Passage
Each time a eukaryotic cell replicates its DNA, the telomeres at the ends of each chromosome become slightly shorter. When telomeres reach a critically short length, the cells enter the senescent state, in which cell division ceases (Figure 1) . In humans, conditions such as hypertension, liver disease, and Alzheimer disease have been associated with the decreased telomere length and resulting senescence that occur as part of the normal aging process.
Figure 1Cells that must be able to divide indefinitely (stem cells and germ cells) express an enzyme known as telomerase. Functional telomerase consists of a protein subunit called telomerase reverse transcriptase (TERT) and a noncoding RNA subunit called telomerase RNA (TR) . Using a portion of TR as a template, telomerase extends telomeres by continuously adding the sequence 5′-TTAGGG-3′ to the ends of chromosomes. The complement strand is then filled in by DNA polymerase. Telomere extension allows these cells to avoid senescence.In healthy somatic cells, however, TERT transcription is generally downregulated by the absence of the oncogene c-Myc and the presence of the tumor suppressor protein WT1. When telomerase is suppressed, the resulting critical telomere shortening has been hypothesized to prevent uncontrolled cell proliferation in adult tissues. In cancerous somatic cells, on the other hand, increased telomerase activity is often associated with tumorigenesis. WT1 is frequently absent and c-Myc is frequently overexpressed in these cells, leading to increased TERT expression.In addition to transcriptional control, TERT can be regulated at the post-transcriptional level. The TERT gene consists of 16 exons and 15 introns that can be spliced into more than 20 different isoforms in humans. Only one isoform (the active isoform) is known to extend telomeres. However, the i2 isoform, in which intron 2 is partially included in the mature mRNA (Figure 2) , has been hypothesized to downregulate residual TERT activity in somatic cells.
Figure 2
Adapted from Cifuentes-rojas C, Shippen DE. Telomerase regulation. Mutat Res. 2012;730(1-2) :20-7.
-Some eukaryotic organisms have multiple TERT genes with high sequence similarity that are expressed at different times in the organism's life cycle. These genes could have arisen by:

Question

Passage
Each time a eukaryotic cell replicates its DNA, the telomeres at the ends of each chromosome become slightly shorter. When telomeres reach a critically short length, the cells enter the senescent state, in which cell division ceases (Figure 1) . In humans, conditions such as hypertension, liver disease, and Alzheimer disease have been associated with the decreased telomere length and resulting senescence that occur as part of the normal aging process.

Figure 1Cells that must be able to divide indefinitely (stem cells and germ cells) express an enzyme known as telomerase. Functional telomerase consists of a protein subunit called telomerase reverse transcriptase (TERT) and a noncoding RNA subunit called telomerase RNA (TR) . Using a portion of TR as a template, telomerase extends telomeres by continuously adding the sequence 5′-TTAGGG-3′ to the ends of chromosomes. The complement strand is then filled in by DNA polymerase. Telomere extension allows these cells to avoid senescence.In healthy somatic cells, however, TERT transcription is generally downregulated by the absence of the oncogene c-Myc and the presence of the tumor suppressor protein WT1. When telomerase is suppressed, the resulting critical telomere shortening has been hypothesized to prevent uncontrolled cell proliferation in adult tissues. In cancerous somatic cells, on the other hand, increased telomerase activity is often associated with tumorigenesis. WT1 is frequently absent and c-Myc is frequently overexpressed in these cells, leading to increased TERT expression.In addition to transcriptional control, TERT can be regulated at the post-transcriptional level. The TERT gene consists of 16 exons and 15 introns that can be spliced into more than 20 different isoforms in humans. Only one isoform (the active isoform) is known to extend telomeres. However, the i2 isoform, in which intron 2 is partially included in the mature mRNA (Figure 2) , has been hypothesized to downregulate residual TERT activity in somatic cells.

Figure 2
Adapted from Cifuentes-rojas C, Shippen DE. Telomerase regulation. Mutat Res. 2012;730(1-2) :20-7.
-Some eukaryotic organisms have multiple TERT genes with high sequence similarity that are expressed at different times in the organism's life cycle. These genes could have arisen by:

Quizplus · Accepted Answer

11ecba2d_0ff8_c9da_a3bf_adc1ec7f3341_MD0008_00 Genes with similar sequences, or high sequence similarity, are considered evolutionarily related. In other words, they are believed to have a common origin in a single gene from an ancestor organism. During the process of molecular evolution, portions of a chromosome may be duplicated by mechanisms such as unequal crossing over. Over time, duplicate genes may undergo mutations, allowing them to carry out distinct roles at various times in an organism's life cycle. The question states that some organisms have multiple TERT genes with high sequence identity. These similar genes most likely arose by gene duplication. (Choice A) Alternate splicing produces multiple protein products from the same gene, not multiple similar genes. (Choice B) Conjugation is the exchange of genetic information between prokaryotes, typically in the form of plasmid DNA. It does not occur in eukaryotes. (Choice D) Transformation occurs when prokaryotes pick up foreign genetic material from their surroundings. Eukaryotes do not participate in transformation. Educational objective: Genes with high sequence identity are evolutionarily related, having a common origin. They generally arise by gene duplication and, over time, may mutate and fulfill distinct roles within an organism.

Passage Each Time a Eukaryotic Cell Replicates Its DNA, the Telomeres