Topic 4.1 Chromosomes, Genes, Alleles and Mutations.
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Transcript of Topic 4.1 Chromosomes, Genes, Alleles and Mutations.
Topic 4.1 Chromosomes, Topic 4.1 Chromosomes, Genes, Alleles and Genes, Alleles and MutationsMutations
GeneGene A heritable factor controlling a specific A heritable factor controlling a specific
trait or characteristictrait or characteristic A segment of DNA coding for a A segment of DNA coding for a
polypeptide sequence polypeptide sequence (or tRNA, rRNA or (or tRNA, rRNA or regulates other genes)regulates other genes)
GeneTrait: petal color
Gene: DNA that leads to petal color
AlleleAllele A form of a geneA form of a gene Alleles for a gene are found in the Alleles for a gene are found in the
same locus same locus (location on the (location on the chromosome)chromosome)
They have a similar base sequence They have a similar base sequence (may only be different by one base)(may only be different by one base)
Lead to an alternate version of a trait, Lead to an alternate version of a trait, e.g. blue petals.e.g. blue petals.
DNA polypeptide enzyme pigment pink petalsGATTCGATCTTA
DNA polypeptide enzyme pigment blue petalsGATTCGATTTTA
Eukaryotic Eukaryotic chromosomeschromosomes DNA strings organized by proteinsDNA strings organized by proteins Each contains many genesEach contains many genes
Part of a chromosome with most of the histones removed
Homologous Homologous chromosomeschromosomes Have the same genes (but may have Have the same genes (but may have
different alleles) in the same orderdifferent alleles) in the same order Will have the same size, centromere Will have the same size, centromere
location.location. Found in Found in pairspairs (inherited maternally (inherited maternally
and paternally)and paternally)From mother From father
Locus for petal color gene
Blue petal allele Pink petal allele
Sister chromatids
Chromosome #4
Diploid Diploid HaploidHaploid Two copies of Two copies of
each each chromosome chromosome (one from each (one from each parent)parent)
Somatic (body) Somatic (body) cellscells
2n = 46 in 2n = 46 in humanshumans
One copy of each One copy of each chromosomechromosome
Gametes (sperm Gametes (sperm and egg cells)and egg cells)
n = 23 in n = 23 in humanshumans
Chromosomes in Chromosomes in HumansHumans 22 homologous pairs22 homologous pairs 2 sex chromosomes (usually XX 2 sex chromosomes (usually XX
or XY)or XY)
A karyotype (organized picture of an organism’s chromosomes) of a human.
Could sickle cell anemia be diagnosed using a karyotype? Explain.
Genome: all of the genetic Genome: all of the genetic information of an information of an organismorganism
+ two plasmids
A bacterial genome One of the smallest of the 24 chromosomes in the human genome (simplified)
Mutations: how alleles Mutations: how alleles are formedare formed
Gene mutation: a change in the base Gene mutation: a change in the base sequence in the DNA of a genesequence in the DNA of a gene
Caused by damage to DNA or an error Caused by damage to DNA or an error in copying (replication error)in copying (replication error)
Sickle Cell Sickle Cell AnemiaAnemia
A base substitution A base substitution mutation created the mutation created the sickle-cell allele.sickle-cell allele.
In DNA that codes for In DNA that codes for part of hemoglobin, part of hemoglobin, just one nitrogen just one nitrogen base is changed.base is changed.
Sickle-cell anemia: detailsSickle-cell anemia: details Red blood cells carry Red blood cells carry
oxygen in hemoglobin oxygen in hemoglobin (made of 2 (made of 2 αα and 2 and 2 ββ subunits)subunits)
There are 146 codons in There are 146 codons in ββ – –hemoglobin, the 6hemoglobin, the 6thth is is mutatedmutated
The 6The 6thth codon (sense DNA) codon (sense DNA)– Normal: GNormal: GAAG → glutamic acid G → glutamic acid
(hydrophilic) (hydrophilic)– Sickle cell: GSickle cell: GTTG → valine G → valine
(hydrophobic)(hydrophobic) Causes hemoglobin to Causes hemoglobin to
polymerize, twisting the polymerize, twisting the shape of red blood cellsshape of red blood cells
Two normal Two normal alleles:alleles:No sickle cell, No sickle cell, no protection no protection from malariafrom malaria
Two sickle cell Two sickle cell alleles:alleles:Sickle cell Sickle cell diseasedisease
One normal, One normal, one sickle cell one sickle cell allele: allele: No sickle cell, No sickle cell, protection protection from malariafrom malaria
Malaria and sickle cell Malaria and sickle cell allele distributionsallele distributions
Malaria
Why is the sickle cell allele common where malaria is found?
More information More information about MUTATIONSabout MUTATIONS
Base substitutionBase substitution: one : one nucleotide substituted for nucleotide substituted for anotheranother
TTAGCG TTAGCG →→ T TCCAGCGAGCG
antisense DNA mRNA amino acidSilent: GGA → GGC CCU → CCG pro → pro
Missense: TGA → TTA ACU → AAU thr → asn
Nonsense: ACG → ACT UGC → UGA cys →STOP
FrameshiftFrameshift mutation – mutation – adding or removing adding or removing nucleotides, so the nucleotides, so the “frame” of the codons “frame” of the codons changeschanges
ORIGINAL
Antisense DNATAC GGT CTC ACG TCC ACT
mRNAAUG CCA GAG UGC AGG UGA
Amino acid sequence:Met (start) – pro – glu – cys – arg - STOP
MUTANT
Antisense DNATAC TGG TCT CAC GTC CAC T
mRNAAUG ACC AGA GUG CAG GUG A
Amino acid sequence:Met (start) – thr – arg – val – gln – val