T4 - Introduction to DNA Flashcards
structure of DNA
- DNA is double helix
- has minor and major grooves
- this is because the spacing of the grooves is uneven
- has antiparrallel complimentary DNA strands
- base pairing of strands by hydrogen bonding
- phosphodiester bonds holding sugar phosphate backbone together
- polynucleotides = linear polymers of nucleotides
describe how eukaryotic DNA is packaged into chromosomes
DNA has to be tightly packed in nucleus as each cel contains one copy of all the genes int he body (although they’re not all activated within each cell)
- DNA is wound around histone protein to form nucleosome
- in between different histone proteins, there are linker DNA of nucleosome, linking different histone proteins together
- DNA coils around many histones, like beads on string
- histones coil around each other to save space, forming solenoid
- further coiling and condensation forms the chromatin
- chromatin condense further to form chromosome
DNA has to be unwound to be copied during mitosis
what are histones, euchromatin and heterochromatin
- DNA is wound around histone proteins to condense
- DNA wound around histone protein = nucleosome
- in between different histone proteins, there are linker DNA of the nucleosome, linking different histone proteins together
- histone core of nucleosome is the histone protein
- histone protein is positive so it can bind to negatively charged DNA
- histones are octamers (consist of 8 different proteins)
euchromatin
- when DNA is loosley wound
- appears lighter on electron micrograph
- open configuration that permits DNA transcription
- this is because transcription factors are able to bind to DNA
heterochromatin
- DNA is tightly wound around histones
- appears darker + denser on electron micrograph
- doesn’t allow DNA transcription
- gene is turned off
changes between heterochromatin and euchromatin during the cell cycle
what is the genome
the complete set of genetic material in an organism
- genome contains the entire DNA sequence
- human genome contains 24 chromosomes
- this consists of 22 autosomes and 2 sex chromosomes
- each person has 46 chromosomes in total
- humans are diploid so contain 2 copies of each chromosome
XX = female (homogametic)
XY = male (heterogametic)
what is the difference between nucleoside and nucleotide
nucleoside
base + sugar
nucleotide
base + sugar + phosphate
*eg ribonucleotide (adenosine monophosphate) and deoxyribonucleotide (deoxyadenosine monophosphate)
both DNA and RNA are polynucleotides, but RNA has ribose sugar, whereas DNA has deoxyribose sugar
what is the difference between nucleoside and nucleotide
nucleoside
base + sugar
nucleotide
base + sugar + phosphate
both DNA and RNA are polynucleotides, but RNA has ribose sugar, whereas DNA has deoxyribose sugar
what are the two types of nitrogenous bases
purine
- double ring (therefore bigger)
- adenine DNA/RNA
- guanine DNA/RNA
pyrimidine
- smaller, single ring
- cytosine DNA/RNA
- uracil RNA
- thymine DNA
purine can only bind to pyrimidine when base pairing occurs - this is to maintain a similar size of molecule across the DNA molecule, as purines are larger than pyrimidines
types of nucleic acid in RNA
name base, nucleoside and nucleotide
Base nucleoside nucleotide
- Adenine adenosine adenosine monophosphate (AMP)
- Guanine guanosine guanosine monophosphate (GMP)
- Uracil uridine uridine monophosphate (UMP)
- Cytosine cytidine cytidine monophosphate (CMP)
types of nucleic acid in DNA
name base, nucleoside and nucleotide
Base nucleoside nucleotide
- Adenine deoxyadenosine deoxyadenosine monophosphate (dAMP)
- Guanine deoxyguanosine deoxyguanosine monophosphate (dGMP)
- Cytosine deoxycytidine deoxycitidine monophosphate (dCMP)
- Thymine deoxythymidine *deoxythymidine monophosphate (dTMP)
how many hydrogen bonds between each of the bases and whether they are are purines or pyrimidines
adenine–thymine (DNA)
adenine–uracil (RNA)
guanine—cytosine
C,T,U = pyrimidine
A,G = purine
describe the structural components of nucleic acids
- polynucleotides
- nucleotides are joined by phosphodiester bonds
- the chain has polarity (5’ to 3’) which is based upon the position of the carbon atoms in the background
- the chain has distinct ends 5’ (P, start) and 3’ (OH, end)
phosphodiester bond joins phosphate group of one unit and binds it to the sugar backbone of another unit
which direction are DNA strands written in by convention?
from left to right, in 5’ (P) to 3’ (OH) direction
during what stage of the cell cycle does the DNA get replicated
the S phase
- aka synthesis
- part of interphase
- point during which all the chromosomes are duplicated
- daughter cells have identical DNA from mitosis
DNA replication
- heterochromatin can be relaxed to euchromatin state
- DNA unwound by helicase to point of replication fork
- multiple origins of replication form, making replication bubbles
- on leading strand (3’) DNA polymerase binds to origin ★
- on lagging strand, primase makes short RNA primer to allow DNA polymerase to bind
- ★ DNA polymerase moves along leading strand in 3’ → 5’ direction, forming new + long continuous DNA
- on lagging strand, have to copy in very short segments, forming okazaki fragments
- 3’ end of lagging strand is close to replication fork, so only a short sequence can be copied by DNA polymerase at one time, as it needs 3’ end
- multiple different DNA polymerases have to bind in sequence to make short okazaki fragments
- DNA ligase stitches okazaki fragments together
- newly formed DNA is wound up into helix, leading to 2 perfect copies of original piece of DNA
- eventually two replication bubbles meet ★
- ligase will stitch different pieces of DNA together
★ 1- initiation 2- elongation 3- termination
what are the main enzymes involved in DNA replication
- Helicase - unwinds DNA double helix until replication fork
- Primase - creates new RNA 3’ primer so that polymerase can bind
- Polymerase - forms phosphodiester bonds to create new DNA strands
- Ligase - stitches okazaki fragments together from the lagging strand
