Topic 1 ; the molecular basis of inheritance

Question 1

Who discovered the double helix structure and in what year?

Answer 1

James Watson and Francis Crick in 1953

Question 2

What does nucleotides consist of ?

Answer 2

Deoxyribose (5 carbon sugar), phosphate groups and a base (nitrogenous compound).

Question 3

What are the four bases of DNA?

Answer 3

Adenine, thymine, guanine and cytosine

Question 4

Which base pairs with which?

Answer 4

A with T and G with C

Question 5

What does the information within DNA do?

Answer 5

Guides the cell in making new proteins along with RNA that determine our biological traits.
Gets passed or copied from one generation to the next.

Question 6

What are the two classes of nitrogen bases called?

Answer 6

Purines (double-ringed structures) and Pyrimidines (single ringed structures).

Question 7

What does the sides of the DNA ladder consist of?

Answer 7

Sugar-phosphate portions of adjacent nucleotides bonded together. The phosphate of one nucleotide is covalently bonded to the sugar of the next nucleotide.

Question 8

What causes the DNA structure to twist?

Answer 8

The hydrogen bonds between phosphates.

Question 9

Explain the location of 5’ and 3’.

Answer 9

The 5’ prime (-OH, where a free phosphate group is attached to deoxyribose sugar) carbon is located at the top of the leading strand and the 3’ prime (OH group which is where a free hydroxyl group is attached to a deoxyribose sugar) carbon is located at the lower section of the lagging strand.

Question 10

Explain the anti parralel situation in the double helix structure.

Answer 10

The nucleotide sequences are coplementary and parallel, but they go in opposite direction. Antiparallel means the two DNA strands are arranged in the opposite direction. This structure is important in DNA replication because it replicates the leading strand one way and the lagging stand the other way.

Question 11

If drawn diagrammatically, the two DNA strands will look like:

Answer 11

5’ AGCTTGCAT 3’

3’ TCGAACGTA 5’

Question 12

Explain complementary.

Answer 12

Because A always pairs with T and G always pairs with C, the order of the bases on one strand dictates the order of the bases on the other strand. Hence one chain is the complement of the other. It is important to realize the two chains are NOT the same.

Question 13

What does 5’ and 3’ mean?

Answer 13

Refers to the carbons on the sugar.

Question 14

When a cell reproduces, where does all the information go?

Answer 14

To the daughter cells.

Question 15

In prokaryotes, where does DNA replication take place?

Answer 15

In the cytoplasm.

Question 16

In eukaryotes, where does DNA replication take place?

Answer 16

In the nucleus.

Question 17

What is semi-conservative replication?

Answer 17

Semiconservative replication would produce two copies that each contained one of the original strands and one new strand.

Question 18

What is a replication fork?

Answer 18

The replication fork is a structure that forms within the nucleus during DNA replication. It is created by helicases, which break the hydrogen bonds holding the two DNA strands together. The resulting structure has two branching “prongs”, each one made up of a single strand of DNA. These two strands serve as the template for the leading and lagging strands, which will be created as DNA polymerase matches complementary nucleotides to the templates; the templates may be properly referred to as the leading strand template and the lagging strand template.

Question 19

What is the leading strand?

Answer 19

The leading strand is the strand of nascent DNA which is being synthesized in the same direction as the growing replication fork. A polymerase “reads” the leading strand template and adds complementary nucleotides to the nascent leading strand on a continuous basis.

Question 20

What is the lagging strand?

Answer 20

The lagging strand is the strand of nascent DNA whose direction of synthesis is opposite to the direction of the growing replication fork. Because of its orientation, replication of the lagging strand is more complicated than that of the leading strand.

The lagging strand is synthesized in short, separated segments. On the lagging strand template, a primase “reads” the template DNA and initiates synthesis of a short complementary RNA primer. A DNA polymerase extends the primed segments, forming Okazaki fragments. The RNA primers are then removed and replaced with DNA, and the fragments of DNA are joined together by DNA ligase.

Question 21

What is DNA replication and why does it occur?

Answer 21

DNA replication is the process of producing two identical replicas from one original DNA molecule.It occurs because when we divide, we need enough DNA to pass to daughter cells.

Question 22

Explain the DNA replication process.

Answer 22

1. DNA gyrase (enzyme) makes a nick in the double helix and each side seperates.
2. Helicase (enzyme) unwinds the double stranded DNA.
3. Several small proteins called single strand binding proteins temporarily bind to each side and keep them seperated.
4. DNA polymerase walks down the DNA strands and adds new nucleotides to each strand. The nucleotides pair with the complementary nucleotides on the existing strand.
5. A subunit of the DNA polymerase proofreads the new DNA.
6. DNA ligase (enzyme) seals up the fragments into one long continuous strand.
7. The new copies automatically wind up again.

Question 23

What are Okazaki fragments?

Answer 23

Short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication. They are complementary to the lagging template strand, together forming short double stranded DNA sections.

Question 24

What is transcription?

Answer 24

DNA never leaves the nucleus (in eukaryotes) so the information must be copied. It is the first step of gene expression, in which a particular segment of DNA is copied into RNA by the enzyme RNA polymerase.

Question 25

What is mRNA?

Answer 25

Messenger RNA (mRNA) is a large family of RNA molecules that convey genetic information from DNA to the ribosome, where they specify the amino acid sequence of the protein products of gene expression.

Question 26

Which enzyme is responsible for performing transcription?

Answer 26

RNA polymerase.

Question 27

How does RNA polymerase make mRNA?

Answer 27

1. Binds to the DNA strand at a specific sequence of the gene called a promotor.
2. Unwinds and unlinks the two strands of DNA
3. Uses one of the DNA strands as a guide or template.
4. Matches new nucleotides with their components on the DNA strand
5. Binds these new RNA nucleotides together to form a complementary copy of the DNA strand (mRNA).
6. Stops when it encounters a terination sequence of bases

Question 28

What is a promotor?

Answer 28

In genetics, a promoter is a region of DNA that initiates transcription of a particular gene. Promoters are located near the transcription start sites of genes, on the same strand and upstream on the DNA (towards the 5’ region of the sense strand). Promoters can be about 100–1000 base pairs long.[1]

Question 29

mRNA is happy to live in a what state?

Answer 29

single stranded

Question 30

First step of transcription:

Answer 30

One or more sigma factors (protein needed for RNA synthesis) initiate transcription of a gene by enabling binding of RNA polymerase to promotor DNA.

Question 31

Second step of transcription:

Answer 31

RNA polymerase moves a transcription bubble like the slider of a zipper, which splits the double helix DNA molecule into two strands of unpaired DNA nuleotides, by breaking the hydrogen bonds between complementary DNA nucleotides.

Question 32

Third step of transcription:

Answer 32

RNA polymerase adds matching RNA nucleotides that are paired with complementary DNA nucleotides of one DNA strand.

Question 33

Fourth step of transcription:

Answer 33

RNA sugar-phosphate backbones forms with assistance from RNA polymerase to form an RNA strand.

Question 34

Fifth step of transcription:

Answer 34

Hydrogen bonds of the untwisted RNA and DNA helix break, freeing the newly synthesized RNA strand.

Question 35

Sixth step of transcription:

Answer 35

If the cell has a nucleus, the RNA may be further processed and exits to the cytoplasm through the nuclear pore complex.

Question 36

What is translation?

Answer 36

The process in which cellular ribosomes creates proteins. It is part of the process of gene expression. mRNA produced by transcription is decoded by a ribosome complex to produce a specific amino acid chain (polypeptide) that will later fold into an active protein.

Question 37

Where does translation occur in eukayotes and prokaryotes?

Answer 37

Eukaryotes: In the cytosol or across the membrane of the enoplasmic reticulum in a process called vectorial synthesis.
Prokaryotes: In the cell’s cytoplasm

Question 38

What is vectorial synthesis?

Answer 38

Vectorial synthesis is synthesis of exported proteins by ribosomes in which the ribosome-nascent chain complex is bound directly to the endoplasmic reticulum (ER) and the nascent peptide chain moves through the ER membrane as it emerges from the ribosome .[1]