Chapter 4 Part I DNA Biology & Cell Division.pdf

8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A 4 DNA BIOLOGY AND CELL DIVISION: PART 1 Figure 4.1 Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellul...

8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A 4 DNA BIOLOGY AND CELL DIVISION: PART 1 Figure 4.1 Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters; credit b: modification of work by Ken Cole, USGS; credit c: modification of work by Martin Pettitt) 0 CHAPTER OBJECTIVES After studying this chapter, you will be able to: Explain how the genetic code stored within DNA determines the protein that will form Describe the process of transcription Describe the process of translation Discuss the function of ribosomes List the stages of the cell cycle in order, including the steps of cell division in somatic cells List the stages of nuclear division that forms haploid cells, which is called meiosis. Describe the process of gametogensis chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 1/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A genetically identical The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resembles its parent or parents. Hippopotamuses give birth to hippopotamus calves; Monterey pine trees produce seeds from which Monterey pine seedlings emerge; and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same. While many singlecelled organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitotic cell division, many single-celled organisms and most multicellular organisms reproduce regularly using another method. Sexual reproduction is the production by parents of haploid cells and the fusion of a haploid cell from each parent to form a single, unique diploid cell. In multicellular organisms, the new diploid cell will then undergo mitotic cell divisions to develop into an adult organism. A type of cell division called meiosis leads to the haploid cells that are part of the sexual reproductive cycle. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms can or must employ some form of meiosis and mitotic diploid cell fertilization to reproduce. 88 4.1 Organization of the Nucleus and its DNA and i t.EEtid Like most other cellular organelles, the nucleus is surrounded by a membrane called the nuclear envelope. This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A nuclear pore is a tiny passageway for the passage of proteins, RNA, and solutes between the nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into and out of the nucleus. Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There also can be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The rRNA nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes. Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores. chromatin DNA histone rRNA The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins called histones (Figure 4.2). A nucleosome is a single, wrapped DNA-histone complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace, in which the string is the DNA and the beads are the associated histones. When a cell is preparing for division, the chromatin condenses into chromosomes, so that the DNA can be safely transported to the “daughter cells.” The chromosome is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes. DNA histone wrapped together chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# I 2/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A DNA risk DNA DNA Hiiiii.am a 3 Figure 4.2 DNA Macrostructure Strands of DNA are wrapped around supporting histones forming the folded threads of chromatin. Chromatin is condesed and packed tightly into chromosomes when the cell is ready to divide. Video 4.1 DNA Structure 4.2 DNA Replication E In order for an organism to grow, develop, and maintain its health, cells must reproduce themselves by dividing to produce two new daughter cells, each with the full complement of DNA as found in the original cell. Billions of new cells are produced in an adult human every day. Only very few cell types in the body do not divide, including nerve cells, skeletal muscle fibers, and cardiac muscle cells. The division time of remain f finterphaseccellcyclet chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 3/18 frequent division 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A different cell types varies. Epithelial cells of the skin and gastrointestinal lining, for instance, divide very frequently to replace those that are constantly being rubbed off of the surface by friction. A DNA molecule is made of two strands that “complement” each other in the sense that the molecules that compose the strands fit together and bind to each other, creating a double-stranded molecule that looks much like a long, twisted ladder. Each side rail of the DNA ladder is composed of alternating sugar and phosphate groups (Figure 4.3). The two sides of the ladder are not identical, but are complementary. These two backbones are bonded to each other across pairs of protruding bases by hydrogen bonds. The four DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Because of their shape and charge, the two bases that compose a pair always bond together. Adenine always binds with thymine, and cytosine always binds with guanine. The particular sequence of bases along the DNA molecule determines the genetic code. Therefore, if the two complementary strands of DNA were pulled apart, you could infer the order of the bases in one strand from the bases in the other, complementary strand. For example, if one ÑFÉAcGGA strand has a region with the sequence AGTGCCT, then the sequence of the complementary strand would be TCACGGA. d g protein synthesis DNA cosines DNArefit DNA RNA Transcription RNA ProteinTranslation Figure 4.3 Molecular Structure of DNA The DNA double helix is composed of two complementary strands. The strands are bonded together via their nitrogenous base pairs using hydrogen bonds. chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 4/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A DNA replication is the copying of DNA that occurs before cell division can take place. After a great deal of debate and experimentation, the general method of DNA replication was deduced in 1958 by two scientists in California, Matthew Meselson and Franklin Stahl. This method is illustrated in Figure 4.4, video 4.2 and described below. Figure 4.4 DNA Replication DNA replication faithfully duplicates the entire genome of the cell. During DNA replication, a number of different enzymes work helicase together to pull apart the two strands so each strand can be used as a template to synthesize new complementary strands. The two new daughter DNA molecules each contain one pre-existing strand and one newly synthesized strand. Thus, DNA replication is said to be “semiconservative.” welica Ʃ Stage 1: Initiation. The two complementary strands are separated, much like unzipping a zipper. Special enzymes, including helicase, untwist and separate permanently the two strands of DNA. eat we Stage 2: Elongation. Each strand becomes a template along which a new complementary strand is built. DNA polymerase brings in the correct bases to complement the template strand, synthesizing a new strand base by base. A DNA polymerase is an enzyme that adds free nucleotides to the end of a chain of DNA, making a new double strand. This growing strand continues to be built until it has fully complemented the template strand. Stage 3: Termination. Once the two original strands are bound to their own, finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete. Each new DNA molecule contains one strand from the original molecule and one newly synthesized strand. The term for this mode of replication is “semiconservative,” because half of the original DNA molecule is chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 5/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A conserved in each new DNA molecule. This process continues until the cell’s entire genome, the entire complement of an organism’s DNA, is replicated. As you might imagine, it is very important that DNA replication take place precisely so that new cells in the body contain the exact same genetic material as their parent cells. Mistakes made during DNA replication, such as the accidental addition of an inappropriate nucleotide, have the potential to render a gene dysfunctional or useless. Fortunately, there are mechanisms in place to minimize such mistakes. A DNA proofreading process enlists the help of special enzymes that scan the newly synthesized molecule for mistakes and corrects them. If a mistake is not corrected it can result in a mutation which is a permanenet change in the DNA sequence. The mechanism involves detecting errors, cutting out the damaged or incorrect nucleotide base and reconnecting the DNA backbone. Once the process of DNA replication is complete, the cell is ready to divide. You will explore the process of cell division later in the chapter. Video 4.2 DNA Replication It was mentioned earlier that DNA provides a “blueprint” for the cell structure and physiology. This refers to the fact that DNA contains the information necessary for the cell to build one very important type of molecule: the protein. Most structural components of the cell are made up, at least in part, by proteins and virtually all the functions that a cell carries out are completed with the help of proteins. One of the most important classes of proteins are enzymes, which help speed up necessary biochemical reactions that take place inside the cell. Some of these critical biochemical reactions include building larger molecules from smaller components (such as occurs during DNA replication or synthesis of microtubules) and breaking down larger molecules into smaller components (such as when harvesting chemical energy from nutrient molecules). Whatever the cellular process may be, it is almost sure to involve proteins. Just as the cell’s genome describes its full complement of DNA, a cell’s proteome is its full complement of proteins. Protein synthesis begins with genes. A gene is a functional segment of DNA that provides the genetic information FILE necessary to build a protein. Each particular gene provides the code necessary to construct a particular protein. Gene expression, which transforms the information coded in a gene to a final gene product, ultimately dictates the structure and function of a cell by determining which proteins are made. chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 6/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A The interpretation of genes works in the following way. Recall that proteins are polymers, or chains, of many amino acid building blocks. The sequence of bases in a gene (that is, its sequence of A, T, C, G nucleotides) translates to an amino acid sequence. A triplet is a section of three DNA bases in a row that codes for a specific amino acid. For example, the DNA triplet CAC (cytosine, adenine, and cytosine) specifies the amino acid histidine. Therefore, a gene, which is composed of multiple triplets in a unique sequence, provides the code to build an entire protein, with multiple amino acids in the proper sequence (Figure 4.5). The mechanism by which cells turn the DNA code into a protein product is a two-step process, with an RNA molecule as the intermediate. Esters Figure 4.5 The Genetic Code DNA holds all of the genetic information necessary to build a cell’s proteins. The nucleotide sequence of a gene is ultimately translated into an amino acid sequence of the gene’s corresponding protein. 4.3 From DNA to RNA: Transcription DNA is housed within the nucleus, and protein synthesis takes place in the cytoplasm, thus there must be some sort of intermediate messenger that leaves the nucleus and manages protein synthesis. This intermediate messenger is messenger RNA (mRNA), a single-stranded nucleic acid that carries a copy of the genetic code for a single gene out of the nucleus and into the cytoplasm where it is used to produce proteins. differences between RNA and DNA There are several different types of RNA, each having different functions in the cell. The structure of RNA is similar to DNA with a few small exceptions. For one thing, unlike DNA, most types of RNA, including mRNA, are single-stranded and contain no complementary strand. Second, the ribose sugar in RNA contains an additional oxygen atom compared with DNA. Finally, instead of the base thymine, RNA contains the base uracil. This means that adenine will always pair up with uracil during the protein synthesis process. Gene expression begins with the process called transcription, which is the synthesis of a strand of mRNA that is complementary to the gene of interest. This process is called transcription because the mRNA is like a transcript, or copy, of the gene’s DNA code. Transcription begins in a fashion somewhat like DNA chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 7/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A replication, in that a region of DNA unwinds and the two strands separate temporarily, however, only that small portion of the DNA will be split apart forming a region called transcription bubble. The triplets within the gene on this section of the DNA molecule are used as the template (non coding) to transcribe the complementary strand of RNA (Figure 4.6). A codon is a three-base sequence of mRNA, so-called because they directly encode amino acids. Like DNA replication, there are three stages to transcription: initiation, elongation, and termination. Figure 4.6 Transcription: from DNA to mRNA In the first of the two stages of making protein from DNA, a gene on the DNA molecule is transcribed into a complementary mRNA molecule. Stage 1: Initiation. A region at the beginning of the gene called a promoter—a particular DNA sequence of nucleotides—triggers the start of transcription. Stage 2: Elongation. Transcription starts when RNA polymerase unwinds the DNA segment. One strand, referred to as the non-coding strand, becomes the template with the genes to be coded. The polymerase then aligns the correct nucleic acid (A, C, G, or U) with its complementary base on the non-coding strand of DNA. RNA polymerase (the main transcription enzyme) is an enzyme that adds new nucleotides to a 1 When the polymerase has reached the end of the gene, A sequence in growing strand of RNA. This process builds a strand of mRNA. Stage 3: Termination. DNA called terminator that signals termination of transcription and release the mRNA transcript. Before the mRNA molecule leaves the nucleus and proceeds to protein synthesis, it is modified in a number of ways. For this reason, it is often called a pre-mRNA at this stage. For example, your DNA, and thus complementary chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# mRNA, contains protein-coding sequences 8/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A called exons (ex-on signifies that they are expressed) and intervening sequences called introns (int-ron denotes their intervening role) that do not code functional proteins. Their function is still a mystery, but a process called splicing precisely removes these introns from the pre-mRNA transcript before protein synthesis (Figure 4.7). The exons- which are the segments of RNA that remain after splicing- are pasted together to code for the correct amino acids. Interestingly, some introns that are removed from mRNA are not always noncoding. When different coding regions of mRNA are spliced out, different variations of the protein will eventually result, with differences in structure and function. This process results in a much larger variety of possible proteins and protein functions. When the mRNA transcript is ready, it travels out of the nucleus and into the cytoplasm. exon Figure 4.7 Splicing RNA In the nucleus, a structure called a spliceosome cuts out introns (noncoding regions) within a pre-mRNA transcript and reconnects the exons. 4.4 From RNA to Protein: Translation Like translating a book from one language into another, the codons on a strand of mRNA must be translated into the amino acid alphabet of proteins. Translation is the process of synthesizing a chain of amino acids called a polypeptide. Translation requires two major aids: first, a “translator,” the molecule that will conduct chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# RNA 9/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A the translation, and second, a substrate on which the mRNA strand is translated into a new protein, like the Ribosome translator’s “desk.” Both of these requirements are fulfilled by other types of RNA. The substrate on which translation takes place is the ribosome. Remember that many of a cell’s ribosomes are found associated with the rough ER, and carry out the synthesis of proteins destined for the Golgi apparatus. Ribosomal RNA (rRNA) is a type of RNA that, proteins together with proteins, composes the structure of themRNA ribosome. Ribosomes exist in the cytoplasm as two distinct components, a small and a large subunit. When an mRNA molecule is ready to be translated, the two subunits come together and attach to the mRNA. The ribosome provides a substrate for translation, bringing together and aligning the mRNA molecule with the molecular “translators” that must decipher its code. The other major requirement for protein synthesis is the translator molecules that physically “read” the carries mRNA codons.Transfer RNA (tRNA) is a type of RNA that ferries the appropriate corresponding amino acids to the ribosome, and attaches each new amino acid to the last, building the polypeptide chain one-byone. Thus tRNA transfers specific amino acids from the cytoplasm to a growing polypeptide. The tRNA molecules must be able to recognize the codons on mRNA and match them with the correct amino acid. The tRNA is modified for this function. On one end of its structure is a binding site for a specific amino acid. On the other end is a base sequence that matches the codon specifying its particular amino acid. This sequence of three bases on the tRNA molecule is called an anticodon. For example, a tRNA responsible for shuttling the amino acid glycine contains a binding site for glycine on one end. On the other end it contains an anticodon that complements the glycine codon (GGA is a codon for glycine, and so the tRNAs anticodon would read CCU). Equipped with its particular cargo and matching anticodon, a tRNA molecule can read its recognized mRNA codon and bring the corresponding amino acid to the growing chain (Figure 4.8). chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 10/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A Figure 4.8 Translation from RNA to Protein During translation, the mRNA transcript is “read” by a functional complex consisting of the ribosome and tRNA molecules. tRNAs bring the appropriate amino acids in sequence to the growing polypeptide chain by matching their anticodons with codons on the mRNA strand. Much like the processes of DNA replication and transcription, translation consists of three main stages: initiation, elongation, and termination. Initiation takes place with the binding of a ribosome to an mRNA transcript. The Initiator tRNA (carrying methionine) binds to the smaller ribosome subunit and to the mRNA transcript.The tRNA and ribosome subunit move along the mRNA until they encounter a start codon “AUG”, join the large subunit to form intact ribosome and initiation complex forms. The elongation stage involves the recognition of a tRNA anticodon with the next mRNA codon in the sequence. Once the anticodon and codon sequences are bound (remember, they are complementary base pairs), the tRNA presents its amino acid cargo and the growing polypeptide strand is attached to this next amino acid. This attachment takes place with the assistance of various enzymes in the large ribosomal chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 11/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A subunit and requires energy. The tRNA molecule then releases the mRNA strand, the mRNA strand shifts one codon over in the ribosome, and the next appropriate tRNA arrives with its matching anticodon. This process continues until the final codon on the mRNA is reached which provides a “stop” message that signals termination of translation and triggers the release of the complete, newly synthesized protein. Thus, a gene within the DNA molecule is transcribed into mRNA, which is then translated into a protein product (Figure 4.9). Figure 4.9 From DNA to Protein: Transcription through TranslationTranscription within the cell nucleus produces an mRNA molecule, which is modified and then sent into the cytoplasm for translation. The transcript is decoded into a protein with the help of a ribosome and tRNA molecules. Commonly, an mRNA transcription will be translated simultaneously by several adjacent ribosomes. This increases the efficiency of protein synthesis. A single ribosome might translate an mRNA molecule in approximately one minute; so multiple ribosomes aboard a single transcript could produce multiple times the number of the same protein in the same minute. A polyribosome is a string of ribosomes translating a single mRNA strand. chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 12/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A Video 4.3 DNA Transcritption and Translation 4.5 The Genetic Code To summarize what we know to this point, the cellular process of transcription generates messenger RNA (mRNA), a mobile molecular copy of one or more genes with an alphabet of A, C, G, and uracil (U). Translation of the mRNA template converts nucleotide-based genetic information into a protein product. Protein sequences consist of 20 commonly occurring amino acids; therefore, it can be said that the protein alphabet consists of 20 letters. Each amino acid is defined by a three-nucleotide sequence called the triplet codon. The relationship between a nucleotide codon and its corresponding amino acid is called the genetic code. Given the different numbers of “letters” in the mRNA and protein “alphabets,” combinations of nucleotides corresponded to single amino acids. Using a three-nucleotide code means that there are a total of 64 (4 × 4 × 4) possible combinations; therefore, a given amino acid is encoded by more than one nucleotide triplet (Figure 4.10). chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 13/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A Figure 4.10 This figure shows the genetic code for translating each nucleotide triplet, or codon, in mRNA into an amino acid or a termination signal in a nascent protein. (credit: modification of work by NIH) Three of the 64 codons terminate protein synthesis and release the polypeptide from the translation machinery. These triplets are called stop codons. Another codon, AUG, also has a special function. In addition to specifying the amino acid methionine, it also serves as the start codon to initiate translation. The reading frame for translation is set by the AUG start codon near the 5' end of the mRNA. The genetic code is universal. With a few exceptions, virtually all species use the same genetic code for protein synthesis, which is powerful evidence that all life on Earth shares a common origin. chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 14/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A Key Terms anticodon consecutive sequence of three nucleotides on a tRNA molecule that is complementary to a specific codon on an mRNA molecule centromere region of attachment for two sister chromatids chromatin substance consisting of DNA and associated proteins chromosome condensed version of chromatin codon consecutive sequence of three nucleotides on an mRNA molecule that corresponds to a specific amino acid DNA polymerase enzyme that functions in adding new nucleotides to a growing strand of DNA during DNA replication DNA replication process of duplicating a molecule of DNA exon one of the coding regions of an mRNA molecule that remain after splicing gene functional length of DNA that provides the genetic information necessary to build a protein gene expression active interpretation of the information coded in a gene to produce a functional gene product genome entire complement of an organism’s DNA; found within virtually every cell helicase enzyme that functions to separate the two DNA strands of a double helix during DNA replication histone family of proteins that associate with DNA in the nucleus to form chromatin intron chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 15/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A non-coding regions of a pre-mRNA transcript that may be removed during splicing mutation change in the nucleotide sequence in a gene within a cell’s DNA nucleus cell’s central organelle; contains the cell’s DNA polyribosome simultaneous translation of a single mRNA transcript by multiple ribosomes promoter region of DNA that signals transcription to begin at that site within the gene proteome full complement of proteins produced by a cell (determined by the cell’s specific gene expression) ribosomal RNA (rRNA) RNA that makes up the subunits of a ribosome ribosome cellular organelle that functions in protein synthesis RNA polymerase enzyme that unwinds DNA and then adds new nucleotides to a growing strand of RNA for the transcription phase of protein synthesis sister chromatid one of a pair of identical chromosomes, formed during DNA replication somatic cell all cells of the body excluding gamete cells splicing the process of modifying a pre-mRNA transcript by removing certain, typically non-coding, regions transcription process of producing an mRNA molecule that is complementary to a particular gene of DNA transfer RNA (tRNA) molecules of RNA that serve to bring amino acids to a growing polypeptide strand and properly place them into the sequence translation process of producing a protein from the nucleotide sequence code of an mRNA transcript End of Chapter Questions chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 16/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A Which of the following sequences on a DNA molecule would be complementary to GCTTATAT? a. TAGGCGCG b. ATCCGCGC c. CGAATATA d. TGCCTCTC Place the following structures in order from least to most complex organization: chromatin, nucleosome, DNA, chromosome a. DNA, nucleosome, chromatin, chromosome b. nucleosome, DNA, chromosome, chromatin c. DNA, chromatin, nucleosome, chromosome d. nucleosome, chromatin, DNA, chromosome Which of the following is part of the elongation step of DNA synthesis? a. pulling apart the two DNA strands b. attaching complementary nucleotides to the template strand c. untwisting the DNA helix d. none of the above Which of the following is not a difference between DNA and RNA? a. DNA contains thymine whereas RNA contains uracil b. DNA contains deoxyribose and RNA contains ribose c. DNA contains alternating sugar-phosphate molecules whereas RNA does not contain sugars d. RNA is single stranded and DNA is double stranded Transcription and translation take place in the ________ and ________, respectively. a. nucleus; cytoplasm b. nucleolus; nucleus c. nucleolus; cytoplasm d. cytoplasm; nucleus chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 17/18 8/29/2021 Chapter 4: Part 1 - DNA Biology and Cell Division - PART A How many “letters” of an RNA molecule, in sequence, does it take to provide the code for a single amino acid? a. 1 a. 2 a. 3 a. 4 Which of the following is not made out of RNA? a. the carriers that shuffle amino acids to a growing polypeptide strand b. the ribosome c. the messenger molecule that provides the code for protein synthesis d. the intron Submit Answers Clear Answers chrome-extension://bealfgabldnbfgljffhjgghogpeejfep/pkg/index.html# 18/18