In this chapter, we are talking about Membrane Structure and Function. Give an example and relate that to the BIB (Big Ideas in Biology).
Added by David C.
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One example could be the role of membrane proteins in cell signaling. Membrane proteins act as receptors for signaling molecules, allowing cells to communicate with each other and respond to changes in their environment. This example supports BIB 5: Information Show more…
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Discuss the biological efficiencies gained by organizing protein function on a membrane (hint: one critical point is the membrane serving as an interface between environments). Provide an example for each.
Sri K.
how can you relate the structures and composition of the cell membrane to its function? * own words explanation *
DNA Replication The protein that catalyzes formation of new covalent bonds between nucleotides as new DNA is being made is called DNA polymerase, but the same type of bond is also made by the enzyme ligase when it is needed to join together sections of new DNA such as Okazaki fragments on the lagging strand of the replication fork. The name of the covalent bond between those nucleotides is phosphodiester linkage. Every new replication bubble is formed from sites on the DNA called origins of replication. Each replication bubble has a replication fork at each end that looks a bit like a fork in the road. There, the strands are being separated by an enzyme called helicase. Before new DNA can be made, a short RNA primer must be made by an enzyme called primase. Then, the enzyme that makes new DNA can use the 3' end of this RNA piece to attach the new DNA nucleotides. The leading strand at each replication fork is made much faster because it is made continuously towards the replication fork. The lagging strand at the same fork is made more slowly because it is made away from the fork, so must be made in short fragments, as mentioned in statement 1. DNA replication only occurs during one stage of the cell cycle, called S phase, which is in the middle of interphase, but only occurs if the cell receives a signal indicating cell division is necessary. Gene Expression Each single unit of information in our chromosomes is called a gene. All genes must be transcribed to make RNA. When a mRNA is made from the gene, it must then be translated to make a protein. Other genes contain information for a functional RNA molecule, such as rRNA or tRNA, so do not require this second step. At the start of every gene is a region of DNA called the promoter that signals what type of gene is encoded there and requires the binding of proteins called transcription factors to attract RNA polymerase so it can actually make the RNA. This regulation ensures that cells only make the proteins and RNA molecules they need, rather than wasting energy expressing every gene in the entire genome. In eukaryotes, a newly made mRNA transcript must be modified by addition of a 5' cap, a 3' poly-A tail, and then a process called splicing removes introns and links the remaining exons together to make the mRNA sequence ready for reading during protein synthesis. Once the mRNA gets to the cytoplasm, protein synthesis requires a ribosome to grab the mRNA and initiate the process by binding of an initiator tRNA to the start codon of the mRNA. Each tRNA is folded into a 3-D shape with a binding site for an amino acid at one end and an anticodon sequence that binds to a specific mRNA codon at the other end. By matching a codon with the correct amino acid, these molecules ensure the right protein sequence is assembled. During elongation, amino acids are joined together by the ribosome, which is the giant complex that grabbed and held the mRNA in place. The covalent bond formed between amino acids is called a peptide bond. The genetic code is a chart of codons that cells use to determine the sequence of amino acids, and it contains many different codons that call for amino acids, one codon that initiates the process and is called the start codon, and three different stop codons that signal the end of the protein sequence and cause release of the finished protein so it can finish folding and go do its job. If the DNA contains a mutation that causes the protein made from that gene to differ in one amino acid, that mutation is called a missense mutation. In contrast, if the DNA change causes one of the codons to be changed to a stop codon and the protein made is much shorter than normal, it is called a nonsense mutation.
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