topic 1.1 central dogma Flashcards
what is a gene?
segment of DNA that encodes a polypeptide chain or RNA molecule
what is a sequence of each gene?
provides information to form a product (typically a protein, but NOT ALL genes code for proteins)
what is central dogma?
- flow of genetic information in cells
- replication
- transcription
- translation
what is replication?
- 2 strands of nucleotides separate (double helix/1 strand of DNA)
- the strands act as “templates” for other free nucleotides to bind to
- 1 strand = 1/2 parent DNA and 1/2 new DNA
- creating 2 identical daughter strands
what is transcription?
- RNA is synthesised
- 1 strand of DNA acts as the “template” for the synthesis of a complementary strand of RNA
- RNA polymerase “reads” the bases of the DNA template to form a RNA transcript (a string of RNA nucleotides/mRNA)
what is translation?
- the mRNA strand is like a set of “instructions” denoting how to line up a chain of AA to obtain a certain protein.
- ribosomes “read” mRNA nucleotides in groups of 3 (each group of 3 ribonucleotides = 1 codon)
what is 1 codon?
- a string of 3 consecutive ribonucleotides (triplet codon)
- “codes” for only 1 type of amino acid (but 1 type of AA can be “coded” by many codons)
unambiguous
- each codon specifies a single amino acid only
degenerate
- a given amino acid can be specified by more than 1 codon
- this is the case for 18 out of 20 standard AA. only 2 AA are specified by only 1 codon
start and stop codons
- for initiation/termination of a translation (does not code for any AA)
DNA complementary base pairing
- Guanine (G) and Cytosine (C) - 3 H bonds
- Adenine (A) and Thymine (T) - 2 H bonds
- in RNA, Thymine (T) is changed to Uracil (U)
- each base can only bond with a specific base partner.
chromosomal proteins
- histones
- nonhistone chromosomal proteins (not the focus of this syllabus)
- chromosomal protein + DNA = chromatin
first level of chromosome organisation: nucleosome
- “beads on a string”
- involves histones
- 1 strand of DNA (double helix/2 complementary strands of nucleotides) wraps around 8 histone molecules + 1 more histone molecule attached on the outside = nucleosome
Linker DNA
- DNA strand that links 1 nucleosome to the next
- can vary in length depending on the need of the cell at the time
- Shorter linker DNA = more tightly packed nucleosomes
- Longer linker DNA = less tightly packed nucleosomes
How does DNA wrap around the core?
- the sugar-phosphate backbone of DNA and AA histones are able to form H bonds
- AA are the building blocks of protein/polypeptides - some chains have hydrogen atoms which allows for the formation of a H bond
Further levels of organisation: nucleosomes packed on top of one another
- histone proteins (chains of AA) have “tails” that extend out from the DNA-histone cores
- allows the nucleosomes to interact with one another
- linker DNA connects the nucleosomes
- other nonhistone chromosomal proteins also help with packing the DNA
Forms of chromatin in interphase (resting phase of nuclei, no cell division)
Heterochromatin and euchromatin
what is heterochromatin?
- highly condensed form of chromatin
- 10% of genome in mammalian cells
- present in many locations along chromosomes
- concentrated in centromere (cell division) and telomere (DNA replication)
- does not contain genes as they are very tightly compact and resistant to gene expression (hard to tear apart to access the gene sequences)
Why is there a need for heterochromatin if the gene sequences cannot be accessed?
- Centromere and telomeres play important roles
- prevent them from being damaged by the surroundings/mutated
- heterochromatin is hard to tear apart and acts as protection for the important sequences
what is euchromatin?
- Less condensed form of chromatin
- majority of genome in eukaryotic cells
- Contains genes
- Can be easily unfolded and transcribed into RNA products
Why can’t DNA remain compacted all the time?
- access to the DNA is required during replication, repair and gene expression
Euchromatin can transform to heterochromatin if needed by the cell. Why?
- At different times, due to the fluctuating external environment, the cell’s needs may change
- thus, different sets of genes must be exposed to be expressed
- Euchromatin is able to transform to heterochromatin to control gene expression
Why must nucleosome structures be re-modeled when a certain gene needs to be expressed?
- To better expose the section of gene needed to be expressed, the nucleosome structures can be re-modeled
What are chromatin remodelling complexes?
- PROTEINS
- loosens DNA-histone contacts
- changes positions of nucleosomes along DNA (depending on whether DNA needs to be expressed/accessed by other proteins)
- Re-form nucleosomes when access to DNA is no longer required
*comes into play when euchromatin transforming to heterochromatin / nucleosomes are being re-modelled
