SESSION 2 Flashcards
What are the two types of nuclei acids?
- DNA: double stranded
- RNA: single stranded
What are the two types of pentode sugars?
- ribonuclease acid (RNA contains ribose)
- deoxyribonucelic acid (DNA contains 2- deoxyribose)
What is the difference between a nucleoside and nucleotide?
Nucleoside= base + sugar Nucleotide = base+ sugar + phosphate
What is the structure of DNA?
- nucleotides are joined via phosphodiester bonds
- chains have polarity: 5’- 3’
- two polynucleotide strands run antiparticle to each other
- the antiparallel strands twist to form a right- handed soluble helix
- the strands are held by hydrogen bonds between the nitrogenous bases
- the nitrogenous bases in DNA store the instructions fro making polypeptide chains
What are the two types of nitrogenous bases?
- purine: two ring structures
- pyrimidine : one ring structure
What are the two types of purines in both RNA and DNA?
- adenine
- guanine
What are the three main pyrimidines?
- cytosine
- uracil (RNA)
- thymine (DNA)
State the complementary base pairs
A- T base pair = 2 hydrogen bonds
A- U base pairs = 2 hydrogen bonds
G- C base pairs = 3 hydrogen bonds
What are genes?
A gene is a short section of DNA
Each gene codes for a specific protein
Genes have a chromosomal location
What is genome?
Genome is the entire DNA sequence
Human genome is 24 chromosomes
- 22 autosomal
- 2 sex chromosomes
Explain the polarity of a DNA or RNA chain
Polynucleotides
Nucleotides are covalently linked via phosphodiester bonds
Each single- strand nuclei acid chain has a polarity
Two distinct ends- 5’ end with free phosphate and a 3’ end with a free - OH
Explain the importance of hydrogen bonding and base- pairing in defining nuclei acid secondary structure
O atoms in carbon ring are electronegative - N group is dipole positive (H- bonding)
Interaction occurs between electronegativity of oxygen and dipole of nitrogen
Base pairing occurs when hydrogen bonds from between nucleotides on different strands
RNA stem- loop structure
Single stand loop back on itself thus one side will run antiparallel and hydrogen bonds will from between complementary bases
Describe the key features of the DNA double helix
- 2 independent polymers
- entirely complimentary and antiparallel to each other (top strand 5’- 3’ bottom 3’- 5’)
- one complete turn is 10 base pairs
- space between base pairs is 0.34nm
- purines and pyramid Ines are planar and unsaturated
- within structure of sugar- phosphate backbone, major (exposed bases) and minor grooves exist
Explain how eukaryotic DNA is condensed in nucleosomes and relate this to the structure of chromosomes
During S phase DNA is replicated
Chromatin is present in interphase- ‘beads on a string’ (nucleosomes)
Nucleosomes- DNA is wound twice round histone core, which is charged
Each nucleosomes is coiled to form solenoid structure
Mitotic chromosomes are highly condensed fibres
Genome is unexpressed cannot replicate as condensed
Interphase chromosomes (chromatin) de- condensed (long thin beads on a string)
Genome is expressed therefore can replicate
Describe the process and role of DNA replication
Initiation:
Recognition of an origin of replication
Helical unravels the DNA double helix- hydrogen bonds between bases are broken
Enzyme primase is required to initiate each strand
The new nucleotides are bonded together by the enzyme DNA polymerase
Elongation
Leading strand is replicated from 5’- 3’
The lagging strand is replicated discontinuously in Okazaki fragments
These fragments are joined by the enzyme ligase from OH group to phosphate group covalently
Termination-
Replication forms two new DNA molecules - double helix ashes
The new molecules contain one strand of the original DNA and one new strand
This type of replication is called semi- conservative replication
Explain the role of DNA polymerase and other enzymes in DNA replication
Requires activated precursors/ substrates: dNTPS:
- dATPs
- dCTPs
- dTTPs
- dGTPs
Each region- driven by the hydrolysis of ATP- extends the existing chain by one unit
Chain growth is 5’- 3’
Helical enzyme
Uses energy of ATP to unwind the DNA helix and allow DNA replication
Around the replication fork helicases reform the DNA helix
Describe the process and the role of cell cycle
Mitotic division is used to repair and maintain the body
Interphase
G1- stage begins immediately after mitosis
RNA, proteins and organelles are synthesised
S- DNA is replicated and chromosomes form sister chromatids
G2- mitochondria divide, precursors of spindle fibres form
Mitosis:
Prophase- chromosomes condense
Nuclear envelop disappears
Centrioles divide and migrate to opposite poles
Spindle fibres form and attach to chromosomes
Metaphase- chromosomes line up on the middle of the dividing cell
Anaphase- chromosomes begin to separate
Telophase- chromosomes each opposite poles
New nuclear envelop
Chromosomes de- condense
Cytokinesis
Cleavage furrow forms and deepens
Cytoplasm divides
Describe chromosome structure
One chromosome = one DNA molecule
One chromosome= two DNA molecules (sister chromatids)
The classical X shape is a replicated chromosome containing two sister chromatids
Q arm telomere= long arm
P arm centromere- short arm
Define telomere and state its function
Telomeres are repeated sequences at each end of chromosome
Function:
Protecting the molecules
Define centromere
Centromere link the sister chromatid and consists of repetitive sequences
Different chromosomes have different centromere positions
What are the different centromere positions on chromosomes?
Metacentric- middle
Submetacentric- between middle and end
Acrocentric- close to end
Telocentric- at the end
Define exonuclease
An enzyme which removes successive nucleotides from the end of a polynucleotide molecule
Define endonuclease
An enzyme which cleaves a polynucleotide chain by separating nucleotides other than the two end ones
What is the function of an exonuclease domain in a DNA polymerase?
Exonuclease is responsible for recognition and excision of mis paired bases- proof- reading
