Cell Cycle 1

define gene

transcriptional unit that encodes a protein

central dogman

describes the one-way flow of genetic information within a biological system: DNA → RNA (transcription) → Protein (translation)

DNA v RNA

DNA is double stranded, stable hereditary storage (sugar has hydrogen)

RNA is single stranded, transient info carrier with a sugar that has a 2 prime hydroxyl making it more reactive and less stable

Location of transcription

nucleus (also in mitochondria)

what does transcription do?

produces protein coding mRNA and several non messenger RNAs (regulate gene expression and transfer RNA)

List the steps of transcription:

1. initiation and promoter recognition

2. elongation and template usage

3. termination

describe initiation and promoter recognition

RNA polymerase (protein enzyme) is recruited to promoter located upstream (5’); promoter DNA doesn’t code for protein but recruits transcriptional machinery

describe elongation and template usage

RNA polymerase open small bubble in dsDNA and synthesise complementary RNA using DNA template strand; RNA sequence matches DNA except U replaces T

describe termination

transcription ceases when a termination signal is encountered; RNA polymerase disassociates

describe RNA processing

pre mRNA include exons (coding segments) and introns (non coding segments); spliceosome removes introns, ligate exons to maintain reading frame

alternative splicing generates multi proteins from 1 gene by including /skipping specific exons

what happens during translation?

• mRNA exits nucleus and is translated by ribosomes

• genetic code read in codons; each codon specifies an amino acid

• start codon: AUG (methionine)

• stop codons: UAA, UAG, UGA

• redundancy in code buffers some mutations

what is the role of tRNA?

• each tRNA carries an anticodon complementary to an mRNA codon

• anticodon pairs with codon (A-U, G-C)

• tRNA is charged with its amino acid using energy

describe the ribosome mechanics during translation

initiations: small ribosomal subunit scans from 5’ to start codon; large subunit joins; methionine tRNA binds

elongation: A (acceptor) site admits next aminoacyl-tRNA; P (peptide) site forms peptide bonds; E (exit) site releases deacylated tRNA

termination: at stop codon

what are key mRNA features that aid translation:

• 5’ cap on mRNA aids ribosome recruitment

• can adopt circular shape during translation, bringing 5’ and 3’ ends in contact → ribosome recycling

• polyribosomes enable concurrent protein synthesis

what are regulatory DNA elements?

steroid hormones diffuse thru cell membrane, bind receptors and enter nucleus, scan for DNA motifs and promote transcription

what is the role of non coding RNA in gene regulation?

micro RNA: 22 nucleotide binds complementary sequences in mRNAs to block translation/promote mRNA degradation

long non coding RNA: diverse functions in nuclear organisation and gene regulation

x inactivation: in female mammals, 1 X chromosome in each cell is silenced via long non coding RNA that coats it, leading to condensed chromatin

what are epigenetic?

control chromatin packaging and DNA accessibility influencing whether transcription factors and RNA polymerase can engage gene loci

open chromatin…

is accessible and transcriptionally active

condensed chromatin…

is least accessible and silenced