Another important component of transcription is a molecular complex called mediator, which binds directly to Pol II and is required for the transcription initiation complex TIC to operate. Additional non coding areas of the gene can act to increase or decrease the activity of the proximal promoter. They are known as enhancer or repressor elements.
The mediator complex has binding sites for many transcription factors that bind specifically to enhancer and repressor elements of the gene. Binding to these regulatory transcription factors appears to alter the shape of the mediator, which can in turn radically alter the activity of the TIC. The regulatory sequences present in the gene, and the specific transcription factors present in the cell, comprise yet another code that allows different cells to produce different proteins, and to produce different proteins at different times.
Another form of gene regulation is controlled by DNA modifications that can either expose DNA for transcription or make it unavailable. Addition of too many methyl groups to DNA can render it incapable of binding transcription factors, or can attract DNA silencing proteins. Acetylation or methylation of histones, proteins around which DNA can coil, can similarly influence the accessibility of genes for transcription.
Silencing of genes by these epigenetic mechanisms is an important form of long-term gene regulation. The mediator complex can also alter histone architecture, providing an additional regulatory mechanism Figure 2.
Figure 2. Gene transcription and regulation. The mediator complex connects the TIC to regulatory transcription factors TFs that are bound to enhancer or repressor DNA elements, which can be distant from the promoter.
The particular TFs that bind to the mediator help to regulate in which cells genes are transcribed, as well as the time and rate of transcription. Figure adapted from Nature Education, Gene expression. Protein production can also be regulated by genes that do not themselves encode proteins.
Production of these microRNAs is another important mechanism of gene control. DNA replication is required so that each cell within an organism possesses the same genetic code. Starting from a single fertilised egg, billons of cell divisions are required to form an individual. The genetic code is also the basis for reproduction. Passing genetic information from one generation to the next ensures that the traits of the parents are passed faithfully to their offspring. In the sex organs, cell division occurs in such a manner that only one chromosome from each pair is passed on to a gamete, which is either an egg or sperm.
During fertilisation, these gametes combine to produce a single cell with the normal number of chromosomes. Thus the genetic code of the offspring gets equal contributions from each parent. Which of the two chromosomes from each pair that a parent passes on to an individual gamete is random, so there are many possible combinations of genes that can come from any set of parents.
As noted above, replication of DNA can result in mutations. If this occurs in divisions leading to a gamete, the defect in DNA can be passed on to the offspring. Mutations in our DNA can have many consequences, from no effect whatsoever to devastating changes in health, or even death. Finally, the DNA double helix has the strong sugar-phosphate backbone on the outside with the bases on the inside, making the molecule stable and protecting the vital coding bases from damage.
Describe the structure of a DNA molecule and explain how this relates to its function. Answered by Emily O. Need help with Biology? One to one online tuition can be a great way to brush up on your Biology knowledge. Answered by Charlie H. Answered by Fazna R. Answered by Matthew A. Telomeres are stretches of repetitive DNA sequences that are found at the ends of your chromosomes.
Telomere shortening has been associated with the aging process. Perhaps making healthy lifestyle choices like maintaining a healthy weight , managing stress , and not smoking can slow telomere shortening? This question continues to be of great interest to researchers. The DNA molecule is made up of nucleotides.
Each nucleotide contains three different components — a sugar, a phosphate group, and a nitrogen base. Each sugar in a nucleotide has a nitrogen base attached to it. There are four different types of nitrogen bases found in DNA.
They include:. The two strands of DNA form a 3-D structure called a double helix. In a prokaryotic cell, the DNA forms a circular structure. DNA contains the instructions that are necessary for an organism — you, a bird, or a plant for example — to grow, develop, and reproduce.
These instructions are stored within the sequence of nucleotide base pairs. Your cells read this code three bases at a time in order to generate proteins that are essential for growth and survival. The DNA sequence that houses the information to make a protein is called a gene. Each group of three bases corresponds to specific amino acids , which are the building blocks of proteins.
For example, the base pairs T-G-G specify the amino acid tryptophan while the base pairs G-G-C specify the amino acid glycine. This tells the cell not to add any more amino acids to the protein. Proteins are made up of different combinations of amino acids. When placed together in the correct order, each protein has a unique structure and function within your body. But what happens in between? Simply put, this occurs via a two-step process:. First, the two DNA strands split apart.
Then, special proteins within the nucleus read the base pairs on a DNA strand to create an intermediate messenger molecule. It travels outside of the nucleus, serving as a message to the cellular machinery that builds proteins. This process is called translation. There are two types of cell — eukaryotic and prokaryotic. Humans and many other organisms have eukaryotic cells. This means that their cells have a membrane-bound nucleus and several other membrane-bound structures called organelles.
In a eukaryotic cell, DNA is within the nucleus. A small amount of DNA is also found in organelles called mitochondria, which are the powerhouses of the cell. There are several different stages of packaging, however the final products are the structures that we call chromosomes. Organisms like bacteria are prokaryotic cells.
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