintock,