Cell Biology BIO 203 Quiz 3 & Exam 3

promoter

DNA sequence that initiates gene transcription; includes sequences recognized by RNA polymerase

template

a molecular structure that serves as a pattern for the production of other molecules. ex. one strand of DNA directs the synthesis of the complementary DNA strand

nonhomologous end joining

a quick and dirty mechanism for repairing double-stand breaks in DNA that involves quickly bringing together, trimming, and rejoining the two broken ends; results in a loss of information at the site of repair.

mutation

a randomly produced, permanent change in the nucleotide sequence of DNA.

ribozyme

an RNA molecule with the catalytic activity

primase

an RNA polymerase that uses DNA as a template to produce an RNA fragment that serves as a primer for DNA synthesis; letter I in picture

DNA polymerase

an enzyme that catalyzes addition of subunits to a nucleic acid polymer; makes DNA; letter D & F in picture

karyotype

an orderly display of the full set of chromosomes of a cell arranged with respect to size, shape, and number

leading strand

at a replication fork, the DNA strand that is made by continuous synthesis in the 5'-to-3' direction.

lagging strand

at a replication fork, the DNA strand that is made discontinuously in short separate fragments that are later joined together to form one continuous new strand.

cancer

disease caused by abnormal and uncontrolled cell proliferation, followed by invasion and colonization of body sites normally reserved for other cells

DNA (deoxyribonucleic acid)

double stranded polynucleotide formed from two separate chains of covalently linked deoxyribonucleotide units, it serves as the cells' store of genetic information that is transmitted from generation to generation.

aminoacyl-tRNA synthetase

during protein synthesis, an enzyme that attaches the correct amino acid to a tRNA molecule to form a "charged" aminoacyl-tRNA

RNA polymerase

enzyme that catalyzes the synthesis of an RNA molecule from a DNA template using nucleoside triphosphate precursors

protease

enzyme that degrades proteins by hydrolyzing their peptide bonds

telomerase

enzyme that elongates telomeres, synthesizing the repetitive nucleotide sequences found at the ends of eukaryotic chromosomes.

DNA ligase

enzyme that reseals nicks that arise in the backbone of a DNA molecule; letter G in picture

chromatin-remodeling complex

enzyme that uses the energy of ATP hydrolysis to alter the arrangement of nucleosomes in eukaryotic chromosomes, changing the accessibility of the underlying DNA to other proteins, including those involved in transcription

chromosome

long, threadlike structure composed of DNA and proteins that carries the genetic information of an organism; becomes visible as a distinct entity when a plant or animal cell prepares to divide

translation

process by which cells take up DNA molecules from their surroundings and then express genes on that DNA

transcription

process in which RNA polymerase uses one strand of DNA as a template to synthesize a complementary RNA sequence

RNA splicing

process in which intron sequences are excised from RNA molecules in the nucleus during the formation of a mature messenger RNA

translation initiation factor

protein that promotes the proper association of ribosomes with mRNA and is required for the initiation of protein synthesis

general transcription factors

proteins that assemble on the promoters of many eukaryotic genes near the start site of transcription and load the RNA polymerase in the correct position

cell cycle

the orderly sequence of events by which a cell duplicates its contents and divides into two

proofreading

the process by which DNA polymerase corrects its own errors as it moves along DNA

DNA replication

the process by which a copy of a DNA molecule is made

gene expression

the process by which a gene makes a product that is useful to the cell or organism by directing the synthesis of a protein or an RNA molecule with a characteristic activity

alternative splicing

the production of different mRNAs (and proteins) from the same gene by splicing its RNA transcripts in different ways

base pair

two complementary nucleotides in an RNA or a DNA molecule that are held together by hydrogen bonds

gene

unit of heredity containing the instructions that dictate the characteristics or phenotype of an organism; in molecular terms, a segment of DNA that directs the production of a protein or functional RNA molecule

False. The polarity of a DNA strand commonly refers to the orientation of its sugar-phosphate backbone, one end of which contains a phosphate group and the other a hydroxyl group.

Q5-1 True or False? A DNA strand has a polarity because its two ends contain different bases

True. G-C base pairs are held together by three hydrogen bonds, whereas A-T base pairs are held together by only two.

Q5-1 True or False? G-C base pairs are more stable than A-T base pairs.

True.

Q5-11 True or False? Each eukaryotic chromosome must contain the following DNA sequence elements: multiple origins of replication, two telomeres, and one centromere.

False. Nucleosome core particles are approximately 11 nm in diameter.

Q5-11 True or False? Nucleosome core particles are 30 nm in diameter.

DNA assembles with specialized proteins to form chromatin. At a first level of packing, histones form the core of nucleosomes. In a nucleosome, the DNA is wrapped almost twice around this core. Between interphase, the chromatin of the interphase chromosomes is in a relatively extended form in the nucleus, although some regions of it, the heterochromatin, remain densely packed and are transcriptionally inactive. During mitosis replicated chromosomes become condensed into mitotic chromosomes, which are transcriptionally inactive and are designed to be readily distributed between the two daughter cells.

Q5-12 Define the following terms and their relationships to one another: A. interphase chromosome B. mitotic chromosome C. chromatin D. heterochromatin E. histones F. nucleosome

The complementary strand reads 5'-TGATTGTGGACAAAAATCC-3'. Paired DNA strands have opposite polarity and the convention is to write a single-stranded DNA sequence in the 5'-to-3' direction.

Q5-5 The nucleotide sequence of one DNA strand of DNA double helix is 5'-GGATTTTTGTCCACAATCA-3' What is the sequence of the complementary strand?

The statement is correct. If the DNA in somatic cells is not sufficiently stable, the organism dies and because this may often happen before the organism can reproduce, the species will die out. If the DNA in reproductive cells is not sufficiently stable, many mutations will accumulate and be passed on to future generations so that the species will not be maintained.

Q6-12 What, if anything, is wrong with the following statement: "DNA stability in both reproductive cells and somatic cells is essential for the survival of a species."

Because DNA polymerase requires a 3'-OH synthesize DNA, without telomeres and telomerase, the ends of linear chromosomes would shrink during each round of DNA replication. For bacterial chromosomes which have no ends, the problem does not arise; there will always be a 3'-OH group available to prime the DNA polymerase that replaces the RNA primer with DNA. Telomeres and telomerase prevent the shrinking of chromosomes because they extend the 3' end of a DNA strand. This extension of the lagging-strand template provides the "space" to begin the final Okazaki fragments.

Q6-14 Explain why telomeres and telomerase are needed for replication of eukaryotic chromosomes but not for replication of a circular bacterial chromosome. Draw a diagram to illustrate your explanation.

Telomeres and telomerase are still needed even if the last fragment of the lagging strand were initiated by primase at the very 3' end of chromosomal DNA, inasmuch as the RNA primer must be removed.

Q6-14 Would you still need telomeres and telomerase to compel eukaryotic chromosome replication if primate always laid down the RNA primer at the very 3' end of the template for the lagging strand?

A. incorrect - The bonds are not covalent and their formation does not require input of energy. B. correct - the aminoacyl-tRNA enters the ribosome at the A site and forms hydrogen bonds with the codon in the mRNA. C. correct - As the ribosome moves along the mRNA, the tRNAs that have donated their amino acid to the growing polypeptide chain are ejected form the ribosome and the mRNA. The ejection takes place tow cycles after the tRNA first enters the ribosome.

Q7-10 "The bonds that form between the anticodon of a tRNA molecule and the three nucleotides of a codon in mRNA are _____________." Complete this sentence with each of the following options and explain why each of the resulting statements is correct or incorrect. A. Covalent bonds formed by GTP hydrolysis B. Hydrogen bonds that form when the tRNA is at the A site C. Broken by the translocation of the ribosome along the mRNA

A. If the single origin of replication were located exactly in the center of the chromosome, it would take more than 8 days to replicate the DNA. The rate of replication would severely limit the rate of cell division. If the origin were located at one end, the time required to replicate the chromosome would be approximately double this. B. A chromosome end that is not "capped" with a telomere would lose nucleotides during each round of DNA replication. and would gradually shrink. Eventually, essential genes would be lost, and the chromosomes ends might be recognized by the DNA damage-response mechanisms, which would stop cell division or induce cell death. C. Without centromeres, which attach mitotic chromosomes to the mitotic spindle, the two new chromosomes that result form chromosome duplication would not be partitioned accurately between the two daughter cells. Many daughter cells would die, because they would not receive a full set of chromosomes.

Q6-15 Describe the consequences that would arise if a eukaryotic chromosome: A. contained only one origin of replication (i) at the exact center of the chromosome (ii) at one end of the chromosome B. lacked one or both telomeres C. had no centromere Assume that the chromosome is 150 million nucleotide pairs in length, a typical size for an animal chromosome, and that DNA replication in animal cells proceeds at about 100 nucleotides per second.

It is not impossible to proofread during the initial stages fo primer synthesis. To start a new primer on a piece of single-stranded DNA, one nucleotide needs to be put in place and then linked to a second and then to a third and so on. Even if these first nucleotides were perfectly match to the template strand, they would bind with very low affinity and it would consequently be difficult to distinguish the correct from incorrect bases by a hypothetical primate with proofreading activity; the enzyme would therefore stall. The task of the primase is to "just polymerize nucleotides that bind reasonably well to the template without worrying too much about accuracy." Later, these sequences are removed and replaced by DNA polymerase which uses newly synthesized and proofread DNA as its primer.

Q6-2 Discuss the following statement: "Primase is a sloppy enzyme that makes many mistakes. Eventually, the RNA primers it makes are disposed of and replaced with DNA synthesized by a polymerase with higher fidelity. This is wasteful. It would be more energy-efficient if a DNA polymerase made an accurate copy in the first place."

A. Without DNA polymerase, no replication can take place. RNA primers will be laid down at the origin of replication. B. DNA ligase links the DNA fragments that are produced on the lagging strand. In the absence of ligase, the newly replicated DNA strands will remain as fragments, but no nucleotides will be missing. C. Without the sliding clamp, the DNA polymerase will frequently fall of the DNA template. In principle, it can rebind and continue, but the continual falling off and rebinding will be time-consuming and will greatly slow down DNA replication. D. In the absence of RNA-excision enzymes, the RNA fragments will remain covalently attached to the newly replicated DNA fragments. No ligation will take place, because the DNA ligase will not link DNA to RNA. The lagging stand will consist of fragments composed of both RNA and DNA. E. Without DNA helicase, the DNA polymerase will stale because it cannot separate the strands of the template DNA ahead of it. Little or no new DNA will be synthesized. F. In the absence of primase, RNA primers cannot begin on either the leading or the lagging strand. DNA replication cannot begin.

Q6-3 A gene encoding one of the proteins involved in DNA replication has been inactivated by a mutation in a cell. In the absence of the protein, the cell attempts to replicate its DNA. What would happen during the DNA replication process if each of the following proteins were missing? A. DNA polymerase B. DNA ligase C. Sliding clamp for DNA polymerase D. Nuclease that removes RNA primers E. DNA helicase F. Primase

The statement is incorrect. DNA damage by deamination and deprivation reactions occurs spontaneously. This type of damage is not the result of replication errors and is equally likely to occur on either strand. If DNA repair enzymes recognized such damage only on newly synthesized DNA strands, half of the defects would go uncorrected.

Q6-4 Discuss the following statement: "The DNA repair enzymes that fix deamination and deprivation damage must preferentially recognize such damage on newly synthesized DNA strands."

If the old strand were "repaired" using the new strand that contains a replication error as the template, then the error would become a permanent mutation in the genome. The old information would be erased in the process. If repair enzymes did not distinguish between the two strands, there would be only a 50% chance that any given replication error would be corrected.

Q6-5 DNA mismatch repair enzymes preferentially repair bases on the newly synthesized DNA strand, using the old DNA strand as a template. If mismatches were simply repaired without regard for which strand served as template, would this reduce replication errors? Explain.

False. Identical DNA polymerase molecules catalyze DNA synthesis on the leading and lagging strands of a bacterial replication fork. The replication fork is asymmetrical because the lagging strand is synthesized in pieces that are then stitched together.

Q6-7 True or False? A bacterial replication fork is asymmetrical because it contains two DNA polymerase molecules that are structurally distinct.

True. usually multiple mutations of specific types need to accumulate in a somatic cell lineage to produce cancer. A mutation in a gene that codes for a DNA repair enzyme can make a cell more liable to accumulate further mutations, accelerating the onset of cancer.

Q6-7 True or False? Cancer can result from the accumulation of mutations in somatic cells.

True. Mutations would accumulate rapidly, inactivating many genes.

Q6-7 True or False? In the absence of DNA repair, genes are unstable.

True. If a damaged nucleotide also occurred naturally in DNA, the repair enzyme would have no way of identifying the damage. It would have only a 50% chance of fixing the right strand.

Q6-7 True or False? None of the aberrant bases formed by deamination occur naturally in DNA.

False. Only the RNA primers are removed by an RNA nuclease; Okazaki fragments are pieces of newly synthesized DNA on the lagging strand that are eventually joined together by DNA ligase.

Q6-7 True or False? Okazaki fragments are removed by a nuclease that degrades RNA.

True. With proofreading, DNA polymerase has an error rate of one mistake in 10^7 nucleotides polymerized; 99% of its errors are corrected by DNA mismatch repair enzymes, bringing the final error rate to one in 10^9.

Q6-7 True or False? The error rate of DNA replication is reduced by both proofreading by DNA polymerase and by DNA mismatch repair.

Replication - the creation of an exact copy; the act of duplicating DNA Transcription - the act of writing out a copy especially form one physical form to another; the act of copying the info stored in DNA into RNA. Translation - the act of putting words into different language; the act of polymerizing amino acids into a defined linear sequence using the information in mRNA.

Q7-11 List the ordinary, dictionary definitions of the terms: replication, transcription, and translation. By their side, list the special meaning each term has when applied to the living cell.

It is likely that in early cells the matching between codons and amino acids was less accurate than it is in present-day cells. The feature of the genetic code described in the question may have allowed early cells to tolerate this inaccuracy by allowing a blurred relationship between sets of roughly similar codons and roughly similar amino acids.

Q7-13 One remarkable feature of the genetic code is that amino acids with similar chemical properties often have similar codons. Thus codons with U or C as the second nucleotide tend to specify hydrophobic amino acids. Can you suggest a possible explanation for this phenomenon in terms of the early evolution of the protein-synthesis machinery?

The codon for Trp is 5'-UGG-3'. A normal Trp-tRNA contains the sequence 5'-CCA-3' as its anticodon. If this tRNA contains a mutation so that its anticodon is changed to UCA, it will recognize a UGA codon and lead to the incorporation of a tryptophan residue instead of causing translation to stop Many other protein-encoding sequences contain UGA codons as their natural stop sites, and these stops would also be affected by the mutant tRNA. Depending on competition between the altered tRNA and the normal translation release factors some of these proteins would be made with additional amino acids at their C-terminal end. The additional lengths would depend on the number of codons before the ribosomes encounter a non-UGA stop codon in the mRNA in the reading frame in which the protein is translated.

Q7-14 A mutation in DNA generates a UGA stop codon in the middle of the mRNA coding for a particular protein. A second mutation in the cells' DNA leads to a single translation of the protein; that is the second mutation "suppresses" the defect caused by the first. The altered tRNA translates the UGA as tryptophan. What nucleotide change has possibly occurred in the mutant tRNA molecule? What consequences would the presence of such a mutant tRNA have for the translation of the normal genes in this cell?

One effective way of driving a reaction to completion is to remove one of the products, so that the reverse reaction cannot occur. ATP contains two high-energy bonds that link the three phosphate groups. In the reaction, PPi is released consisting of two phosphate groups linked by one of these high-energy bonds. PPi can be hydrolyzed with a considerable gain of free energy and can be efficiently removed. This happens rapidly in cells and reactions that produce and further hydrolyze PPi are virtually irreversible.

Q7-15 The charging of a tRNA with an amino acid can be represented by the following equation: amino acid + tRNA + ATP ---> animoacyl-tRNA + AMP + PPi where PPi is pyrophosphate. In the aminoacyl-tRNA, the amino acid and tRNA are linked with a high-energy covalent bond; a large portion of the energy derived from the hydrolysis of ATP is thus stored in this bond and is available to drive peptide bond formation at the later stages of protein synthesis. The free-energy change of the charging reaction shown in the equation is close to zero and therefore would not be expected to favor attachment of the amino acid to tRNA. Can you suggest a further step that could drive the reaction to completion?

Mutations of the type described in B and D are often the most harmful in both cases, the reading frame would be changed and because this frameshift occurs near the beginning or in the middle of the coding sequence, much of the protein will contain a nonsensical or truncated sequence of amino acids. A reading frameshift that occurs toward the end of the coding sequence as described in A will result in a largely correct protein that may be functional. Deletion of three consecutive nucleotides as described in C leads to the deletion of an amino acid but does not alter the reading frame. The deleted amino acid may or may not be important for the folding or activity of the protein; in many cases the mutation are silent. Substitution of one nucleotide for another as described in E is often completely harmless. In some cases it will not change the amino acid sequence of the protein but in other cases it will change a single amino acid. At worst is may create a new stop codon giving rise to a truncated protein.

Q7-17 Which of the following types of mutations would be predicted to harm an organism? Explain. A. insertion of a single nucleotide near the end of the coding sequence. B. removal of a single nucleotide near the beginning of the coding sequence. C. deletion of four consecutive nucleotides in the middle of the coding sequence. D. substitution of one nucleotide for another in the middle of the coding sequence.

The RNA polymerase were moving left to right as indicated by the gradual lengthening of the RNA transcripts. The RNA transcripts are shorter because they begin to fold up as they are synthesized, where as DNA is an extended double helix.

Q7-2 Are the RNA polymerase molecules moving from right to left or from left to right? Why are the RNA transcripts so much shorter than the DNA segments (genes) that encode them?

The RNA polymerase used to make primers would need to initiate every few hundred bases, which is much more often than promoters are spaced on the DNA. Initiation would need to occur in a promoter-independent fashion or many more promoters would have to be present in the DNA, -- problematic for the control of transcription. The RNA primers used in DNA replication are much shorter then mRNAs. The RNA polymerase would need to terminate much more frequently than during transcription. Termination would need to occur spontaneously or many more terminators would need to be present -- problematic for control of transcription. Evolution has solved this problem by using separte enzymes with specialized properties. Although, some small DNA viruses utilize the host RNA polymerase to make DNA primers for their replication.

Q7-3 Could the RNA polymerase used for transcription be used as the polymerase that makes the RNA primer required for DNA replication?

The mRNA will have a 5'-to-3' polarity, opposite to that of the DNA strand that serves as the template. The mRNA sequence will read 5'-GAAAAAAGCCGUUAA-3'. The N-terminal amino acid coded for by GAA is glutamic acid. UAA specifies a stop and is an arginine. The convention in describing the sequence of a gene is to give the sequence of the DNA strand that is not used as a template for RNA synthesis; this sequence is the same as that of the RNA transcript, with T written in place of U.

Q7-5 A sequence of nucleotides in a DNA strand -- 5'-TTAACGGCTTTTTTC-3' -- was used as a template to synthesize an mRNA that was then translated into protein Predict the C-terminal amino acid and the N-terminal amino acid of the resulting polypeptide. Assume that the mRNA is translated without the need for a start codon.

The first statement is probably correct. RNA is though to have been the first self-replicating catalyst, and in modern cells, is no longer self-replicating. RNA now serves many roles in the cell: as messengers, as adaptors for protein synthesis, as primers for DNA replication, and as catalysts for some of the most fundamental reactions like RNA splicing and protein synthesis.

Q7-6 Discuss the following: "During the evolution of life on Earth, RNA lost its glorious position as the first self-replicating catalyst. Its role now is as a mere messenger in the information flow from DNA to protein."

False. mRNAs are translated as linear polymers; there is no requirement that they have any particular folded structure. Such structures that are formed by mRNA can inhibit its translation, because the ribosome has to unfold the mRNA in order to read the message it contains.

Q7-7 True or False? All mRNAs fold into particular three-dimension structures that are required for their translation.

False. Ribosomes can make any protein that is specified by the particular mRNA that they are translating. After translation, ribosomes are released from the mRNA and can then start translating a different mRNA. It is true that a ribosome can only make one type of protein at a time.

Q7-7 True or False? An individual ribosome can make only one type of protein.

False. RNA contains uracil but not thymine.

Q7-7 True or False? An mRNA may contain the sequence: ATTGACCCCGGTCAA

False. The position of the promoter determines the direction in which transcription proceeds and which DNA strand is used as the template. Transcription int eh opposite direction would produce an mRNA with a completely different sequence.

Q7-7 True or False? Because the two strands of DNA are complementary, the mRNA of a given gene can be synthesize using either strand as a template.

False. Ribosomes are cytoplasmic organelles but they are not individually enclosed in a membrane.

Q7-7 True or False? Ribosomes are cytoplasmic organelles that are encapsulated by a single membrane.

False. The level of a protein depends on its rate of synthesis and degradation but not on its catalytic activity.

Q7-7 True or False? The amount of a protein present in a cell depends on its rate of synthesis, its catalytic activity, and its rate of degradation.

False. Ribosomal subunits exchange partners after each round of translation. After a ribosome is released from an mRNA, its two subunits dissociate and enter a poll of free small and large subunits from which new ribosomes assemble around a new mRNA.

Q7-7 True or False? The large and small subunits of an individual ribosome always stay together and never exchange parts.

Sequence 1 and 4 both code for the peptide Arg-Gly-Asp. Because the genetic code is redundant, different nucleotide sequences can encode the same amino acid sequence.

Q7-9 Use the genetic code shown in fig. 7-25 to identify which of the following nucleotide sequences would code for the polypeptide sequence arginine-glycine-aspartate: 1. 5'-AGA-GGA-GAU-3' 2. 5'-ACA-CCC-ACU-3' 3. 5'-GGG-AAA-UUU-3' 4. 5'-CGG-GGU-GAC-3'

small nuclear RNA (snRNA)

RNA molecule of around 200 nucleotides that participates in RNA splicing

RNA transcript

RNA molecule produced by transcription that is complementary to one strand of DNA

ribosomal RNA (rRNA)

RNA molecule that forms the structural and catalytic core of the ribosome

messenger RNA (mRNA)

RNA molecule that specifies the amino acid sequence of a protein; letter E

RNA processing

broad term for the modification that a precursor mRNA undergoes as it matures into an mRNA. It typically includes 5' capping, RNA splicing, and 3' polyadenylation

DNA repair

collective term for the enzymatic processes that correct deleterious changes affecting the continuity or sequence of a DNA molecule

chromatin

complex of DNA and proteins that makes up the chromosomes in a eukaryotic cell

complementary

describes two molecular surfaces that fit together closely and form non covalent bonds with each other. examples: complementary base pairs and the two complementary strands of DNA molecule

codon

group of three consecutive nucleotides that specifies a particular amino acid or that starts or stops protein synthesis; applies to the nucleotides in an mRNA or in a coding sequence of DNA

heterochromatin

highly condensed region of an interphase chromosome; generally gene-poor and transcriptionally inactive

RNA world

hypothetical period in Earth's early history in which life-forms were thought to use RNA both to store genetic information and to catalyze chemical reactions.

spliceosome

large assembly of RNA and protein molecules that splices introns out of pre-mRNA in the nucleus of eukaryotic cells

ribosome

large macromolecular complex, composed of ribosomal RNAs and ribosomal proteins, that translates messenger RNA into a protein

proteasome

large protein machine that degrades proteins that are damaged, misfiled, or no longer needed by the cell; its target proteins are marked for destruction primarily by the attachment of a short chain of ubiquitin

nucleolus

large structure within the nucleus where ribosomal RNA is transcribed and ribosomal subunits are assembled

homologous recombination

mechanism by which double-stranded breaks in a DNA molecule can be repaired flawlessly; uses an undamaged, duplicated, or homologous chromosome to guide the repair. During meiosis the mechanism results in an exchange of genetic info between the maternal and parental homologs.

mismatch repair

mechanism for recognizing and correcting incorrectly paired nucleotides--those that are noncomplementary

centromere

microtubule-organizing center that sits near the nucleus in an animal cell; during the cell cycle, this structure duplicates to form the two poles of mitotic spindle

RNA

molecule produced by the transcription of DNA; usually single-stranded, it is a polynucleotide composed of covalently linked ribonucleotide subunits. Serves a variety of structural, catalytic, and regulatory functions in cells

RNA (ribonucleic acid)

molecule produced by the transcription of DNA; usually single-stranded, it is a polynucleotide composed of covalently linked ribonucleotide subunits. Serves a variety of structural, catalytic, and regulatory functions in cells; picture A

intron

noncoding sequence within a eukaryotic gene that is transcribed into an RNA molecule but is then excised by RNA splicing to produce an mRNA

replication origin

nucleotide sequence at which DNA replication is initiated

histone

one of a group of abundant highly conserved proteins around which DNA wraps to form nucleosomes, structures that represent the most fundamental level of chromatin packing

reading frame

one of the three possible ways in which a set of successive nucleotide triplets can be translated into protein, depending on which nucleotide serves as the starting point

euchromatin

one of the two main states in which chromatin exists within an interphase cell; prevalent in gene-rich areas, its less compact structure allows access for proteins involved in transcription

telomere

repetitive nucleotide sequence that caps the ends of linear chromosomes; counteracts the tendency of the chromosome otherwise to shorten with each round of replication

exon

segment of a eukaryotic gene that is transcribed into RNA and dictates the amino acid sequence of part of a protein

genetic code

set of rules by which the information contained in the nucleotide sequence of a gene and its corresponding RNA molecule is translated into the amino acid sequence in a protein

anticodon

set of three consecutive nucleotides in a transfer RNA molecule that recognizes, through base-pairing, the three-nucleotide codon on a messenger RNA molecule; this interaction helps to deliver the correct amino acid to a growing polypeptide chain

Okazaki fragment

short length of DNA produced on the lagging strand during DNA replication. Adjacent fragments are rapidly joined together by DNA ligase to form a continuous DNA strand; letter E in picture

transfer RNA (tRNA)

small RNA molecule that serves as an adaptor that "reads" a codon in mRNA and adds the correct amino acid to the growing polypeptide chain

initiator tRNA

special tRNA that initiates the translation of an mRNA in a ribosome. It always carries the amino acid methionine.

polyadenylation

the addition of multiple adenine nucleotides to the 3' end of a newly synthesized mRNA molecule.

RNA capping

the modification of the 5' end of a maturing RNA transcript by the addition of an atypical nucleotide

genome

the total genetic information carried by all the chromosomes of a cell or organism

double helix

the typical structure of a DNA molecule in which the two complementary polynucleotide strands are wound around each other with base-pairing between the strands

replication fork

y-shaped junction that forms at the site where DNA is being replicated; entire picture as a whole