Lecture 5- Genotype to Phenotype

The building blocks of DNA

nucleotides

Deoxyribonucleic acid (DNA) is a nucleic acid:

• resides in the nucleus of the cell

• stores and transmit information

• made up of smaller molecules called nucleotides

Nucleotide Bases

Adenine, Guanine, Thymine, Cytosine

DNA is compromised of Nucleotides:

• phosphate

• sugar (deoxyribose)

• nitrogenous base

Complementary Base Pairing

Adenine- Thymine

Cytosine-Guanine

Implications of DNA structure

• complementary chains (specificity of base pairing) provides a mechanism by which the molecule can replicate itself

• the sequence of nucleotide bases varies, and contains important information (i.e., the genetic code)

DNA Functions

• replication (cell division)

• protein synthesis (genotype to phenotype)

Genotype

underlying genes

Phenotype

physical outcome

Amino acid

• subunits linked together to form linear polypeptide chains

• can be combined in different amounts and sequences to produce different proteins

The Genetic Code

information to make proteins is encoded in the nucleotide base sequence

Codon

a sequence of three nucleotide bases, which code for one amino acid

Gene

sequence of DNA bases that carriers information for synthesizing a particular protein, and occupies a specific chromosomal locus

Genome

the complete set of genes or genetic material present in a cell or organism

What percentage of the human genome codes for protein?

1%

How many protein coding genes are there in the human genome?

~ 25,000

RNA (ribonucleic acid)

• single stranded

• ribose sugar

• Uracil base

Protein Synthesis l: Transcription

• happens inside the nucleus

• DNA splits in region of gene, and attracts complementary ribonucleotides

• complementary messenger RNA (mRNA) strand is synthesized; this mRNA will leave the nucleus and travel to the ribosome for protein synthesis

Protein Synthesis ll: Translation

• happens outside the nucleus, at the ribosome

• transfer RNA (tRNA) molecules bind to the complementary mRNA strand at the ribosome, bringing with them amino acids specified by the mRNA codon

• as amino acids are brought to the ribosome, they bind together to form the amino acid chain of the synthesized protein

DNA Mutation

an alteration in the genetic code; source of new variants

Examples of mutations

chromosomal mutations, point mutations, duplication, inversion, deletion (frame-shift)

Mendelian Inheritance

• single gene, autosomal dominant-recessive model

• useful for examining traits with qualitative variation (discrete categories)

Codominance

both alleles in the heterozygous condition are fully expressed, with neither being dominant over the other

Sex Linkage (X-linked traits)

controlled by genes on the X chromosome, more commonly expressed in males

Traits that do not follow Mendel’s rules

• polygenic traits (stature, skin color, eye color)

• pleiotropy (Marfan syndrome)

Polygenic traits

traits with quantitive (continuous) variation, influenced by two or more genes

Pleiotropy

a single gene influences the expression of multple traits simulatenously

Pseudogenes

DNA sequences, related to known genes, but as a result of a mutation, they have been rendered nonfunctional- they have lost their protein-coding ability and are no longer expressed

Two things to consider when thinking about genes producing proteins

1. Gene coding

2. Gene regulation/gene expression

Basic structure of a gene

promotor, polymerase, transcription factor

Epigenetics

modifying the regulation of genes without changing the DNA sequence itself

DNA Methylation

certain bases (particularly Cytosine) can get a methyl group (CH3) attached

What causes these DNA Methylation ‘tags’?

• physical environment: temperature

• chemical environment: BPA

• social environment: maternal behavior

• nutritional stress environment: diet