genetics test 3

RNA polymerase

catalyzes the addition of each ribonucleotide to the 3' end of the nascent strand; forms phosphodiester bonds between nucleotides

transcription

the process by which genetic information encoded in DNA is copied into RNA by RNA polymerase

messenger RNA (mRNA)

serves as the intermediate between DNA and protein synthesis; undergoes translation to produce a polypeptide chain

ribosomal RNA (rRNA)

combines with proteins to form ribosomes which catalyze protein synthesis

transfer RNA (tRNA)

brings amino acids to ribosomes during translation and aligns them according to codon-anticodon pairing

telomerase RNA

provides a template for telomere DNA synthesis

small nuclear RNA (snRNA)

functions in mRNA splicing and processing in the nucleus

micro RNA (miRNA( and small interfering RNA (siRNA)

regulate gene expression post-transcriptionally by degrading or repressing specific mRNAs

promotor

controls access of RNA polymerase to the gene; located upstream of the +1 start site

coding region

contains the sequence to be transcribed

termination region

defines the endpoint of transcription

holoenzyme

RNA polymerase + sigma unit; binds promoters and initiates transcription

consensus sequences

RNA polymerase attracted to promotors via the presence of this

intrinsic termination

hairpin loop formation followed by poly-U sequence causes polymerase to dissociate

rho-dependent termination

requires the rho protein to separate RNA from DNA at the rut site

RNA polymerase I

transcribes rRNA genes

RNA polymerase II

transcribes mRNA and snRNA genes

RNA polymerase III

transcribes tRNA, one rRNA gene, and other small RNAs

TATA box

core promotor sequence recognized by TBP

TBP

TATA binding protein

TAF

TBP associated factor

PIC

preinitiation complex

enhancer sequences

increase transcription rates by interacting with promotor-bound proteins

silencer sequences

repress transcription by DNA bending and preventing activator access

introns

noncoding regions removed from pre-mRNA

spliceosome

complex of snRNAs and proteins catalyzing splicing

group 1 intron

self-splicing; eukaryotes, bacteria, bacteriophages

group 2 intron

self-splicing; eukaryotic organelles, bacteria, archaea

pre-mRNA intron

spliceosome; eukaryotic nuclear genes

rRNA and tRNA intron

enzymatic; eukaryotes, bacteria, archaea

5' splice site

at 5' intron end; contains consensus sequence with nearly invariant GU dinucleotide at 5'-most end of the intron

3' splice site

at 3' end of intron; has 11-nucleotide consensus with pyrimidine-rich region; nearly invariant AG at 3'-most end

branch site

3rd consensus region 20-40 nucleotides upstream of 3' splice site; pyrimidine-rich containing invariant adenine

carboxyl terminal domain (CTD) of RNA polymerase II

regulates pre-mRNA processing machinery

thyroid cells

use exon 4 polyadenylation site, producing calcitonin

neuronal cells

splice out exon 4 and used exon 6 polyadenylation site, producing CGRP

alternative RNA processing

one gene can produce multiple mRNA variants through differential processing

alternative splicing

inclusion/exclusion of exons

alternative promotors

transcription starts at different sites

alternative polyadenylation

multiple tail addition sites

rRNA processing

are transcribed as large precursors, then cleaved to form small units; fold and assemble with proteins to form ribosomal subunits

tRNA processing

produces as precursors that are trimmed and chemically modified

RNA editing

posttranscriptional alteration of RNA nucleotides

translation

cellular process by which the genetic information encoded in mRNA is decoded to synthesize a polypeptide chain (protein)

N-terminus

corresponds to the start codon (AUG); amino end

C-terminus

corresponds to the stop codon; carboxyl end

5' untranslated region

aids in ribosome binding and regulation; upstream of the start codon

coding region

contains the codons specifying amino acids

3' untranslated region

affects stability and translation efficiency; follows the stop codon

function of ribosome

macromolecule machines that coordinate mRNA decoding and peptide bond formation

A (aminoacyl) site

binds incoming aminoacyl-tRNA

P (peptidyl) site

holds tRNA carrying the growing polypeptide

E (exit) site

releases uncharged tRNA

translation initiation

small ribosomal subunit binds the mRNA near its 5' end to identify the start codon; the initiator tRNA carrying MET binds to the start codon and large subunit joins to form a complete ribosome

translation elongation

adds amino acids sequentially to the growing polypeptide chain

translation termination

when a stop codon enters the A site, no tRNA can pair with it; completed polypeptide is released

monocistronic mRNA

one mRNA translates into one protein in eukaryotes

polycistronic mRNA

single mRNA contains multiple coding regions, each with its own start and stop codon in prokaryotes

operons

allow coordinated expression of multiple genes in prokaryotes

intercistronic spacers

separate cistrons and may allow ribosomes to reinitiate translation in prokaryotes

genetic code

links mRNA codons (triplets) to specific amino acids enabling translation

phosphorylation

addition of phosphate groups by kinases; regulates activity

methylation/acetylation/hydroxylation

modifies charge, stability, or localization

glycosylation

addition of carbohydrate side chains; critical for secreted and membrane proteins

cleavage

some polypeptides are cut into smaller segments which may form multiple functional proteins

chromosome variation

the number and shape of chromosomes vary widely among species; though closely related species tend to have similar chromosome counts

karyotype

organized visual display of chromosomes arranged by size, shape, and banding pattern

p arm

short arm

q arm

long arm

fluorescent in situ hybridization (FISH)

uses fluorescent DNA probes to locate specific DNA sequences of chromosomes; detects rearrangements, deletions, or duplications

chromosome banding (G-banding)

giemsa dye stains chromosomes to produce distinct banding patterns for identification; preparation involves cell culture, metaphase arrest, and slide preparation

euchromatin

less condensed, transcriptionally active

heterochromatin

densely packed, transcriptionally inactive, often found near centromeres and telomeres

nondisjunction

failure of homologous chromosomes or sister chromatids to separate properly during cell division

aneuploidy

abnormal chromosome number (n+1 or n-1)

euploidy

normal chromosome set (haploid, diploid, polyploid)

gene dosage

number of copies of a gene directly affects the level of its expression

gene balance

disruption in gene dosage causes imbalance between gene products; animals more sensitive than plants

trisomy 21 (down syndrome)

extra copy of chromosome 21; cognitive impairment and heart defects

turner syndrome (XO)

monosomy of the X chromosome; haploinsufficiency of SHOX gene; short stature, infertility, developmental abnormalities

mosaicism

presence of two or more genetically distinct cell lines in one individual; mitotic nondisjunction early in embryogenesis

uniparental disomy (UPD)

both homologous chromosomes inherited from the same parent; arises from dual nondisjunction in sperm and egg or trisomy rescue where one chromosome is ejected post-fertilization

polyploidy

possession of three or more full sets of chromosomes

autopolyploidy

chromosome duplication within one species

allopolyploidy

combination of chromosome sets from different species through hybridization

meiotic nondisjunction

diploid gametes form; cause of polyploidy

mitotic nondisjunction

chromosome doubling in somatic cells; cause of polyploidy

hybrid vigor

enhanced growth, fertility, and resistance in polyploid hybrids

partial (terminal deletion)

loss of chromosome end

interstitial deletion

two breaks remove an internal segment

unequal crossover

misalignment during meiosis results in one chromosome with a duplication and another with a deletion

partial duplication/deletion heterozygotes

organisms with one altered and one normal homolog

cytological methods

observation of chromosomal loops in prophase I pairing

deletion mapping

used to locate genes by studying pseudodominance (recessive alleles expressed when dominant allele deleted)

inversions

occur when chromosome segment breaks and reinserts in reverse orientation

paracentric inversion

centromere outside inverted region

pericentric inversion

centromere within inverted region

inversion heterozygotes

pairing requires formation of an inversion loop during meiosis

translocation

exchange of chromosome segments between nonhomologous chromosomes

nonreciprocal translocation

one-way transfer