Chapter 2 Flashcards
Protein
-a molecule composed of one or more long chains of amino acids occurring in a specific sequence or order.
-This very specific sequence is coded for by the sequence of DNA within the gene for that protein.
-Thus, the purpose of a gene is to provide directions for assembling (synthesizing) a very specific protein.
-All hormones, enzymes, growth facrors, and other protein-based chemicals needed for normal human physiologic function are protein gene products that are produced when the correct genes are activated and expressed.
Protein synthesis
-the selective activation of a gene, eventually resulting in the production of me appropriate protein.
-For this reason, proteins are called gene products.
-Each gene provides me code for making one specific protein.
Gene expression
The activation of a gene, leading to its transcription and translation and, ultimately to the synthesis of a specific protein.
Protein synthesis
Transcription and translation
Figure 2-1
Structure of a protein
-The basic structure of a protein is its amino acid sequence. -The 20 different amino acids are commonly called the building blocks of life.
- the sequencing order of me amino acids is what makes one protein different in structure and function from anomer protein. If one amino acid is out of order or completely deleted from the sequence, thr protein will be affected and may not perform its function well.
-the order of the amino acids is critical for the final function of any protein, and even one amino acid change can alter the protein’s function.
Table DNA Triplets and RNA Codons for the 20 Amino Acids
DNA triplets
-Each amino acid has at least one specific code within the DNA
-these codes are each three nucleotide bases long and are called DNA triplets.
-For example, the final active form of beta globin has 146 amino acids. Thus, the minimum number of bases needed in the gene for beta globin is 438 (3 bases per amino acid multiplied by 146 amino acids).
-[The gene for beta globin (the HBB gene) is located on the short arm (p arm) of chromosome II. The synthesis of beta globin occurs only in immature RBCs, although the HBB gene is present in the nucleus of every cell. This means that the HBB gene is part of every cell’s genome, and beta globin is part of the cellular proteome for RBCs. Synthesis of beta globin, just like for any protein, involves the processes of transcription, translation, and protein modification.]
Transcription
-Transcription is the process of making a strand of ribonucleic acid (RNA) that contains the same amino acid codes as the DNA sequence of the gene for the protein needed.
-This phase of protein synthesis takes place completely within the nucleus.
DNA coding regions
-Examining DNA reveals DNA coding regions separated by noncoding regions.
-DNA coding regions contain many genes, and the sequences of these genes are largely the same from one person to another.
-For example, the gene for insulin has the same DNA sequence in all healthy humans.
DNA noncoding regions
- sections of DNA that contain multiple repeat sequences that are not genes or parts of genes and that do not code for specific proteins.
-These noncoding regions, sometimes called redundant DNA or desert DNA, make up about 95% of nuclear DNA.
-These regions vary from one person to another and are used to identify the DNA from a specific individual.
-The noncoding regions of DNA influence how genes are expressed, but not all of their functions are yet understood.
Difference between DNA synthesis and protein synthesis
-During DNA replication, both double strands of all the DNA within one cell are entirely copied, resulting in two new strands of DNA.
- Key difference: Extent of the process.
- DNA replication copies both double strands resulting in two new complete strands.
- Protein synthesis only involves the gene-containing DNA segment.
- A segment of one DNA strand is transcribed into RNA.
Protein Synthesis Process
-In protein synthesis, only the area of DNA that contains the actual “recipe” for the protein is read (transcribed), and a complementary strand of RNA is synthesized.
-RNA is a single strand of nitrogenous bases constructed during transcription from a segment of DNA containing the template for a specific protein.
-Several types of RNA exist, and the ultimate purpose of all types is to ensure that the information held in the genes reaches cell areas where formation of the actual proteins needed for normal human funcrion can occur.
-In this sense, RNA is a molecular interpreter of the DNA information stored in the genes.
-Newly transcribed RNA functions as the initial pattern for protein synthesis.
RNA and DNA differences
-RNA is very similar to DNA, with a few differences.
-First, functional RNA is single stranded (ss) rather than double stranded (ds).
-The sugar component of RNA is ribose rather than deoxyribose, which just means that it contains one more oxygen molecule than does the sugar in DNA.
-Another difference is that RNA does not contain the pyrimidine base thymine.
The base uracil is used in place of thymine.
-It is a pyrimidine base almost identical to thymine except that uracil does not contain the methyl group (CH3) that thymine has (Fig. 2-3).
-However, this difference is important because molecules in the nucleus that contain a methyl group remain trapped inside the nucleus.
-Because the remaining phases of protein synthesis occur outside the nucleus, the newly transcribed RNA must be able to exit the nucleus.
Sense Versus Antisense
-Some of the same enzymes involved in DNA synthesis assist in the loosening and unwinding of the DNA coding region that contains the beta globin gene.
-Once this DNA is loosened and unwound, the two strands are slightly separated into a sense strand and an anti sense strand (Fig.2-5).
-After the strands are slightly separated, an enzyme known as RNA polymerase II reads and then transcribes the gene sequence on the anti sense strand of DNA (template strand), resulting in the formation of a complementary strand of RNA that will have exactly the same amino acid codes as the gene in the sense strand.
-Thus, the antisense strand is the “template” used to direct RNA synthesis.
-The DNA information is transcribed using the bases adenine, guanine, cytosine, and uracil into a single RNA strand that amino acid codes identical to the gene (see Fig. 2-5).
-Because this RNA is used as a recipe to direct the building of the actual protein coded for by the gene, it is known as messengerRNA, or mRNA.
-The instructions in the gene have now been converted into RNA codons by the process of transcription of the template (antisense) strand (Fig. 2-6).
DNA sense strand
-The DNA sense strand, also known as sense DNA, contains the actual gene-coding sequence for the protein to be synthesized
DNA antisense strand
-The DNA antisense strand, also known as antisense DNA, contains the complementary base sequence to this gene, not the gene itself.
Starting and Stopping
- DNA contains start and stop codes for RNA synthesis.
- Start signal is upstream from gene triplets that code for specific amino acid.
- It can be over 100 base pairs upstream.
- These regions are known as promoter regions for RNA synthesis (term is different when ref. to cancer .
- A common promoter sequence is the TATA box (cont. many thymine and adenine).
- downstream from the gene, there are transcription stop signals.
- They halt DNA-to-RNA codon transcription.
- A poly-A tail is added to the RNA, known as polyadenylation.
- Poly-A tail has mainly adenine and isn’t part of the protein.
Codon
-A codon is a specific RNA base sequence containing the complementary code to each amino acid’s DNA triplet.
-For example, the DNA triplet for the amino acid methionine is TAC-thymine, adenine, and cytosine (see Table 2-1).
-Remember that RNA contains uracil instead of thymine. Thus, everywhere an adenine is positioned in the gene’s DNA, a uracil is positioned in the complementary strand of RNA.
-This makes the RNA codon for methionine AUG-adenine. uracil. and guanine.
Exons
The sectional parts of the gene that actually belong in the gene are known as exons (for expressed sequences).
Introns
The additional sequences that do not code for part of that protein are introns (for interoening sequences).
Posttranscriptional Modification
-Once the gene has been initially transcribed into mRNA, the RNA must be further processed to its mature form through posttranscriptional modification.
-This is a process that eliminates the introns before the mRNA can be translated and used to direct the precise synthesis of the protein coded for by the gene (Fig. 2-7).
-Removing the introns and conneccing the exons is known as RNA splicing.
-After the initial mRNA transcript has been processed and the introns eliminated, the mature mRNA is moved out from the cell nucleus into the cytoplasm, where actual translation into a protein occurs.