Genetics

Chromosome

Discrete unit of chromatin that is a physical part of the genome

Gene

Segment of DNA that codes for the production of a protein product

Allele

A version of the gene

Phenotype

Physical expression of the gene

Genotype

Genetic makeup of a gene that codes for a trait

Principal of dominance

Some traits have the ability to mask or hide other traits

Principal of segregation

Genes come in pairs (one from the father and one from the mother) which separate during gamete formation to form haploid cells

What accounts for variations in inherited genes?

Alternative versions of genes

Principal of independent assortment

Genes separate into gamete cells independently from other genes

What allows for the principal of independent assortment?

Random alignment of chromosomes along the metaphase plate during metaphase I of meiosis I

Penetrance

The degree to which a dominant trait is expressed

Full penetrance

Results in complete expression of dominant gene (many of our important cell regulatory genes are full penetrance)

Incomplete dominance

Results in a blending of traits (Black x white = grey)

Codominance

Both alleles are equally dominant such that both are expressed completely but in equal amounts (Black x white = black and white spots)

Multiple alleles

Prescence of more than two alleles for a single gene (works in a game of rock, paper)(e.g eye color)

Pleiotropy

Multiple phenotypic effects of a single gene, opposite of polygenes (eg. cystic fibrosis)

Epistasis

One “master gene” that regulates and can override other genes (eg. how homozygous albinism overrides all other melanin genes)

Polygenes

Single trait that is controlled by more than one gene results in higher phenotypic variation, manifests as a bell curve of features, opposite of pleiotropy, eg. skin coor, height, and hair color

Phenotypic plasticity

Environmental stimuli can result in the production of different protein products, eg. hydrangeas are either pink or blue based on soil pH

What are the characteristics of cystic fibrosis?

1. Recessive autosomal

2. due to deletion in base

3. results in dysfunctional Na channels

Characteristics of sickle-cell disease

1. Recessive autosomal

2. codominant: rr results in all blood cells being misshapen, Rr results in only some blood cells being misshapen

3. Issues

   1. Cannot carry O2 or hemoglobin efficiently

   2. Can clog in veins and cause pain

4. Sited commonly as co-evolution as SCD results in a lower likelihood of contracting malaria

How did we discover the phenomenon of transformation?

The Griffith Experiment

Describe the Griffith Experience

1. Two strands of bacteria (fatal S strand and non-fatal R strand)

2. Heat killed S strand bacteria (so that it was no longer fatal on its own)

3. Mixture of heat killed S bacteria and living R strand bacteria was injected in to mice, which later died

   1. This was because the S strand bacteria was able to up take the R strand DNA

4. This phenomenon was known as transformation

How was transformation confirmed?

Avery and MacLeod experiment

1. Ruled out the possibility of that the R cells were just using the dead S cell capsules by showing that the S cell capsules were still present

What was the Hershey Chase experiment and what did it confirm?

1. Radioactively labelled proteins in one sample and DNA in another (this was in a virus) then exposed e coli bacteria to the virus (to see which substance was passed from virus to bacteria). DNA is the substance that made it through

2. Confirmed that DNA is the carrier of genetic information

What are the three functional sites of a ribosome and what do they do?

A- Site : area where tRNA molecules arrive and bind to the ribosome

P-Site: area where tRNA molecules transfer amino acid to growing polypeptide chains

E-Site: area where tRNA molecules leave the ribosome

What direction in DNA read in?

3’ —→ 5’

What direction is DNA synthesized in?

5’ ——> 3’

As a rule of thumb how many proteins does a single gene code for? what is this rule known as?

Generally we say that each gene codes for one polypeptide, this is known as Beadle and Tatum’s one-gene one-polypeptide theory (very aptly named)

Which strand is read in order to get genetic information? Coding or template? Why?

Template, as the coding strand has the information we want, but reading it directly would produce the exact opposite result (due to complimentary base pairing during RNA translation)

Why is there a redundancy in code?

We have more codons than we need since 4³ = 64 but 4² = 16 and we need at least 20 codons for the amount of amino acids that exist

Central dogma theory

DNA —→ RNA —→ Protein

Introns vs Exons

Introns are “nonsense” DNA, exons represent a domain of a polypeptide

Structural gene

Segment of a gene that holds transcribable information

Promotor sequence

Short segment of a gene upstream to the structural gene that serves to be the binding site for transcriptional enzymes

TATA box

Segment of promotor sequence that serves as the binding site for RNA polymerase

What are the differences between RNA and DNA

1. RNA is single stranded

2. Ribose sugar

3. Uracil

4. Can have enzyme like abilities (called a ribozyme)

5. Thought to be an “older” molecule than DNA

Why is RNA single stranded?

Since it is much shorter lived it does not require a protective strand

What are the three types of RNA? What are their functions?

mRNA: carries gene code for translation

tRNA: carries the appropriate amino acid to the ribosome for protein synthesis

rRNA: structural component of the ribosome

Transcription

Production of mRNA from a DNA template

Describe the process of transcription

1. chromatin slightly uncoils to allow for DNA to be accessed

2. Transcription factors attach to the promotor section of the gene

3. RNA polymerase attaches to the TATA box

4. RNA polymerase moves along the strand until it reaches the initiation sequence

5. RNA polymerase unzips DNA beginning at the initiation sequence

6. RNA polymerase begins adding complimentary RNA nucleotides

   1. As this continues RNA peels away from the template DNA and the double helix retwists

7. RNA polymerase continues transcription until it reaches a termination sequence

What is the first product of transcription?

Pre-RNA

What direction does RNA elongation occur in?

5’ —→ 3’ as new nucleotides can only be added to the 3’ end

What must pre-RNA go through to become functional mRNA?

1. add a 5’ cap

2. add a poly-a tail

3. go through RNA splicing

UTR

Untranslated region, never translated by the ribosome

RNA splicing

removal of introns

How do snurps work?

1. recognize and bind to introns

2. combines with a larger protein complex to form a spliceosome that then cuts the RNA strand, removes the intron, and splices the remaining exons together

What is the purpose of introns?

1. regulatory, so proteins are not made accidentally

2. in alternative RNA splicing may allow an intron to serve as a coding exon to produce a different protein product

Exon shuffling

order of the exons is altered so that one gene may produce more than one polypeptide

Describe the structure of tRNA

1. distal loop holds anti codon

2. 3’ end is the binding site for a specific amino acid

   1. this is done through the enzyme aminoacyl-tRNA synthetase

Wobble

1. due to loose H-bonding at the 3rd codon position, many tRNA molecules can bond to more than one codon

2. This is why we only have 45 tRNA molecules, rather than 61

What are the differences between eukaryotic and prokaryotic ribosomes?

1. eukaryotic ribosomes are larger

2. prokaryotic ribosomes may be affected by certain antibiotics

Describe the process of translation

1. mRNA beings to the small ribosomal subunit at the 5’ cap and the ribosome scans until it reaches the start codon

2. protein initiation factors bring the large subunit and the 1st tRNA that carries methionine together to form a complex Ribosome

   1. the first tRNA arrive directly at the P-Site

3. The ribosome reads the next mRNA codon and the corresponding tRNA arrives to the A-site

4. GTP is hydrolyzed to release energy and the amino acid methionine is transferred over to the 2nd tRNA to form a dipeptide

5. the tRNA molecules move over one site

6. ribosome continues until it reaches the stop codon

7. Protein release factor binds to the A-site

8. Ribosomal complex is disassembled and the protein is moved to the RER and then to the golgi

Polyribosomes

Ribosomes that can read and translate the same mRNA to produce multiple copies of proteins