genetic information Flashcards
define gene
a sequence of DNA nucleotide bases that codes for a polypeptide or a functional RNA
structure of DNA
double stranded molecule
each strand is a polynucleotide made up of individual nucleotides
what is a DNA nucleotide composed of?
phosphate group
deoxyribose sugar
nitrogenous base
describe the DNA in prokaryotes
circular DNA
some have smaller loops of plasmid
DNA is naked
not associated with histone proteins
describe the DNA in eukaryotic chloroplasts and mitochondria
short circular molecule
not associated with histone proteins
describe the DNA in the nuclei of eukaryotic cells
very long linear molecule
associated with histone proteins
DNA molecule and histone proteins forms a chromosome
describe a homologous chromosome
-same genes of same loci
-centromeres at same place so same shape and size
-one maternal, one paternal
how is a chromosome formed?
-DNA molecule is wrapped around histone proteins to fix in place
-DNA histone complex is coiled into solenoids
this means a very long molecule of DNA can be condensed into a single chromosome (a lot of info)
what do humans have a diploid number of?
2n=46
what do genes code for?
either:
-the amino acid sequence of a polypeptide
-a functional RNA (including rRNA and tRNA)
where do genes occupy?
locus (fixed position)
what is an exon?
parts of a gene which codes for amino acid sequences
what are introns?
non-coding sequences
define genome
complete set of all the genes in a cell
define proteome
the full range of all the proteins that a cell is able to produce using its genome
define allele
two or more versions of a genetic sequence coding for a different polypeptide
define gene pool
all the different alleles of all the genes found within a population
which amino acid does the start triplet always code for
methionine
define triplet
every three bases in the sequence of a gene
what does a triplet code for?
a specific amino acid
how many different nucleotides are there in DNA?
4
how many possible combinations of triplets are there to code for 20 amino acids?
4^3= 64
define degenerate
in the genetic code, most amino acids are coded for by more than one triplet
how many triplets dont code for amino acids in DNA?
what are they called?
3
stop triplets
define universal
the same triplet always codes for the same amino acid
define non-overlapping
each base is only read once, each base is part of only one triplet
RNA structure
single stranded polynucleotide
nucleotides composed of phosphate,ribose sugar and nitrogenous base (Uracil instead of Thymine)
how long is messenger RNA?
few thousand nucleotides long
what is every 3 bases called in mRNA?
codon
what is the shape of mRNA?
linear molecule as bases don’t bond to each other
how long is transfer RNA?
about 75 nucleotides long
describe the structure of tRNA
-strands fold back on themselves
-hydrogen bonds form between complementary base pairs
-shape is called clover leaf
-one end is an amino acid binding site
-3 of the bases are exposed called an anticodon
how many different types of tRNA are there?
61
why are there not 64 triplets?
because there are 3 stop codons which dont code for any specific amino acid
how is a protein made in eukaryotes?
-specific gene is transcribed (introns and extrons)
-this makes pre-mRNA
-this is spliced to form mRNA
-this is a copy of the exons of the gene only
-mRNA is translated to make polypeptides
describe transcription in the nucleus
-gene unwinds as hydrogen bonds between complementary bases broken by RNA polymerase
-one strand acts as a template
-complimentary free RNA nucleotides bind to exposed bases
-in a condensation reaction, sugar phosphate backbone is joined using RNA polymerase
-both inton and exon are transcribed to make pre-mRNA
what disengages the RNA polymerase?
the stop triplet marking the end of a gene
describe splicing in eukaryotic cells
pre-mRNA is spliced
the non-coding sections (transcribed from the introns) are cut out and coding sections are edited together to produce mRNA
what types of DNA dont need splicing?
why?
prokaryotic DNA
chloroplast and mitochondria DNA
because they dont have introns
where does translation happen? and what does it make?
cytoplasm
makes polypeptides
describe translation
-mRNA moves out of nucleus and attaches to ribosome.
-tRNA carrying amino-acids bind by their anticodons to the mRNA - by complementary base pairing.
-Another molecule of tRNA with another amino-acid attaches to the mRNA on the next codon along.
- The two amino-acids on the tRNA form a peptide bond.
-The first tRNA now moves away and a new tRNA attaches.
- This process repeats, adding more amino-acids to the polypeptide-chain. -Process ends when a ‘stop signal’ is reached and completed polypeptide detaches.
what does a gene mutation involve?
a change in the base sequence of a chromosome
what are neutral mutations?
when the change in amino acid sequence does not affect the function of the protein
why might some substitutions not result in any change?
genetic code is degenerate
the new triplet may code for the same amino acid
what are silent mutations?
in substitution mutation when the mutation causes no change
what happens to the length of the polypeptide chain when the new triplet is a stop triplet?
shorter
what does addition result in?
a frame shift
what’s the consequence of a frame shift?
new sequence of amino acids
changes position of hydrogen and disulphide bonds
what 2 mutations result in a frame shift?
addition
deletion
how can the rate of mutations be increase?
mutagenic agents
x4 examples of mutagenic agents?
-high energy radiation
-ionising radiation
-chemicals eg tar in cigarette smoke
-viruses
what are carcinogens?
substances which cause mutations in oncogenes which cause the cell to become cancerous
whats non-disjunction?
when homologous chromosomes do not separate in meiosis
whats down syndrome caused from?
non-disjunction of chromosome 21
