Lesson 2: DNA replication

Question 1

In the Meselson and Stahl experiment, what pattern of banding would you expect to find in the centrifuge tubes after one generation in the regular nitrogen medium, if DNA replication were, in fact, conservative?

Answer 1

If replication were conservative, there would be one band of heavy DNA and one band of regular DNA in the centrifuge tubes. This would be because one copy would contain the original, heavy nitrogen, while the other copy would contain only the regular-weight nitrogen

Question 2

What does it mean that DNA replication is semi-conservative?

Answer 2

Semiconservative replication means that during DNA replication, each strand of DNA from the original cell is “conserved”, or not changed, while a complementary copy is made from “new” nucleotides.

Question 3

Prokaryotes and eukaryotes in DNA replication

Answer 3

• the process in prokaryotes (organisms whose cells dont contain a nucleus) and eukaryotes ( do have a nucleus) is similar even though they have differently shaped DNA
• Prokaryotes have circular strands of DNA and contain about a thousand times fewer nucleotides than the long twisted strands of DNA found in eukaryotes
• This is why DNA replication in prokaryotes is much faster

Question 4

What are the 3 main phases of DNA replication?

Answer 4

1. Initiation: DNA is unwound and separated to expose each strand in the pair at multiple sites of replication along the DNA molecule
2. Elongation: Enzymes attach complementary nucleotides onto the 3 prime end of each exposed strand in a linear sequence and check for errors
3. Termination: Nucleotide addition stops, enzymes are removed, and the newly formed strands of DNA coil back into the double helix shape.

Question 5

Hydrogen bonds are quite weak compared to compared to covalent bonds. Explain why this fact is actually advantageous to DNA in its role as the hereditary material in cells.

Answer 5

This is an advantage because it allows these bonds to be easily broken, thus facilitating the process of DNA replication. In order for DNA replication to occur, helicase must break the hydrogen bonds formed between nitrogenous bases.

Question 6

DNA replication occurs in virtually all cells, prior to cell division. Why is DNA replication essential?

Answer 6

Its essential because it preserves the genetic information from generation to generation, in each new daughter cell. This includes the information for an organisms genes, growth, repair and reproduction.

Question 7

What role do telomeres play in aging?

Answer 7

Telomeres protect the genetic information near the ends of a DNA molecule. With each replication, short segments of them break off, resulting in the telomeres getting shorter over time. This sets a limit to how many times a cell can divide because, at some point, the telomeres get too short and DNA replication stops, When this happens, the cell dies. Cells with longer telomeres live longer.

Question 8

What is a somatic cell?

Answer 8

any cell of a living organism other than the reproductive cells

Question 9

What is the role in helicase

Answer 9

it continues to unwind the DNA forming a structure called the replication fork, which is named for the forked appearance of the two strands of DNA as they are unzipped apart. The process of breaking the hydrogen bonds between the nucleotide base pairs in double-stranded DNA requires energy.

Question 10

What is telomerase?

Answer 10

an enzyme that adds nucleotides to telomeres, especially in cancer cells.

Question 11

What do telomeres do?

Answer 11

• They are a DNA sequence
• An enzyme made of protein and RNA subunits that elongates chromosomes by adding TTAGGG sequences to the end of existing chromosomes
• So telomeres allow cells to divide without losing genes. Cell division is necessary for growing new skin, blood, bone, and other cells. Without telomeres, chromosome ends could fuse together and corrupt the cell’s genetic blueprint, possibly causing malfunction, cancer, or cell death.

Question 12

Describe a nucelotide and its main function.

Answer 12

A nucleotide consists of three things: A nitrogenous base, which can be either adenine, guanine, cytosine, or thymine (in the case of RNA, thymine is replaced by uracil). A five-carbon sugar, called deoxyribose because it is lacking an oxygen group on one of its carbons. One or more phosphate groups.

Purine and pyrimidine nucleotides fill a variety of metabolic roles. They are the “energy currency” of the cell. In some cases, they are signaling molecules, acting like hormones directly or as transducers of the information. They provide the monomers for genetic information in DNA and RNA.

Question 13

Describe the 3 models suggestions for DNA replication.

Answer 13

There were three models suggested for DNA replication: conservative, semi-conservative, and dispersive.
conservative: parental DNA remains together and newly-formed daughter strands are also together.
semi-conservative: the two parental DNA strands serve as a template for new DNA and after replication, each double-stranded DNA contains one strand from the parental DNA and one new (daughter) strand.
dispersive: after replication, the two daughter DNAs have alternating segments of both parental and newly-synthesized DNA interspersed on both strands.
SEMI CONSERVATIVE IS RIGHT

Question 14

isotope

Answer 14

any of two or more forms of an element where the atoms have the same number of protons, but a different number of neutrons within their nuclei

Question 15

prokaryotes vs eukaryotes

Answer 15

prokaryotes: Prokaryotic cells do not contain a nucleus or any other membrane-bound organelle; example is bacteria

eukaryotes: cells contain membrane-bound organelles, including a nucleus. They can be single-celled or multi-celled
examples: humans, plants, fungi, and insects

Question 16

explain DNA replication in prokaryotes

Answer 16

• they are replicated by DNA polymerase III in the 5 prime to 3 prime direction
• Helicase separates the DNA to form a replication fork at the origin of replication where DNA replication begins.
• origin of replication: a particular sequence in a genome at which replication is initiated
• Replication forks extend bi-directionally (2 opposite directions) as replication continues.
• Okazaki fragments are formed on the lagging strand, while the leading strand is replicated continuously.
• DNA ligase seals the gaps between the Okazaki fragments.
• Primase synthesizes an RNA primer with a free 3′-OH, which DNA polymerase III uses to synthesize the daughter strands.

Question 17

DNA replication in Eukaryotes: there are three stages

1. Initiation
2. Elongation
3. Termination

Answer 17

• During initiation, proteins bind to the origin of replication while helicase unwinds the DNA helix and two replication forks are formed at the origin of replication.
• During elongation, a primer sequence is added with complementary RNA nucleotides, which are then replaced by DNA nucleotides.
• During elongation the leading strand is made continuously, while the lagging strand is made in pieces called Okazaki fragments.
• During termination, primers are removed and replaced with new DNA nucleotides and the backbone is sealed by DNA ligase.

Question 18

Leading Strand

Answer 18

the template strand of the DNA double helix that is oriented so that the replication fork moves along it in the 3′ to 5′ direction

Question 19

Lagging Strand

Answer 19

the strand of the template DNA double helix that is oriented so that the replication fork moves along it in a 5′ to 3′ manner

Question 20

Telomere replication

Answer 20

• The ends of linear chromosomes, called telomeres, protect genes from getting deleted as cells continue to divide.
• The telomerase enzyme attaches to the end of the chromosome; complementary bases to the RNA template are added on the 3′ end of the DNA strand.
• Once the lagging strand is elongated by telomerase, DNA polymerase can add the complementary nucleotides to the ends of the chromosomes and the telomeres can finally be replicated.
• Cells that undergo cell division continue to have their telomeres shortened because most somatic cells do not make telomerase; telomere shortening is associated with aging.

Question 21

What is a telomere and and telomerase

Answer 21

telomere: either of the repetitive nucleotide sequences at each end of a eukaryotic chromosome, which protect the chromosome from degradation
telomerase: an enzyme in eukaryotic cells that adds a specific sequence of DNA to the telomeres of chromosomes after they divide, giving the chromosomes stability over time

Question 22

Explain the lagging strand.

Answer 22

The lagging strand in DNA replication is the strand of the double-stranded DNA molecule that has to be synthesised discontinuously. This is because the two strands of DNA molecules are antiparallel, so run in opposite directions. DNA polymerase can only polymerise DNA nucleotides in a 5’ to 3’ direction, so can only use a template strand that runs in a 3’ to 5’ direction. The DNA strand that runs in this direction is synthesised continuously, but the other DNA strand must be synthesised in Okazaki fragments joined with the help of DNA ligase and DNA polymerase 1 (in prokaryotes).