Why is Dna Synthesis Continuous on One Strand and Iscontinuous on the Other

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The formation of Okazaki fragments in eukaryotic DNA__________.

Possible Answers:

is required in both the leading and the lagging strands

is required in the lagging strand only

is required in the leading strand only

is required in neither the leading or the lagging strands

Correct answer:

is required in the lagging strand only

Explanation:

Replication of DNA is both continuous and discontinuous, each form of replication occurring simultaneously. Continuous DNA synthesis occurs in the 3'  5' direction on the parent strand. This is often referred to as the leading strand with new nucleotides being added to the 3' end. Discontinuous DNA synthesis occurs in the 5'  3' direction on the parent strand. This strand is often referred to as the lagging strand. It is completed in short sequences of nucleotides called Okazaki fragments. Replication on the lagging strand begins with the addition of an RNA primer by the enzyme primase. Primase adds the RNA primers ahead of the 5' end of the lagging. This allows DNA polymerase III to add the Okazaki fragments to fill in the space between primers. This process repeats itself until the entire strand has been replicated. DNA polymerase I then comes to exchange the RNA primer with DNA nucleotides, then DNA ligase reinforces the bonding between the fragments and the DNA nucleotides that replaced the RNA primer. Note that in both leading and lagging strand synthesis, nucleotides are added to the 3' end of the growing chain, thus synthesis always occurs in the 5' 3' direction of the growing strand.

The leading strand is replicated __________, and the lagging strand is replicated __________.

Possible Answers:

continuously . . . continuously

continuously . . . discontinuously

discontinuously . . . continuously

discontinuously . . . discontinuously

Correct answer:

continuously . . . discontinuously

Explanation:

Replication of DNA is both continuous and discontinuous, each form of replication occurring simultaneously. Continuous DNA synthesis occurs from the 3' end to the 5' end of the parent strand. This is often referred to as the leading strand with new nucleotides being added to the 3' end. Discontinuous DNA synthesis occurs from the 5' end to the 3' end of the parent strand. This strand is often referred to as the lagging strand. It is completed in short sequences of nucleotides called Okazaki fragments. Replication on the lagging strand begins with the addition of an RNA primer by the enzyme primase. Primase adds the RNA primers ahead of the 5' end of the lagging. This allows DNA polymerase III to add the Okazaki fragments to fill in the space between primers. This process repeats itself until the entire strand has been replicated. DNA polymerase I then comes to exchange the RNA primer with DNA nucleotides, then DNA ligase reinforces the bonding between the fragments and the DNA nucleotides that replaced the RNA primer. Once both the leading and lagging stranded have completed replication, the result is two identical strands of the original DNA molecule.

The enzyme helicase is required to__________.

Possible Answers:

produce RNA primers on the lagging strand so that DNA Polymerase III may add new nucleotides

unzip the double helix structure of DNA to commence replication

bind Okazaki fragments to one another

prevent the strands beyond the replication fork from supercoiling

Correct answer:

unzip the double helix structure of DNA to commence replication

Explanation:

The enzyme helicase unzips the two strands of the double helix. Once unzipped, single stranded binding (SSB) proteins stabilize the newly single strands. The enzyme DNA gyrase ensure the double stranded areas beyond the replication fork do not supercoil onto one another. After stabilization of the replication fork, an enzyme complex known as DNA polymerase III commences the addition of nucleotides to the new strand. Proteins such as the beta clamp and clamp loader assist in keeping DNA polymerase III in its place on the strand of DNA, The enzyme primase adds sequences of RNA primers to the DNA strand to begin replication. DNA Polymerase III cannot begin replication without this primer. DNA ligase reinforces the bonding between the Okazaki fragments and the DNA nucleotides that replace the RNA primer.

DNA synthesis always occurs in the __________ direction, so one new strand is synthesized continuously towards the replication fork, producing the __________ strand. The other strand, known as the __________ strand, forms away from the replication fork in small fragments.

Possible Answers:

3' to 5' . . . lagging . . . leading

5' to 3' . . . lagging . . . leading

3' to 5' . . . leading . . . lagging

5' to 3' . . . leading . . . lagging

4' to 3' . . . leading . . . Okazaki fragments

Correct answer:

5' to 3' . . . leading . . . lagging

Explanation:

DNA synthesis always occurs in the 5' to 3' direction, so one new strand is synthesized continuously towards the replication fork, producing the leading strand. The other strand, known as the lagging strand, forms away from the replication fork in small fragments.

DNA replication occurs both continuously and discontinuously at the same time. Nucleotides can only be added to a new strand of DNA on the 3' end, so the process has to start with the 5' end. As DNA continues to be split apart, the leading strand (growing in the direction towards the replication fork) can continuously add new nucleotides. However, for the lagging strand, the 5' to 3' direction is away from the replication fork, so new nucleotides are added in small chunks called Okazaki fragments as the DNA strand continues to separate.

The enzyme helicase opens the double helix of DNA at points called __________.

Possible Answers:

replication forks

DNA split holes

primer points

telomere tears

Okazaki fragments

Correct answer:

replication forks

Explanation:

The enzyme helicase opens the double helix of DNA at points calledreplication forks.

The unwinding of the double helix of DNA is caused by an enzyme called helicase, which breaks the hydrogen bonds holding the complementary base pairs together, creating two template strands of DNA ready to begin the next step of replication. The place where this enzyme 'unzips' the DNA is called the replication fork.

Which enzyme is responsible for forming a new strand of DNA?

Possible Answers:

RNA Polymerase

Helicase

DNA Polymerase

Amylase

Synthase

Correct answer:

DNA Polymerase

Explanation:

DNA polymerase is responsible for joining nucleotide subunits to form the new strand of DNA during replication. In contrast, RNA polymerase will join nucleotides to form strands of RNA during transcription. Amylase is found in saliva and catalyzes the breakdown of starch and carbohydrates. Synthases are a class of enzyme that act as catalysts for joining two molecules. Helicase uncoils double-stranded DNA, allowing the formation of the replication fork.

When considering the regulatory processes behind DNA replication, what is special about chloroplast and mitochondrial genomes?

Possible Answers:

Replication occurs independently from the cell cycle.

There are no regulatory checkpoints on these types of replication.

Cyclins and cyclin-dependent kinases are strengthened in these types of replication.

There is no well-defined cycle of replication for chloroplasts or mitochondria, and their DNA instead replicates continuously.

The cell checkpoints in these types of replication are only limited by available fuel.

Correct answer:

Replication occurs independently from the cell cycle.

Explanation:

Replication of chloroplast cells in plants and mitochondrial DNA in advanced cells occurs independently of the cell cycle, follwing instead the process known as D-Loop Replication.

To what process does the term "transcription" refer?

Possible Answers:

Transcription is the process of "writing" proteins from DNA

Transcription is the process of "writing" RNA from DNA

Transcription is the process of tRNA carrying amino acids to a polypeptide being built

Transcription is the process of "writing" proteins from RNA

Transcription is the process of rRNA building ribosomes

Correct answer:

Transcription is the process of "writing" RNA from DNA

Explanation:

The conversion of DNA into RNA is known as transcription. A DNA template is read to produce a complementary RNA strand.

The conversion of RNA to protein is described by translation, and is completed with the help of mRNA. You cannot transition from DNA straight to protein without these intermediary steps. tRNA is used to carrying amino acids to a growing polypeptide, and rRNA is used to build ribosomes, but these processes are not considered parts of transcription.

Which sugar is found in the back bone of DNA?

Possible Answers:

Dextran

Dextrose

Ribose

Deoxyribose

Correct answer:

Deoxyribose

Explanation:

DNA stands for "deoxyribonucleic acid." The backbone of DNA is comprised of alternating sugar and phosphate units, in which the sugar is deoxyribose. The backbone of RNA is also comprised of sugar and phosphate units, but uses the sugar ribose.

Which of the following might you find in the backbone of DNA?

I. Phosphate group

II. Hexose sugar

III. Adenine

Possible Answers:

I only

II only

I, II, and III

I and II

Explanation:

A DNA molecule has two primary structural domains: the DNA backbone and the DNA bases. Recall that all DNA molecules are made from nucleotides. One nucleotide of a DNA molecule consists of a phosphate group, a pentose (five-carbon) sugar called deoxyribose, and a nitrogenous base (adenine, thymine, guanine, cytosine). Several of these nucleotide monomers are joined together by phosphodiester bonds to create a DNA molecule.

The backbone of a DNA molecule consists of the phosphate groups and the deoxyribose sugars, whereas the base region of the DNA molecule consists of the nitrogenous bases; therefore, the backbone of DNA is made up of phosphate groups and pentose sugars. Adenine is part of the base region of the molecule. DNA does not contain any hexose (six-carbon) sugars.

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