Bio 12 Lec 3 Genetics

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sex-linked genes

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genes inherited with the X or Y chromosome

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Duchenne Muscular Dystrophy

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X-linked

>weakening of the muscles and loss of control

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Hemophilia

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X-linked

>blood does not clot properly

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Hypertrichosis

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Y-linked

>abnormal/excessive hair growth in ear

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Barr body

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Xchromosome that becomes inactive and lies along the inside of thenuclear envelope in females, reactivatedin cells that give rise to eggs

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Linked genes

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tend to be inherited together because theyare located near each other on the same chromosome

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unlinked genes

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geneson separate chromosomes are never linked, genes on thesame chromosome but are far apart are also unlinked, results to more variation

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parental types

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same combination as P generation

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recombinant types

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new combinations not seen in P generation

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nondisjunction in meiosis 1

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Failure of one or more pairs of homologous chromosomes or sister chromatids toseparate normally during nuclear division

> 1/2 (2n+1) and 1/2 (2n-1)

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nondisjunction in meiosis 2

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Failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division> 1/4 (2n+1) and 1/4 (2n-1) and 1/2 (n)

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Aneuploidy

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abnormal chromosome number

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monosomy

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2n-1

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Trisomy

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2n+1

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Polyploidy

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more than 2 complete sets of chromosomes in all somatic cells (3n, 4n, etc)

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Down Syndrome

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Trisomy 21 2n+1 in chromosome 21; characteristic facialfeatures, heart defects, developmental delays, hypotonia (low muscle tone → lowmuscle strength)

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RockerBottomFeet

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Trisomy 18 trisomicchromosome 18; clenched hand, usuallyresults to early death

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Polydactyly

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Trisomy 13 trisomicchromosome 13; extra finger

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Klinefelter Syndrome

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47,XXY abnormally small penis and underdeveloped testes,some female characteristics ( gynecomastia ), subnormal intelligence, sterile

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Meta-female

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47.XXX female with some male characteristics such as broadshoulders

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Turner's Syndrome

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45, XO only known monosomy (2n1) in humans, sterility and mental retardation

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Jacob's Syndrome

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47, XYY said to exhibit more violent / criminal behaviors than normal men but has not been proven

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45,YO

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lethality (fetus dies)

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deletion

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lose a chromosomal fragment

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duplication

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repetition of fragment

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inversion

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attachment of a fragment in reverse orientation

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translocation

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segmentmoves to a nonhomologouschromosome, usuallyreciprocal

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Cri du Chat

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deletionof part of chromosome 5; severe intellectual disability,unusual facial features, cries like a cat

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Chronic Myelogenous Leukemia

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reciprocaltranslocation betweenchromosome 22 and 9 resulting to unusually short chromosome 9 (akaPhiladelphia chromosome)

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Fragile X Syndrome

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repetition of many CGG segments; big ears, prominent jaws, associated with mental retardation in worst cases

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replication

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DNA --> DNA

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transcription

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DNA --> mRNA

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translation

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mRNA --> proteins

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DNA → DNA → mRNA → proteins ( amino acid building blocks) → genotype →phenotype (may be affected by environment)

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Transformation

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change in genotype and phenotype due to assimilation of external DNA

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components of DNA

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sugarphosphate backbone + nitrogenous bases ( purines A and G, pyrimidines T and C), antiparallel strands linked by hydrogen bonds

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Unwinding

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● Replication fork Y-shaped region of unwinding

● Helicase enzymes that untwist the double helix at the replication fork

● Single strand binding proteins bind unpaired DNA and stop repairing

● Topoisomerase relieves strand strain

● RNA Primase primer for DNA polymerase, makes the initial segments for starting replication

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Synthesis

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● DNA polymerase catalyzes synthesis of new DNA, add nucleotides to preexisting chain

● Leading strand continuously made in 5’ → 3’ direction towards replication fork

● Lagging strand okazaki fragments building in 5’ → 3’ direction as enough nucleotides are revealed (away from replication fork)

● DNA ligase fuses halves of lagging strand to form one continuous strand

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Chromatin packing

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● Nucleoid dense region of DNA in bacteria

● Chromatin complex of DNA and protein

● DNA → histones bind DNA → nucleosomes → histones and DNA wound twice around them → fiber → looped domain → metaphase chromosome

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Transcription

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RNA polymerase pries DNA strands apart at promoter region → Transcription begins, RNA molecule is slowly formed → Elongation; nucleotides added at 3’ end, RNA peels away from the DNA strand → Termination ; signal to terminate transcription, polymerase detaches from DNA → Polyadenylation ; signal sequence added to premRNA

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Alteration of mRNA ends (for protection of mRNA)

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5’ end → 5’ cap

● 3’ end with polyadenylation signal → polyAtail

● Both alterations facilitate mRNA export from nucleus, protect it fromdegradation by hydrolytic enzymes, help ribosomes attach once in cytoplasm

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RNA splicing (cutting out unnecessary information in mRNA)

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Necessary due to noncoding stretches ( introns ) on premRNA between coding stretches ( exons )

● Small nuclear ribonucleoproteins ( snRNPs ) recognize splicing sites → several join to form spliceosome (cuts out introns and joins exons)

● Alternative RNA Splicing one gene gives rise to 2 or more different polypeptides depending on RNA processing (which are considered as exons) e.g. fruit fly sex difference

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Translation

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mRNA binds with small ribosomal subunit, tRNA with anticodon bonds with amino acid via aminoacyltRNA synthetase → tRNA binds with mRNA at Asite → translocation within large ribosomal subunit has tRNA with growing polypeptide chain moving into the Psite as it waits for the next tRNA to bind → release factor binds with stop codon + water molecule → tRNA exits via Esite

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Point mutations

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Nucleotidepair substitution

■ Silent mutation change still codes for the same amino acid

■ Missense mutation change codes for a different amino acid

■ Nonsense mutation terminate translation prematurely

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Hardy Weinberg Law

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● p 2 + 2pq + q 2 = 1

● p + q = 1