DNA Replication Flashcards
Describe Nucleic Acids
macromolecules that encode genetic material and direct gene expression
What are the two nucleic acids used by human cells?
Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA)
What are the monomers of Nucleic Acids?
Nucleotides
What are the three parts a Nucleotide consists of?
A phosphate, a pentose sugar and a nitrogenous base
Describe a Pentose sugar
It contains 5 carbons, and is used in both DNA and RNA
it is different from most sugars in our diet, which are hexose (6C)
Which sugar is used in DNA?
Deoxyribose
Which sugar is used in RNA?
Ribose
What is the difference between Deoxyribose and Ribose sugar?
Ribose has an -OH on carbon 2
Deoxyribose has an H on carbon 2
(hence de oxy)
Describe Phoshodiester Bonds and what do they result in making?
The C5 of each sugar bonds do a phosphate molecule, then react with the OH group on the C3 of another nucleotide sugar
they result in making the Sugar Phosphate Backbone
What are the four nucleotide bases?
Adenine (A), Thymine (T), Cytosine (C) and Guanine (G)
Where do the bases bond?
they bond to the C1 of the nucleotide sugar and project perpendicular to the backbone
What is the primary structure of DNA?
The sequence of nucleotide bases in a single strand of DNA
(think primary as 1D)
What are the two types of bases?
Purine and Pyrimidines
Describe Purines
The largest bases, containing two rings
adenine and guanine are purines
Describe Pyrimidines
contain only in ring, making them smaller than purines
Thymine and Cytosine are pyrimidines
What do all bases contain?
Nitrogen, making them nitrogenous
Describe Complimentary Base Pairing
The bases of one DNA strand bond to the bases of another strand, however each base can only bond with its compliment
a purine must always bond to a pyrimidine to keep the space constant
A+T and G+ C
What bond are formed between bases and how are they formed?
Hydrogen Bonds
formed when an atom (polar positive) of one base is attracted to the nitrogen of oxygen (polar negative) of another base
What makes hydrogen bonds different from ionic and covalent bonds?
they are weaker and can be broken easily
Describe the amount of hydrogen bonds in between the bases
Adenine and Thymine form 2 hydrogen bonds
Cytosine and Guanine form three hydrogen bonds
What is the secondary structure of DNA?
Complimentary Base Pairing
Describe Antiparallel
Complimentary DNA strands are antiparallel
this means that the sugar phosphate backbones face opposite directions
Describe the Antiparallel Strands
one strand is aligned 5’ to 3’ while the complimentary strand is aligned 3’ to 5’
What part of the DNA is hydrophilic and what part is hydrophobic?
The sugar phosphate backbone is hydrophilic (likes water)
the bases are hydrophobic
Why does DNA form a double helix?
to keep bases protected in the middle of the helix while the sugar phosphate backbone faces outwards
the phosphates are attracted to the hydrogens of the sugars on the following twist of the helix
When does DNA complete one turn of the helix?
every 10 base pairs
Describe the Minor Groove
the smallest groove that occurs where the overlapping backbones are closest together
Describe the Major Groove
occurs where the gap between backbones is greatest
allows easy access for Protein Attachment
What is the Tertiary Structure of DNA
the double helix
What do DNA attach to that cause supercoiling?
Histone Proteins
which spool up DNA like a fishing reel to keep it compact inside the nucleus
Describe Chromatin
the combination of DNA and histone proteins
What is the Quaternary Structure of DNA?
Chromatin
(DNA + histones)
What are the three steps of DNA Replication?
initiation
elongation
termination
where does DNA replication occur and what are the three things it requires?
occurs in the nucleus
requires
1. pre-resisting DNA
2. free nucleotides
3. enzymes
Describe Initiation
the first step of DNA Replication
involves unwinding the double helix and breaking hydrogen bonds
What does the enzyme Topoisomerase do?
unwinds the double helix
which exposes the bases and makes it easier for other enzymes to do their job
What does the enzyme Helicase do?
breaks apart the hydrogen bonds that are holding the double helix together, separating the two strands
referred to as “unzipping” the DNA
Where does Helicase Attach?
to specific sites in the DNA called “origins of replication” or “Ori”
What does Primer do?
after the DNA is unzipped, primers attach
primer is a short sequence of RNA that serves as a starting point for DNA synthesis (making a new strand)
(primer is usually about 20 base long)
What is different about Primer compared to other enzymes?
Primer is temporary and will be replaced by nucleotides in the next stage of replication
Describe Elongation
second stage of DNA replication
stage where the new DNA stands are synthesized (made)
Describe how DNA replication is semi-conservative
the resulting DNA is a combination of Parent strand (pre existing DNA) and Daughter Strands (new DNA)
each copy of DNA has 1 parent strand and 1 daughter strand
Where do Free Nucleotides Attach?
Free Nucleotides temporarily attach to the open DNA strand using hydrogen bonds
What does the enzyme DNA Polymerase do?
DNA Polymerase synthesizes the daughter strands by fusing nucleotides together using the sugar phosphate backbone
What is the Replication Fork?
the opening in the DNA behind helicase
Describe the Leading Strand
the daughter strand in the replication fork being synthesized in the 5’ to 3’ direction
DNA polymerase follows continuously behind helicase when forming the leading strand
Describe the Lagging Strand
the daughter strand in the replication fork being synthesized in the 3’ to 5’ direction
What is the issue with DNA polymerase and the Lagging Strand?
DNA polymerase can only add nucleotides in the 5’ to 3’ direction, which is the opposite of the Lagging strand
This means that DNA Polymerase must synthesize behind the lagging strand in the OPPOSITE direction that helicase works
Describe how DNA Polymerase works on the lagging strand
DNA polymerase synthesizes roughly 100 nucleotide sequences at a time before detaching and returning to the replication fork to begin a new sequence
What are Okazaki Fragments?
the short sequences of lagging strand DNA
What does the enzyme DNA Ligase do?
since DNA polymerase can only add one nucleotide at a time, DNA Ligase is required to fuse the Okazaki Fragments together, forming a continuous Lagging Strand.
How do Mutations Occur?
When DNA polymerase fails to catch a mis-matched nucleotide pair while proofreading the daughter strands, resulting in an error
Describe Termination
the third and final stage of DNA Replication
where the DNA copies are finalized and safeguarded from degradation
Describe Termination Sites
Sites along the parent strands that direct helicase and DNA polymerase to detach, ending the Replication Fork
Where do Termination Sites occur?
Before the last nucleotide in a DNA Strand
this means that DNA is lost every time DNA replicates
Describe Telomeres
long sequences of DNA that protect the chromosome from damage
(roughly 15,000 base pairs long at birth)
human telomeres consists of the sequence TTAGGG being repeated
What does the enzyme Telomerase do?
add telomeres (repetitive sequences of base pairs) after the final replication fork
Telomerase do not code for any proteins and do not contain any hereditary information
Describe how genetic information gets lost in Termination
once a telomere is used up due to excessive replications, genetic information begins to get lost, severely impacting the health of the organism
What happens to chromosomes that no longer possess telomeres?
They can’t replicate, meaning that cells can’t heal, grow or reproduce
Describe the Hayflick Limit
the number of times that DNA can reproduce before using up its telomeres
Describe Senescence
the process of shortening your telomeres over time
(cellular aging)
