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