WEEK 2: CHROMOSOMES AND GENES Flashcards
What is DNA?
-DNA is heritable material that codes for all
proteins inside a cell.
What are introns and exons?
-Eukaryote cells’ DNA has genes with introns
( non coding) and exons (coding regions of DNA)
-In eukaryote cells, where is the DNA found?
-In eukaryote cells, DNA is found in the nucleus
and mitochondria (in the cytoplasm)
How much % of the DNA codes for functional genes?
About 2% of human DNA codes for functional
genes while the rest does not
What does DNA stands for?
-DNA stands for Deoxyribonucleic Acid
DNA is made up of three types of molecules.
Name them.
1)A five carbon sugar called deoxyribose
2)A phosphate group
3)A nitrogen containing base (4)
Name the 4 nitrogen containing bases that make up a DNA.
Adenine
Cytosine
Guanine
Thymine
DNA is built from Nucleotides.
What are these nucleotides called?
-DNA is built from Nucleotides
-These nucleotides are called deoxyribonucleoside triphosphates (dNTPs)
What base does dCTP contain?
a. Adenine
b. Guanine
c. Cytosine
d. Thymine
Name the four nucleotides.
The four nucleotides are:
a. deoxyadenosine triphosphate (dATP)
b. deoxythymidine triphosphate (dTTP)
c. deoxyguanosine triphosphate (dGTP)
d. deoxycytidine triphosphate (dCTP)
Which carbons do triphosphates and bases attach on the deoxyribose?
-The triphosphates are attached to deoxyribose at the 5’ carbon.
-The bases are attached to the 1’ carbon
Describe the joining of Nucleotides into a DNA Strand.
What two groups link on the nucleotides to form a DNA?
What is lost during the reaction?
-Nucleotides are joined together by linking a phosphate to the sugar on the adjacent nucleotide
-The phosphate group in one nucleotide (5’ Carbon) attaches the 3’Carbon (containing the hydroxyl group) in the deoxyribose of another nucleotide
-This process results in the loss of the other 2 phosphates, releasing energy
-Any nucleotide can be adjacent to any nucleotide
-The DNS strand can contain thousands and billions of nucleotides strung together
What is the 5’ end?
What is the 3’ end?
Cell build DNA strands (during replication) form what end?
-One end of the strand has phosphates at the 5’ carbon of the first nucleotide.
This is called the 5’ end.
-The other end has a hydroxyl group at the 3’ Carbon of the last nucleotide.
This is called the 3’ end.
-Our cells always build DNA strands (during replication) from the 5’ end to the 3’ end.
How does DNA form double strands?
Through base pairing
State the base pairing rule.
How many hydrogen bonds are in each pair?
Which bond pair is stronger?
*Base Pairing Rule
A always pairs with T
G always pairs with C
*This is accomplished via hydrogen bonds
A-T pair has 2 hydrogen bonds.
G-C pair has 3 hydrogen bonds.
*G-C bond > A-T bond
What is DNA double stranded structure called?
Describe the structure?
*Double helix
*The strands twist around one another in a right-handed double helix like in a spiral staircase.
*The bases are perpendicular to the helix axis
Describe the strand directionality.
Strand Directionality
*The two paired strands are antiparallel to
each other
*This means that one strands goes from 5’ end to the 3’ end while the other strand goes from 3’ end to the 5’ end.
What is DNA replication?
The process of DNA duplication.
State the need for DNA replication.
When does it occur?
Step 1: Replication Fork Formation
*This is performed by an enzyme known as DNA helicase.
DNA helicase disrupts the hydrogen bonding between base pairs to separate the strands into a Y shape known as the replication fork. This area will be the template for replication to begin.
Step 2: Primer Binding
*The leading strand is the simplest to replicate.
*Once the DNA strands have been separated, a short piece of RNA called a primer binds to the 3’ end of the strand.
*The primer always binds as the starting point for replication. Primers are generated by the enzyme DNA primase.
Step 3: Elongation
Enzymes known as DNA polymerases are responsible creating the new strand by a process called elongation.
*DNA polymerase I, II, IV and V are responsible for error checking and repair.
- DNA polymerase III binds to the strand at the site of the primer and begins adding new base pairs complementary to the strand during replication.
*Because replication proceeds in the 5’ to 3’ direction on the leading strand, the newly formed strand is continuous
*The lagging strand begins replication by binding with multiple primers.
- Each primer is only several bases apart.
-DNA polymerase then adds pieces of DNA, called Okazaki fragments, to the strand between primers.
-This process of replication is discontinuous as the newly created fragments are disjointed.
Step 4: Termination
*Once both the continuous and discontinuous strands are formed, an enzyme called exonuclease removes all RNA primers from the original strands.
*These primers are then replaced with appropriate bases.
*Another enzyme called DNA ligase joins Okazaki fragments together forming a single unified strand.
*The ends of the parent strands consist of repeated DNA sequences called telomeres.
Telomeres act as protective caps at the end of chromosomes to prevent nearby chromosomes from fusing.
A special type of DNA polymerase enzyme called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA.
*Once completed, the parent strand and its complementary DNA strand coils into the familiar double helix shape.
- In the end, replication produces two DNA molecules, each with one strand from the parent molecule and one new strand.
The replication fork is bi-directional.
Describe the leading strand and lagging strand.
However, the replication fork is bi-directional; one strand is oriented in the 3’ to 5’ direction (leading strand) while the other is oriented 5’ to 3’ (lagging strand).
What is a primer?
Primers are short RNA molecules that act as templates for the starting point of DNA replication.
How many DNA molecules are produced at the end after replcation.
- In the end, replication produces two DNA molecules, each with one strand from the parent molecule and one new strand.
What are telomeres?
What are their functions?
Name the enzyme that catalyzes the formation of telomeres.
*The ends of the parent strands consist of repeated DNA sequences called telomeres.
Telomeres act as protective caps at the end of chromosomes to prevent nearby chromosomes from fusing.
A special type of DNA polymerase enzyme called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA.
State the function of the following enzymes in DNA replication.
1.DNA helicase
2.DNA Primase
3.DNA Polymerases
4.Topoisomerase / DNA Gyrase
5.Exonucleases
6.DNA ligases
*DNA helicase - unwinds and separates double stranded DNA as it moves along the DNA. It forms the replication fork by breaking hydrogen bonds between nucleotide pairs in DNA.
*DNA primase - a type of RNA polymerase that generates RNA primers.
*DNA polymerases - synthesize new DNA molecules by adding nucleotides to leading and lagging DNA strands.
*Topoisomerase or DNA Gyrase - unwinds and rewinds DNA strands to prevent the DNA from becoming tangled or supercoiled.
*Exonucleases - group of enzymes that remove nucleotide bases from the end of a DNA chain.
*DNA ligase - joins DNA fragments together by forming phosphodiester bonds between nucleotides.
How many base pairs does a human genome have?
How long is a DNA in one cell?
How long is the DNA in the whole body?
*Human genome = 3x10^9 base pairs
*A single haploid cell thus has 6x10^9 bp since a base pair is 0.34x10^-9meters, then 0.34x10^-9meters/bp x 6x109
bp=2.04m per cell
*Since there are about 50x1012 cells in the human body, then total DNA is about 100x10^12 meters long or 10^14meters long.
If total DNA is about 10^14meters long, how is it
hidden away in the human body?
That much DNA must be packaged and compressed somehow in the cell.
Describe the process of DNA packaging into a chromosome.
Step 1:
*Double stranded DNA helix forms a complex with nine protein molecules called histones (8 core histones plus H1 histone)
- Double stranded DNA is wrapped twice around the histones
Formed complex is called a Nucleosome.
A group of nucleosomes form DNA fibrils.
Step 2:
Groups of nucleosomes or DNA fibrils are supercoiled and packed together into 30nm chromatin fibers.
Step 3:
30nm chromatin fibers loops around to form loops that average 300nm in length. These uncondensed loops are bound to a protein scaffold.
Step 4: The 300nm chromatin fibers are compressed resulting in a 250nm wide and 700nm long chromatin fiber.
Step 5: The chromatin fiber is further compressed into
tight coils to produce chromatids of a chromosome.
What are histones?
List all the histones.
- They are rich in lysine and arginine residues, which give them positive charges that enable them to bind to the negatively charged DNA.
*H1, H2A, H2B, H3, H4
*8 core histones plus H1 histone
*H2-H4 ARE CORE HISTONES, THEY ARE PAIRED.
What are euchromatin and heterochromatin?
*Euchromatin is the part of the chromatin involved in the active transcription of DNA into mRNA.
-It is loosely bound nucleosomes and exposed DNA helical sequences that carry active genes accessible for transcription by polymerases and other regulatory protein complexes.
*Heterochromatin is a form of chromatin that is densely packed, it is condensed.
What is a karyotype?
A karyotype is a diagram or photograph of chromosomes
in an organism.
What information does a karyotype provide?
How are the chromosomes arranged?
A karyotype provides information on the number and
shapes of the chromosomes of an organism.
Can be used to identify :
- missing or additional chromosomes
- Chromosomal rearrangements or breaks
● Each chromosome is shown with its homologue
● Chromosomes are the stained and ordered by size
● Sex chromosomes are shown last even though they are
not the smallest
Name the disorders.
1.Trisomy 21
2.Trisomy 18
3.XXY
4.XYY
Trisomy 21: Down Syndrome
Trisomy 18: Edward Syndrome
XXY Karyotype: Klinefelter Syndrome
XYY Karyotype: Jacob’s Syndrome
Characteristics
● Hypogonadism
● Decrease in testiculat size from puberty
● Poor facial hair growth
● Educational difficulties are common
What disorder is this?
Klinefelter
● Growth deficiencies
● Clenched hands with overlapping digits
● Cardiac defects
● Renal abnormalities
What disorder is this?
Edward’s syndrome
State characteristics of Down Syndrome.
Characteristics
● Small hands and feet
● Flattened face
● Poor muscle tone
● Heart Defects
State the abnormal numbers of sex chromosomes.
Abnormal numbers of sex chromosomes: Single X,
triple X chromosomes, double Y chromosome.
What is trisomy?
Trisomy: Incorrect separation of chromosomes
during meiosis can lead to germ cells (usually ova)
with two homologous autosomes:- three copies of
an autosome.
State the 4 types of genetic mutation.
*Deletion
*Duplicate
*Inverse
*Translocation
How is a mitochondrial DNA inherited?
Carried by ovum and is inherited maternally.
What can mutations in Mitochondrion genes lead to?
1) Liver Diseases
2) Hearing impairments
3) Visual Impairments
4) Cognitive Impairments
