Genetics Quiz 1

What is a gene?

a basic biological unit of heredity made of a specific stretch of DNA that acts as an instruction manual for your body

What is an allele?

One of two or more versions of a gene

One allele is inherited from each parent

Combination of alleles determines the trait expressed

What is a genotype?

The genotype of an organism is the collection of alleles present

for each gene in the genome

The genotype of an organism determines the specific

characteristics of an organism (its phenotype)

What is a phenotype?

the set of observable characteristics or traits of an organism, resulting from the interaction between its genetic makeup and the environment

Heredity

The inheritance of genetic traits from one generation to the next

based on the genetic material – DNA

Frederick Griffith, 1928

Discovery of the transforming principle

Transforming principal

the unknown substance from dead bacteria that permanently changes harmless live bacteria into deadly ones

Oswald Avery, Colin MacLeod and Maclyn McCarty, 1944

DNA is the transforming principle

Alfred Hershey & Martha Chase, 1952

DNA is transmitted to progeny phage during a bacterial infection

Lytic

cells break open or lyse to release phage for infection of other bacteria

Lysogenic phase

phage propagates within bacterium through its genome, then enters lytic stage

Label DNA

with radioactive phosphorous (32P)

Label proteins

radioactive sulfur (35S)

Primary structure:

linear sequence of nucleotides

Secondary structure:

double stranded helix

Tertiary structure:

supercoiling, complexes with proteins, packing

Pyrimidines

(C, T, (U in RNA))

Purines

(A,G)

Deoxyribose

DNA

Ribose

RNA (OH-group at C2’)

Nucleotides are connected to one another via

phosphodiester bonds (strong covalent bonds)

A pairs with T bonds

2 H-bonds

G pairs with C bonds

3 H-bonds

Watson’s hypothesis:

Two linear polynucleotide strands held together by hydrogen bonds between the bases

G-C base pairs are

stronger and require more energy to dissociate/melt/denature

B-DNA

Forms a right-handed helix

Forms when water is plentiful

10 base pairs per turn (10 bp/turn)

B-DNA has … grooves

major and minor grooves

Supercoiling

DNA is overwound or underwound (DNA is under strain, requires ATP to make or unwind turns), catalyzed by Topoisomerase (DNA gyrase in E. coli)

Supercoiled DNA takes up

less space (reduces size problem)

Positive

Over-rotation (twisting) of the DNA duplex, increased number of turns, condenses the DNA, less accessibility to enzymes

Negative:

Under-rotation (unwinding) of the DNA duplex, decreased number if turns, DNA is partially denatured

Euchromatin

Chromatin that condenses and decondenses during the cell cycle

Heterochromatin

Chromatin that is highly condensed throughout the cell cycle (even during interphase)

Histones comprise

~50% of chromosomal proteins

Nucleosome:

fundamental repeating unit of chromatin

DNA wrapped

twice around histone core of 8 histone proteins (octamer)

Mild digestion with DNase I

~200 bp of DNA is “protected”

Strong treatment with DNase I

~145-147 bp of DNA is “protected”

A core nucleosome

has 145-147 bp of DNA wrapped ~1.6X around a histone octamer

What is the function of H1 histone?

Not part of nucleosome core

H1 histone clamps DNA to histone octamer

H1 binding requires 20-22 bp DNA beyond the 145-147 bp

This is the 167 bp nucleosome

Linker DNA:

separates neighboring nucleosomes (~40bp)

Extreme condensation:

metaphase of cell division.

Necessary for the cell to be able to properly segregate chromosomes in mitosis and meiosis.

Moderately condensed:

cell is not dividing (interphase)

Chromosomes are extended and much of their chromatin exists as long, thin tangled threads in the nucleus

Centromere

constricted region of chromosome at which chromatids are attached

Kinetochore

binds to centromeres, which attaches to spindle microtubules during mitosis

Centromere function

Composed of heterochromatin

During mitosis and meiosis, spindle fibers attach to the centromere via the kinetochore (a multi-subunit protein complex)

Centromere function through chromosome breakage

Chromosome fragments that lack centromeres are lost in mitosis.

Centromeres do not have a “set sequence”

Telomeres:

Chromosome ends, highly repetitive sequence; heterochromatin

Protects ends from effects of degradation

Telomeric repeats: 5’-TTAGGG-3’ (humans)