Introduction to DNA Flashcards
Explain the ultrastructure of DNA on ‘beads on a string’.
DNA is wrapped around 8 histones also called a histone core. This structure is called a nucleosome. The DNA between the nucleosomes are genes that are expressed and can be read.
How come the DNA wraps itself around the histone core?
Because the histones are positively charged and the DNA is negatively charged.
What other even more dense form of packaging exist?
The solenoid.
What is heterochromatin?
DNA that is in the packaging form of a solenoid (30 nm fibre). These genes are not expressed and cannot be read. The heterochromatin on a TEM is stained much darker than euchromatin.
What is euchromatin?
The DNA that is in the packaging form of ‘beads on a string’. These genes are expressed and can be read. The euchromatin on a TEM is stained much lighter.
Explain the ultrastructure of chromosomes.
They are tightly packed solenoids with a p-arm and a q-arm. Each chromosome has two chromatids.
Explain briefly what genes are.
Genes carry the code for proteins.
Where can you find the genes?
They have a chromosomal location. Certain genes can be found on certain chromosomes.
How many genes can be found in a human?
Around 25000
Define the genome.
The genome is the entire DNA sequence of the 24 chromosomes. 22 autosomes + sex chromosomes X and Y.
It is the total genetic information of an organism.
Explain the structure of the DNA.
It’s a nucleic acid which are polynucleotides = linear polymers of nucleotides.
What are nucleotides?
A nitrogenous base with a pentose sugar and a phosphate group.
What are nucleosides?
A nitrogenous base with a pentose sugar. NO phosphate group!
What are the two types of pentose sugar? How do they differ?
Ribose which can be found in RNA.
2-deoxyribose where there is an absence of the oxygen that can be found on ribose on the second carbon.
2-deoxyribose can be found in DNA.
What is the charge of the phosphate group?
It’s negatively charged so it can bind to the histones.
There are two types of nitrogenous bases (structurally). Which? How do they differ?
Purines which have a 2 ring structure
Pyrimidines which have a 1 ring structure
What are the two types of purines in DNA and RNA?
Adenine and guanine
What are the three types of pyrimidines found in DNA and RNA?
Cytosine, thymine and uracil
What are nucleotides joined via?
Phosphodiesterbonds
What does the polymer chain of DNA or RNA start with at the 5’ end?
A phosphate group
What does the polymer chain of DNA or RNA end with at the 3’ end?
An OH group.
What can be found at the 5’ end?
Phosphate group
What can be found at the 3’ end?
an OH group.
What types of nitrogenous bases bind to which?
Pyrimidines bind to purines.
A binds to T
G binds to C
A also binds to U
What are nitrogenous bases joined via?
Via hydrogen bonds.
How many H-bonds does a G-C binding have?
3
How many H-bonds does a A-T binding have?
2
How many H-bonds does a A-U binding have?
2
How is the DNA sequence usually depicted? (Read and written)
Always written and read from 5’ to 3’ and left to right. So from phosphate group to OH group.
What would be the complementary strand of 5’ACCTCG3’?
3’TGGAGC5’ since it is antiparallel. If you want to write it from 5’ to 3’ it would b 5’CGAGGT3’
What are the different steps in the cell cycle?
(Interphase) G1 S G2 and (Division) M
What happens in G1 interphase?
The cell content is replicated. All organelles and cytoplasm duplicate.
What happens in S interphase?
DNA replication
What happens in G2 interphase?
There’s a double check where everything that has gone wrong goes under reparation.
What happens in M division?
The cell divides into two new daughter cells.
What are the three steps of DNA replication?
Initiation
Elongation
Termination
Explain initiation.
There’s a recognition of the origin of replication.
This requires DNA polymerase and other specific proteins.
It also requires primate to kick-start the replication.
In which direction does DNA polymerase read?
In 3’ to 5’.
In which direction is the new strand formed?
In 5’ to 3’ meaning antiparallel to the template strand.
What is the leading strand?
The 5’3’ strand that is read by the template strand that goes from 3’ to 5’.
What is the lagging strand?
The 5’3’ strand that is read by the template strand that goes from 5’ to 3’.
How can the DNA polymerase read the template strand that goes rom 5’ to 3’?
It reads it in reverse by the use of primase. Primers are added along the way so that the DNA polymerase can read in the right direction backwards. This forms Okazaki fragments that are then connected by DNA ligase.
Explain elongation.
This is what has previously been discussed. Also how the DNA helix unwinds.
How does the DNA helix unwind in order for more replication to occur?
By DNA helicase.
When does DNA replication terminate?
Replication occurs all over the DNA strand so not just in one ‘end’ or something. DNA replication stop when two forks meet each other. Because the direction of replication on one fork might be to the right and the other one to the left.
What does the one chromosome = one DNA molecules, one chromosome = two DNA molecules mean?
Before replication one chromosome is only one DNA molecule or one chromatid.
After replication one chromosome consist of two DNA molecules or two chromatids.
