Molecular Genetics!

Subtopics are highlighted, terms are bolded, and definitions are underlined :)

DNA Structure:

DNA is a type of Nucleic Acid. The monomer unit is called a nucleotide

3 Nucleotide Components

1. 5-carbon sugar - DNA - Deoxyribose sugar or RNA - Ribose Sugar

2. Phosphate Group

3. Nitrogen Base

Nitrogen Bases - encode the genetic information

Purines - Double ring

Adenine (A) and Guanine (G)

Pyrimidines - single ring

Cytosine (C) and Thymine (T)

Complementary Base Pairing:

Adenine bonds with Thymine connected by 2 hydrogen bonds. Guanine bonds with Cytosine connected by 3 hydrogen bonds.

Double Helix - consists of two strands connected in a helical fashion (think spiral staircase)

The DNA strands are antiparallel meaning they are complementary strands run in opposite direction.

DNA Replication:

Semi-conservative Replication - base pairing allows each strand to serve as a template for a new strand

Each new DNA molecule consists of an original template strand and a new complementary strand.

DNA Helicase - unzips the double helix by breaking the hydrogen bonds between the nitrogen bases that connect the two strands together

DNA Polymerase - reads the template strand and adds nucleotides to form the complementary DNA strand

Protein Synthesis:

Genome - consists of all of the genetic material of an organism

Gene - a short segment of DNA found on a chromosome that contains the instructions for building a protein; ex. the insulin gene

It contains the instructions for making the protein insulin and is found on chromosome 11.

Allele - variation of a gene; ex. - Pea Seed color

Locus - location of a gene on a chromosome; the gene’s address

Genotype - genetic makeup, seq. of DNA; ex. gene for keratin

Phenotype - the organism’s physical traits; ex. the protein keratin

The genotype codes for the phenotype.

Gene Expression - refers to the production of the protein the gene codes for

When we say a gene is being expressed, we mean the gene is turned “on.” Gene expression is like a “dimmer knob,” it could be

turned on, off or an variation in the middle, ex. - If gene expression is high for the insulin gene, then a

lot of insulin is being produced.

RNA (Ribonucleic Acid) - a polymer of a nucleic acid that is found in all living cells and plays a role in protein synthesis

Three Major Types of RNA...

1. Messenger RNA (mRNA) is produced from a DNA template, and carries the information for protein production to the ribosome (where proteins are made).

2. Transfer RNA (tRNA) functions as a translator between nucleic acid and protein languages by carrying specific amino acids to the ribosome, where they recognize the appropriate codons in the mRNA.

3. Ribosomal RNA (rRNA) molecules together with proteins makeup ribosomes.

Protein Synthesis - the formation of proteins by using information contained in DNA and carried by mRNA

Two main steps... 

1. Transcription - DNA to RNA

2. Translation - RNA to Protein

Transcription:

Transcription makes a mRNA copy of the DNA gene. It takes place in the nucleus for eukaryotes and takes place in the cytoplasm for prokaryotes.

Step 1 - DNA unzips

Step 2 - Form the mRNA Strand

RNA polymerase unzips the DNA & reads the DNA template strand and adds RNA nucleotides that are complementary with the DNA bases of the template strand.

Step 3 - Edit the mRNA

Prokaryotic cells - message is not edited (prokaryotes lack introns)

Eukaryotic cells - message is edited

Introns - noncoding regions (do not code for the polypeptide)

Exons - code for the polypeptide

Alternative Splicing - genes can code for multiple proteins depending how the mRNA is spliced

It only occurs in eukaryotes.

Translation:

Translation - the synthesis of a protein using the genetic information encoded in an mRNA molecule

It takes place in the ribosome.

Codon - three consecutive nucleotides on mRNA that code for a specific amino acid

Genetic Code - the set of codons and the amino acids they code for

Three Stages...

1. Initiation

Start Codon: AUG

2. Elongation

Amino acids are being attached to each other by peptide bonds to form a polypeptide.

3. Termination

Stop Codon: UAG, UAA, or UGA

Transfer RNA (tRNA) - functions as a translator between nucleic acid and protein languages by carrying specific amino acids to the ribosome, where they recognize the appropriate codons in the mRNA

Anticodon – sequence on the tRNA that is complementary to the codon

It allows the tRNA to bond to the mRNA and bring down the correct amino acid. Different tRNAs have different anticodons and therefore carry different amino acids.

Ribosomes - site of translation

It forms peptide bonds between amino acids in order to form polypeptides. Ribosomes are made from complexes of protein & rRNA.

Mutations:

Mutation - a permanent change in the DNA sequence of a gene

The source of new genes or alleles. Mutations are the main source of genetic variation. DNA mutations can be positive, negative or neutral based on the effect or the lack of effect they have on the resulting protein. Whether or not a mutation is detrimental, beneficial or neutral also depends on the environmental context.

Causes of Mutations...

Mutagen - a chemical or physical agent that causes a mutation in the DNA

Mutations in cells can be triggered by:

1. Radiation,

UV, X-rays, Radioactive elements

2. Chemical mutagens

Reactive chemicals, ex. free-radicals

3. Errors during DNA replication or DNA repair

Most mutations are fixed by DNA repair mechanisms

Two main categories...

1. Point Mutations - affects one nucleotide

2. Chromosomal mutations - affects many nucleotides

Point Mutations - mutations that affect one nucleotide

1. Substitution - Change a nucleotide

2. Insertion - Add a nucleotide

3. Deletion - Remove a nucleotide

Frameshift Mutation - mutation that shifts the “reading” frame of the genetic message by inserting or deleting a nucleotide caused by insertion or deletion point mutations

Epigenetics:

Chromosome - a threadlike, gene-carrying structure found in the nucleus

Each eukaryotic chromosome is made of chromatin condensed further during mitosis.

Chromatin - a complex of DNA and its associated histone proteins

Nucleosome – DNA wrapped twice around 8 histone proteins

Condensed Chromatin - nucleosomes are packed tightly together

DNA is not accessible for transcription, Genes are “off” (no gene expression)

Loose Chromatin - nucleosomes are not packed together

DNA is accessible for transcription, Genes are “on” (genes are expressed)

Epigenetics - refers to all processes that act on DNA and modify the expression of genes

Epigenetic markers: two types

1. Methyl groups – tighten chromatin, DNA is not accessible for transcription. Genes are “off” (no gene expression).

2. Acetyl groups – Loosen chromatin, DNA is accessible for transcription, Genes are “on” (genes are expressed).

Differentiation - the process in which cells become specialized in structure and function

While all cell types within an organism have the same genes, they have different functions because they

express those genes differently due to epigenetic markers.