WEEK 4 Flashcards
Detail the ‘Central dogma’ of molecular biology
DNA⇌DNA⇌RNA→Protein
dna to dna via replication, dna to rna via transcription, rna to dna via reverse transcription (reverse transcriptase in retrovirus), rna to protein via translation
What is DNA?
Deoxyribonucleic acid, double-stranded molecule that forms a helix
What is RNA?
Ribonucleic acid. single-stranded molecule with intra-molecular base pairing
What are the differences between DNA and RNA?
RNA has an OH group at ribose C2, DNA doesn’t
RNA uses uracil (U), DNA uses thymine (T)
Bases of DNA/RNA
Adenine Guanine Cytosine Thymine (DNA) Uracil (RNA)
What effect does RNA having an OH group at ribose C2 but DNA not having one have?
Causes RNA to be unstable in aqueous environments where DNA is more stable
Which are the purine bases?
Adenine and Guanine
Which are the pyrimidine bases?
Cytosine, Thymine and Uracil
How many H bonds between the respective purine and pyrimidine base pairs?
A-T=2 H bonds
G-C=3 H bonds
How do H bonds between base pairs aid DNA replication?
Allows easy replication as H bonds easily broken, so strands separate and complementary bases pair to form two molecules
What is a gene?
Sequence of DNA nucleotides
What is the genome?
An organism’s complete set of genetic information
Excluding genes, what are the other elements of the genome?
regulatory elements→’promoter’ and ‘enhancer’ regions
non-coding DNA→introns, removed from primary RNA transcript via splicing (exons=coding DNA)
repeat elements→regions that don’t endode a protein that occur commonly in tandem arrays
What is the method of DNA replication?
Semi-conservative
How is the DNA helix arranged?
contains two strands that run anti-parallel (5′ and 3′ ends at opposite ends of DNA molecule)
LOOK AT DNA STRUCTURE AND BONDING OF DNA MOLECULES
5′ end=triphosphate
3′ end=hydroxyl
What is the basic DNA building block?
deoxynucleotide triphosphate (dNTPs)
What are the enzymes for DNA synthesis?
DNA helicase-causes separation of DNA helix to form 2 strands DNA polymerase (1, 2 or 3)
Process of DNA synthesis
DNA helicase separates the two strands, H bond formed between complementary bases, a second dNTP comes in which is complementary to the next base so forms a H bond, then forms a covalent bond with the dNTP adjacent via DNA polymerase causing two phosphates to be lost
Characteristics of DNA polymerase
Proceeds in 5′ to 3′ direction
Adds 1000 bases/second to the chain
Requires dNTPs
Must have a template and an RNA primer→provides free 3′ end which receives incoming dNTP’s
What is the replication fork?
Contains leading and lagging strands on which DNA polymerase acts, DNA polymerase synthesis from 3′ end of the leading strand to the 5′ end, forming the 5′ to 3′ new strand-complementary to the leading strand, on the lagging strand, Okazaki fragments are formed in a 5′ to 3′ manner but from the 5′ end to the 3′ end via multiple DNA polymerase, ligation of strands via DNA ligase forms one big new strand complementary to the lagging strand
What initiates whole chromosome replication?
Origin proteins which form a replication ‘bubble’
How does DNA polymerase prevent errors in DNA replication?
It adds a base, moves back to check it and excises it if it’s wrong and moves on
What is the process of DNA forming RNA?
Transcription
What enzyme is needed for RNA synthesis?
RNA polymerase
What are the building blocks of RNA?
Adenine/Guanine/Cytosine/Uracil Triphosphate (ATP/GTP/CTP/UTP)
What is the directionality of RNA synthesis?
5′ to 3′
What is different from DNA replication and RNA synthesis?
Only one strand is copied in RNA synthesis (template/non-coding strand)
Process of transcription
Local DNA melting produces roughly 30bp bubble which moves along the DNA and extends the RNA chain, proteins involved in RNA processing bound to primary RNA transcript (spliceosome-removes introns from pre-mRNA)
What occurs between transcription and translation?
mRNA moves out the nucleus into the cytoplasm via nuclear pores
What is tRNA?
An adaptor molecule with 3 ‘hairpin’ stem-loop structures formed from intramolecular base pairing
What are the two distinct functional regions on tRNA?
Amino acid binding site (specific to anticodon) and anticodon site (complementary to specific codon)
What is an anticodon?
Trinucleotide sequence which is complementary to a codon found on mRNA-relates to DNA sequence
What is a codon?
Trinucleotide sequence found on mRNA which is complementary to DNA sequence
What is a polyribosome?
Multiple ribosomes found along the mRNA strand
What are the two sub-units of a ribosome?
40S and 60S
What is the function of ribosomes?
They are complexes that assemble strings of amino acids as introduced by the mRNA sequence
What are the two binding sites on ribosomes?
A-site→aminoacyl-tRNA binding site
P-site→peptidyl-tRNA binding site
What bonds are formed between AAs to form the polypeptide?
Covalent peptide bond on the C-terminus of the AA
What defines the start of the coding region?
Methionine (MET) AA initiates protein synthesis universally
Characteristics of the genetic code
read in groups of 3 bases
read in 5′ to 3′ direction
each AA is coded for by a codon
some AA coded for by more than one codon (degenerate)
3 codons don’t encode AAs (UAA, UGA and UAG are stop codons)
What does it mean for the genetic code to be non-overlapping?
Each base is read only once
What makes up the human genome?
DNA sequence on 1 chromosome from each of the 22 autosomal pairs and both X + Y (sex) chromosomes
Define the transcriptome
The complete set of RNA transcripts
Define the proteome
The complete set of proteins produced
What is the major difference between the genome and the transcriptome+proteome?
Genome is stable whereas the transcriptome+proteome are dynamic
What are blocks of tandem repeats called?
Satellite DNA
What are the two types of Satellite DNA?
mini satellite-highly polymorphic, up to 1000 copies in one block
micro satellite-small arrays of simple sequence repeats (usually intronic DNA)
Give three roles of multiple repeated sequences
Telomeres, centromeres and mini satellites
What is the role of telomeres?
Allow replication to the tip of chromosomes, deletions immediately below telomeres due to instability of long tracts of repeats
What is the role of centromeres?
Essential for segregation during cell division
Give issues that result from mini satellites
can cause mispairing during cell division to give:
large scale duplication/deletion between homologous chromosomes or translocation of DNA between non-homologous chromosomes
What is a chromosome?
A single DNA molecule
What is a gene?
Sequence of DNA nucleotides (arranged one after the other along a chromosome with stretches of non-coding DNA between→exon-intron-exon-intron-exon)
What are mitotic chromosomes formed from?
Highly packed chromatin
What is the difference between euchromatin and heterochromatin?
Euchromatin contains loose chromatin structure (active)
Heterochromatic contains tightly packed chromatin structure (inactive)
What is the purpose of mitosis?
cell division/organismal growth and genetically identical products
What occurs to chromosomes during interphase and mitosis?
Interphase=duplication via replication origin sites (doubles no. of chromosomes)
Mitosis and Cytokinesis=division and segregation of chromosomes into two genetically identical cells
What does a diploid cell (2n) form when it undergoes mitosis?
Two diploid cells (2n x2)
What are the stages of Mitosis?
(interphase), Prophase, Prometaphase, Metaphase, Anaphase, Telophase, (cytokinesis)
Describe Prophase in mitosis
chromosomes condense, nuclear envelope disappears, spindle assembles outside the nucleus-contains spindle pole and branching of microtubules (some pull DNA apart-kinetochore microtubules, some push microtubules from other pole away-interpolar microtubules)
Describe Prometaphase in mitosis
spindle microtubules bind to chromosomes via kinetochore that develops around the centromere
Describe Metaphase in mitosis
spindle gathers chromosomes to the equator, tension on kinetochores at centromeres facing opposite directions
What is the M-phase checkpoint?
occurs after Metaphase, ensuring all chromosomes attached to all microtubules-preventing non-disjunction (too many chromosomes on one pole, not enough on the other)
Describe Anaphase in mitosis
chromosomes at spindle poles, kinetochore microtubules begin to disappear, interpolar microtubules elongate to push apart poles
Describe Telophase in mitosis
nuclear membrane begins to reform, chromosomes decondense, nucleoli reappear
Describe Cytokinesis
initiation of plasma membrane cleavage splitting the cell into two via contraction of ring of actin and myosin filaments at cleavage burrows
What is the purpose of meiosis?
gametogenesis
What are the functions of meiosis?
reduction division (haploid cell-23 c'somes per gamete (22 autosomal and X/Y) re-assortment of genes (independent segregation of c'somes and crossing over)
What is the only difference seen in Meiosis I from Mitosis?
Crossing over occurs at Metaphase I
What are the two ways in which genetic variability is brought about during Meiosis?
Independent reassortment (possibilities of haploid cells=2^number of c'somes-each c'some may be from mother/father) Re-assortment via crossing over (exchange of genetic material between chromosomes when moving to the cell's equator-physical manifestations/link=chiasmata)
What are the two differences seen in Meiosis II from Mitosis?
Metaphase II→kinetochores point in opposite directions, not the same direction, which cause cohesions in the centromere to split, sister chromatids separate
Anaphase II→sister chromatids at spindle poles
Which requires a greater number of cell divisions to be produced: a human sperm cell or an egg cell?
Human sperm cell
When is Meiosis I completed in an egg cell?
Ovulation
When is Meiosis II completed in an egg cell?
Fertilisation
How is Meiosis used to create an egg cell with large amounts of content?
Every cell division forms a larger cell and a much smaller polar body which is non-functional due to most cytoplasm going to potentially fertilising egg
