Ch3

Deoxyribose

Thymine

Adenine

Thymine

Adenine

Deoxyribose

Deoxyribose

^Deoxyribose

Cytosine

Guanine

FIGURE 2 Complementary base pairing by the formation of hydrogen bonds between thymine and adenine and between cytosine and guanine. RNA contains uracil in place of the thymine found in DNA. Uracil and thymine differ in structure only by the presence of the methyl group (CH3) found in thymine.

^Deoxyribose

Cytosine

Guanine

FIGURE 2 Complementary base pairing by the formation of hydrogen bonds between thymine and adenine and between cytosine and guanine. RNA contains uracil in place of the thymine found in DNA. Uracil and thymine differ in structure only by the presence of the methyl group (CH3) found in thymine.

the sometimes very long stretches of DNA that lie between the regulatory sequences. The promoter is the particular nucleotide sequence located upstream of the 30 end of the transcribed portion that determines where the enzymatic machinery attaches and in which direction it will face. To initiate transcription, a multimolecular complex of nuclear proteins called general transcription factors and RNA polymerase II must be assembled on the promoter. Assembly begins with the binding of one of these transcription factors (called TFIID) to a particular nucleotide sequence, usually TATA, in the promoter. Sequential binding of other transcription factors aligns the RNA polymerase on the promoter, after which phosphorylation of the polymerase launches its movement along the DNA template as it assembles the RNA transcript. Access of the general transcription complex to a promoter generally requires loosening of the tightly coiled DNA strand and is accomplished in part by acetylation of nucleosomal histones. The rate of expression of a particular gene is controlled not only by proteins that bind to DNA sequences in the promoter, but also by proteins that bind to other sequences in more distant regulatory regions of DNA. Regulatory proteins are transcription factors that may act as enhancers or repressors of gene expression (Fig. 3).

There are hundreds of such proteins, which are expressed in different combinations in different cell types and at different times in their life cycles. In many cases, genes for regulatory proteins are themselves targeted for regulation.

RNA Processing

Genetic information is stored in the nucleus, but proteins are synthesized in the cytoplasm. The RNA transcript that emerges from the nucleus to direct synthesis of one or more proteins is called messenger RNA (mRNA), and the process of converting information carried by nucleic acids to proteins is called translation. Each amino acid encoded in mRNA is specified by a codon of three adjacent nucleotides. For example, alanine is encoded as CGA, while serine is specified as AGC. Not all of the nucleotides in the transcribed part of the gene have counterparts in mRNA. This is because the nucleotides that specify portions of the protein product or untranslated regulatory components of mRNA are interrupted by long stretches of DNA that have no known coding function. The portions of the gene that have complementary sequences in mRNA are called exons and the intervening stretches of DNA are called introns (see Fig. 3). The portions of RNA transcribed from introns are excised from the primary RNA transcript and destroyed. The remaining fragments representing the exons are spliced together by a complex apparatus of RNA and protein called a spliceosome to produce the mRNA. In processing the RNA transcribed from some genes, the spliceosomes may sometimes skip over stretches of RNA transcribed from one or more exons with the result that alternately spliced mRNAs are formed. This is one way that a single gene can give rise to several different proteins (Fig. 4).

Translation

Translation of mRNA requires two additional forms of RNA: ribosomal RNA and transfer RNA (tRNA). Each tRNA molecule reacts at one end with a specific amino acid, and at its other end it contains a triplet of nucleotides that are complementary to the codon in mRNA that specifies that amino acid. In this way, complementary base pairing between the tRNA and the mRNA lines up the amino acids in the appropriate sequence. The ribosomes are comprised of a large and a smaller subunit and are complexes of RNA and many different proteins. The ribosomes hold the reaction components together in the correct orientation and catalyze peptide bond formation. Initiation of protein synthesis requires a complex of regulatory and catalytic

2. Control of Cell Function

Transcription General factors and Transcription

Transcription General factors and Transcription

Potential Health Therapy

FIGURE 3 Transcription and RNA processing. The DNA strand contains all of the stored information for expression of the gene including the promoter, distant regulatory elements (not shown), binding sites (response elements) for regulatory proteins, and the coding for the sequence of the protein (exons) interrupted by intervening sequences of DNA (introns). Exons are numbered 1-5. The primary RNA transcript contains the complementary sequence of bases coupled to a poly A tail at the 30 end and a methyl guanosine cap at the 50 end. Removal of the introns and splicing the remaining exons together produce the messenger RNA (mRNA) that contains all of the information needed for translation including the codons for the amino acid sequence of the protein and untranslated regulatory sequences at both ends.

Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

One of the main home remedies that you need to follow to prevent gallstones is a healthy lifestyle. You need to maintain a healthy body weight to prevent gallstones. The following are the best home remedies that will help you to treat and prevent gallstones.

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