BIOL 271 Wolverton Genetics Exam 2

Segregation

2 Alleles at the same locus separate in gamete formation to ensure each gamete receives one allele for each gene.

Independent Assortment

Alleles at 1 locus act independently of alleles at another locus during gamete formation

Recombination

New allele combinations from independent assortment

Linked Genes

Genes located close together on same chromosome travel together during meiosis

Crossing Over

Genes switch from one homologous chromosome to another, resulting in recombination. Max rate of 50% = independent assortment. Closer loci have a lower probability.

Complete Linkage

All offspring show nonrecombinant phenotypes after testcross

Recombination Frequency

(Number of recombinant offspring) / (Total number of offspring)

Coupling

Double dominant or double recessive genes on the same chromosome. After testcross, if homozygous individuals are the majority.

Repulsion

Doubly heterozygous chromosomes. After testcross, if heterozygous individuals are in majority.

Barbara McClintock’s experiment

Demonstrated that recombination was due to physical exchange between chromosomes (crossing over) by pairing counting offspring phenotypes with staining chromosomes to track locations

Chi Square Test of Independence

χ² = Σ((O-E)² / E, but with the fancy table where E = ((row total x column total)/grand total)

Gene mapping

Genetic maps based on recombination frequencies. Double crossover always decreases the distance between genes.

1) Identify nonrecombinants (most common)

2) Identify double-crossover progeny (least common)

3) Determine the middle locus

4) Rewrite alleles in correct order

5) Determine locations of crossovers

6) Determine recombination frequencies

Supercoiling

Lowest energy state for B-DNA (10 bp/turn)

Topoisomerases rotate DNA, requiring energy input

Overrotates: Positive supercoiling

Underrotated: Negative supercoiling

Heterochromatin

Remains condensed throughout cell cycle. Less available to cell. Long-term storage

Euchromatin

Condenses and decondenses throughout cell cycle by energy input. More accessible for cell.

Histone

Proteins that DNA is wrapped around

Nucleosome

Collection of 8 wrapped histones with about 145-147 bp of DNA. 11 nm

Chromatosome

Collection of wrapped nucleosomes

30-nm fiber

Collection of chromatosomes. Main thread knitted into chromosome.

Linker DNA

DNA between histones that is cleaved and destroyed by nuclease

Does chromatin structure change during transcription? (is gene expression altered by chromatin structure?)

Experiment: Chicken DNA- Sensitivity of DNA to DNase I is correlated with gene expression, suggesting that chromatin structure changes during transcription

Control: Brain cells throughout development, which do not produce globin remained INSENSITIVE to DNase

Erythroblasts first 24 hours: before hemoglobin synthesis, no globin genes are sensitive to DNase I digestion.

Erythroblasts 5 days: After globin synthesis has begun, embryonic globin gene U is most sensitive at same time of gene expression for that region.

Erythroblasts 14 days: Adult genes are most sensitive and embryonic gene is insensitive.

Centromere

Defined location where microtubules connect. Defined by specific DNA sequences involved in topology/shape, but not informative and never encoded into mRNA. The sequence acts as a binding site for kinetochore using linking proteins. Often contains tandem repeats of G and C as they have triple H bonds.

Metacentric

Centromere in middle

Submetacentric

Centromere slightly off middle

Acrocentric

Centromere greatly off middle

Telocentric

Centromere at end

Telomeres

Caps on the ends of chromosomes. G-C rich, highly conserved region of DNA throughout eukaryotes.

Central Dogma

DNA→transcription→RNA→translation→PROTEIN

Reverse Transcription

Used by viruses RNA→DNA

Properties of Hereditary Material

1) High-density info storage

2) Faithful replication

3) Able to encode phenotype

4) Capacity to vary

Contents of DNA

Phosphate, sugar, and nitrogenous base form nucleotide

Griffith -1928

Study of pneumonia in mice. Discovered TRANSFORMATION PRINCIPLE (genetic material is transferred/transformed)

Virulent (smooth, sneaky, alive)= dead

nonvirulent (rough, alive)= alive

heat-killed virulent (smooth, sneaky, dead) = alive

heat-killed virulent + nonvirulent (smooth, sneaky container + alive)= DEAD

Avery, MacLeod, McCarty -1944

DNA is the transforming substance! (not RNA or PROTEIN)

1) Took heat killed smooth, sneaky + dead bacteria

2) Treated with RNase, Protease, and DNase.

3) Sample with no DNA=no transformation (no smooth cells)

Hershey and Chase -1955

Changes model systems to bacteria phage viruses.

1) Infect E. coli grown in a medium containing ³⁵S (because Sulfur is a part of proteins but not DNA)

a) ³⁵S PROTEIN + UNLABELED BACTERIA

2) Infect E. coli grown in a medium containing ³²P (because Phosphate is part of DNA and not PROTEINS)

a) ³²P DNA + UNLABELED BACTERIA

3) Shear off protein coats in blender

4) Seperate protein from cells by centrifuging

5) PROTEIN BACTERIA IS UNLABELED, DNA BACTERIA IS LABELED (DNA was transmitted)

Chargaff’s Rules

( A = T ) and ( G = C )

(%A + %G)/(%T + %C) = 1

X-ray crystallography

Allows us to see DNA structure

Rosalind Franklin’s Contribution

DNA has two strands

Sugar-phosphate backbone

Atomic distance measurements

Watson and Crick -1953

DNA is double-stranded

Strands for a double helix

Strands are chemically antiparallel

DNA is complimentary

Purine

Adenine and Guanine, 2 rings

Pyrimidine

Cytosine, Thymine, and Uracil, 1 ring

DNA Primary Structure

Nucleotide

DNA Secondary Structure

Double-stranded, complimentary, antiparallel, double-helix with phospho-diester bonds on outsides and H+ bonds on insides. C-G is TRIPLE H+ bonded, A-T is DOUBLE H+ bonded. 2 nm wide. 10 bases every full revolution is about 3.4 nm. 0.34 nm between each base.

Can only add 5’→3’ on the 3’ strand

Different forms of DNA Helix

A form: more condensed

B form: occurs in high water content, less condensed

Z form: direction of helix is backwards

Chromosomal Sex Determination

Sex determined by genes encoded on sex chromosomes encoding for sexual development and can impact how autosomal influences are presented.

50:50 sex ratio maintained by segregation (XX vs XY)

X and Y chromosomes are homologous only at pseudoautosomal regions, essential for X-Y chromosome pairing in meiosis in the male (XY). Primary region at short arm end. Secondary region at long arm end.

Turner Syndrome

XO genotype (lack of Y chromosome) FEMALE, but underdeveloped traits

SRY gene

Sex-determining region on Y chromosome, acts as master regulator because it encodes for development of male sexual traits

ZZ/ZW

X/Y system, but females are the heterogametic sex.

XX/XO

Female XX, male have XO (no pair)

Hemizygous

Have 1 allele and no homologue on other chromosome (XO)

Haplodiploidy

Sex determined by number of chromosome sets (ploidy level) male: n (no fertilization) female: 2n (fertilization)

Genic Sexual Determination

No difference in chromosomes (no presence of sex chromosomes)

Sex determined by genotype at particular loci

Environmental Sexual Determination

Environmental position influences exposure to sex-determining factor (e.g. temperature, hormones) (e.g. marine mollusks that attract larvae that develop into males on female, mate, and eventually switch into females (hermaphroditism) that attract additional larvae, etc)

Genic Balance System Sexual Determination

Sex determined by ratio of autosomal and sex chromosome genes. (X:A)

X has female-producing genes

Autosomes (A) has male-producing genes

X:A ratio

Sex-linked traits

Typically encoded by gene on sex chromosome

Nondisjunction

Failure of chromosomes to separate properly during cell division, causing an abnormal number of chromosomes in daughter cells.