DNA Structure

Question 1

How many strands does DNA have?

Answer 1

Two strands
Double-stranded

Question 2

What is the alignment of DNA strands?

Answer 2

Antiparallel alignment

Question 3

What does the antiparallel alignment look like?

Answer 3

The two strands lie next to one another with the 5’ end of one strand next to the 3’ end of the other strand. And the bases within those strands are facing each other.

Question 4

What is a DNA helix?

Answer 4

The two strands of DNA coiled around each other. The DNA helix is asymmetrical.

Question 5

How is a DNA helix oriented?

Answer 5

The backbones of the strand are facing outside of the helix
The bases are projecting towards the center of the helix. The bases are also projecting toward the bases of the other strand.

Question 6

What is the overall charge of the DNA?

Answer 6

The backbone facing outside of the helix causes DNA to have an overall negative charge

Question 7

How are the two strands of DNA held together?

Answer 7

Hydrogen bonds

Question 8

What is base pairing?

Answer 8

The formation of hydrogen bonds between bases

Question 9

What are the names of each nitrogenous base?

Answer 9

Adenine
Guanine
Thymine
Cytosine
Uracil

Question 10

Which nitrogenous base binds with adenine? How many H bonds?

Answer 10

Thymine (DNA) and uracil (RNA)
2 H bonds

Question 11

Which nitrogenous base binds with guanine? How many H bonds?

Answer 11

Cytosine
3 H bonds

Question 12

Which bases are purines?

Answer 12

Guanine and adenine

Question 13

Which bases are pyrimidines?

Answer 13

Thymine, uracil, and cytosine

Question 14

What do purines look like?

Answer 14

Two ring structure

Question 15

What do pyrimidines look like?

Answer 15

One ring structure

Question 16

Why do purines only bind with pyrimidines?

Answer 16

To avoid bulkiness or concaving within the helix
Purine + purine = Bulky
Pyrimidine + pyrimidine = Concave

Question 17

What is the consequence of base pairing?

Answer 17

If the bases along one strand are known, the sequence of the bases along the other strand can be predicted

Question 18

Are the sequences of the two strands in DNA identical?

Answer 18

No, they are complementary

Question 19

What conformation does DNA usually assume?

Answer 19

B-DNA conformation

Question 20

What is B-DNA conformation?

Answer 20

The two strands are coiled in such a way that there are 10 base pairs in one complete turn of the helix.

Question 21

How far is each base pair from the next base pair in B-DNA conformation?

Answer 21

0.34 nm

Question 22

How long is each turn in the DNA helix?

Answer 22

3.4 nm

Question 23

In B-DNA, there are indentions on both sides of the helix. What are these called?

Answer 23

Major groove
Minor groove
This is due to the helix being asymmetrical

Question 24

What does each chromosome contain?

Answer 24

There is one DNA molecule in each chromosome

Question 25

What does DNA look like in bacteria?

Answer 25

Circular

Question 26

What does DNA look like in eukaryotic cells?

Answer 26

Linear

Question 27

How is the size of a DNA molecule indicated?

Answer 27

In base pairs

Question 28

What is the length of DNA in a typical bacterial chromosome?

Answer 28

1-10 million base pairs

Question 29

What is the length of DNA in a typical eukaryotic chromosome?

Answer 29

There wasnt a specific number but it sure is longer than bacterial chromosomes

Question 30

What of the length of the largest human chromosome, chromosome 1?

Answer 30

249 million base pairs

Question 31

What is the length of the smallest human chromosome, chromosome 21?

Answer 31

48 million base pairs

Question 32

What does the DNA within a chromosome's centromere consist of?

Answer 32

Repetitive DNA

Question 33

What is repetitive DNA?

Answer 33

Repetitive DNA consists of base sequences that are repeated anywhere from a few hundred to several million times within a DNA molecule

Question 34

Where is repetitive DNA found?

Answer 34

Often in telomeres and centromeres, but can also be found in other parts of the chromosome

Question 35

What percentage of human DNA in chromosomes is repetitive DNA?

Answer 35

59%

Question 36

What are telomeres?

Answer 36

Specialized regions located at the ends of a chromosome

Question 37

What repeated DNA sequence is found in telomeres?

Answer 37

TTAAGGG, which is repeated 500-3000 times

Question 38

What is found at the very end of the DNA molecule in telomeres?

Answer 38

A short region of about 12-16 nucleotides in length that is single-stranded DNA

Question 39

What proteins bind to the single-stranded DNA sequences in telomeres?

Answer 39

... Specific proteins?

Question 40

What is exonuclease?

Answer 40

An enzyme that degrades DNA. The specific proteins that bind to the single-stranded DNA sequences in telomeres prevent exonuclease to degrade the DNA.

Question 41

What happens to the size of linear DNA after each replication?

Answer 41

The size of the DNA gets shorter after each replication

Question 42

What sequences are lost during replication?

Answer 42

Shortening of DNA occurs at the ends of DNA, which are where telomeres are located. This means the sequences that are lost are repetitive sequences.

Question 43

What happens when too much repetitive DNA is lost during replication?

Answer 43

A cell will activate genes that result in the death of a cell

Question 44

What is telomerase?

Answer 44

An enzyme that can prevent shortening by replacing the repeat sequences lost during replication with new repeat sequences

Question 45

Where is telomerase present?

Answer 45

Germ cells Stem cells Tumor cells

Question 46

What are germ cells?

Answer 46

Reproductive cells

Question 47

What does telomerase do to germ cells?

Answer 47

It ensures that the zygote does not receive shortened chromosomes from the parent

Question 48

What are stem cells?

Answer 48

Somatic cells that can divide indefinitely

Question 49

What does telomerase do to stem cells?

Answer 49

The cells are present in most tissues of the body and function to produce new cells that will replace worn-out or damaged cells

Question 50

What are tumor cells?

Answer 50

Cancer cells

Question 51

What does telomerase do to tumor cells?

Answer 51

As cancer progresses, some tumor cells acquire mutations that activate telomerase and immortalize the cells. This allows these cells to survive and spread through the body.