DNA Structure Flashcards
How many strands does DNA have?
Two strands
Double-stranded
What is the alignment of DNA strands?
Antiparallel alignment
What does the antiparallel alignment look like?
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.
What is a DNA helix?
The two strands of DNA coiled around each other. The DNA helix is asymmetrical.
How is a DNA helix oriented?
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.
What is the overall charge of the DNA?
The backbone facing outside of the helix causes DNA to have an overall negative charge
How are the two strands of DNA held together?
Hydrogen bonds
What is base pairing?
The formation of hydrogen bonds between bases
What are the names of each nitrogenous base?
Adenine
Guanine
Thymine
Cytosine
Uracil
Which nitrogenous base binds with adenine? How many H bonds?
Thymine (DNA) and uracil (RNA)
2 H bonds
Which nitrogenous base binds with guanine? How many H bonds?
Cytosine
3 H bonds
Which bases are purines?
Guanine and adenine
Which bases are pyrimidines?
Thymine, uracil, and cytosine
What do purines look like?
Two ring structure
What do pyrimidines look like?
One ring structure
Why do purines only bind with pyrimidines?
To avoid bulkiness or concaving within the helix
Purine + purine = Bulky
Pyrimidine + pyrimidine = Concave
What is the consequence of base pairing?
If the bases along one strand are known, the sequence of the bases along the other strand can be predicted
Are the sequences of the two strands in DNA identical?
No, they are complementary
What conformation does DNA usually assume?
B-DNA conformation
What is B-DNA conformation?
The two strands are coiled in such a way that there are 10 base pairs in one complete turn of the helix.
How far is each base pair from the next base pair in B-DNA conformation?
0.34 nm
How long is each turn in the DNA helix?
3.4 nm
In B-DNA, there are indentions on both sides of the helix. What are these called?
Major groove
Minor groove
This is due to the helix being asymmetrical
What does each chromosome contain?
There is one DNA molecule in each chromosome
What does DNA look like in bacteria?
Circular
What does DNA look like in eukaryotic cells?
Linear
How is the size of a DNA molecule indicated?
In base pairs
What is the length of DNA in a typical bacterial chromosome?
1-10 million base pairs
What is the length of DNA in a typical eukaryotic chromosome?
There wasnt a specific number but it sure is longer than bacterial chromosomes
What of the length of the largest human chromosome, chromosome 1?
249 million base pairs
What is the length of the smallest human chromosome, chromosome 21?
48 million base pairs
What does the DNA within a chromosome’s centromere consist of?
Repetitive DNA
What is repetitive DNA?
Repetitive DNA consists of base sequences that are repeated anywhere from a few hundred to several million times within a DNA molecule
Where is repetitive DNA found?
Often in telomeres and centromeres, but can also be found in other parts of the chromosome
What percentage of human DNA in chromosomes is repetitive DNA?
59%
What are telomeres?
Specialized regions located at the ends of a chromosome
What repeated DNA sequence is found in telomeres?
TTAAGGG, which is repeated 500-3000 times
What is found at the very end of the DNA molecule in telomeres?
A short region of about 12-16 nucleotides in length that is single-stranded DNA
What proteins bind to the single-stranded DNA sequences in telomeres?
… Specific proteins?
What is exonuclease?
An enzyme that degrades DNA. The specific proteins that bind to the single-stranded DNA sequences in telomeres prevent exonuclease to degrade the DNA.
What happens to the size of linear DNA after each replication?
The size of the DNA gets shorter after each replication
What sequences are lost during replication?
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.
What happens when too much repetitive DNA is lost during replication?
A cell will activate genes that result in the death of a cell
What is telomerase?
An enzyme that can prevent shortening by replacing the repeat sequences lost during replication with new repeat sequences
Where is telomerase present?
Germ cells
Stem cells
Tumor cells
What are germ cells?
Reproductive cells
What does telomerase do to germ cells?
It ensures that the zygote does not receive shortened chromosomes from the parent
What are stem cells?
Somatic cells that can divide indefinitely
What does telomerase do to stem cells?
The cells are present in most tissues of the body and function to produce new cells that will replace worn-out or damaged cells
What are tumor cells?
Cancer cells
What does telomerase do to tumor cells?
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.
