Biology

All Bio Topics on the MCAT

Transcription

Process where DNA is copied into RNA

Translation

Process where RNA is converted into Protein

Replication

Process where DNA is duplicated to form new DNA strands

Genetic Code Exceptions

RNA can be replicated (in viruses) and RNA can be transcribed to DNA

Triplet Code

Ribosomes translate mRNA in groups of three nucleotides

Codons

Three-nucleotide sequences on mRNA that code for specific amino acids

tRNA

RNA molecules with anticodons complementary to mRNA codons

Redundancy in Genetic Code

Presence of multiple codons coding for the same amino acid

Start Codon

AUG, codes for methionine and initiates translation

Stop Codon

UAA, UGA, UAG, signals the end of translation

Wobble Pairing

Flexibility in the third base of the codon during translation

Wobble Pairing

Pairing of bases in RNA that deviates from the typical Watson-Crick base pairing

Mutation

A change in the DNA sequence

Point Mutation

One nucleotide substitution, three types (nonsense, missense, and silent)

Silent Mutation

Results in the same amino acid

Missense Mutation

Results in a new amino acid

Nonsense Mutation

Results in a STOP codon

Insertion

Addition of nucleotides

Deletion

Subtraction of nucleotides

Frameshift

Insertion or Deletion of a number of nucleotides that are not divisible by three

Causes of Frameshift

Alters the reading frame, can lead to changes in the protein sequence, and can lead to a premature stop codon

mRNA

Messenger RNA, codes for proteins

tRNA

Transfer RNA, carries amino acid to mRNA/Ribosome during translation

rRNA

Ribosomal RNA, component of ribosomes

Ribozymes

Catalytic, biological enzymes

Transcription Start Site

RNA polymerase binds to promoter

Helicase activity

RNA polymerase unwinds the DNA

The template strand of DNA

What RNA polymerase uses to create a complementary RNA molecule

mRNA is complementary to….

The template strand of DNA

mRNA has the same sequence as….

The coding strand

RNA polymerase

DNA dependent

Transcription Stop Site

When RNA Polymerase reaches this site, transcription stops

Eukaryotic mRNA Processing

The RNA molecule created by RNA polymerase undergoes several modifications to form mRNA

Does RNA polymerase have proofreading activity?

No, and it is error prone!

Where does Eukaryotic mRNA processing occur?

Nucleus

5’ capping

Addition of a methylated guanine at the 5’ end of RNA polymerase

3’ Polyadenylation

Addition of a Poly A-Tail to the 3’ end

What does 5’ capping and 3’ Polyadenylation do?

Protect the RNA from degradation and promote translation

RNA splicing

Non-Coding Regions (Introns) are removed, leaving the coding regions (Exons)

Alternative Splicing

One pre-mRNA molecule can be spliced in multiple ways to produce multiple protein products from a single gene

snRNPs (Small nuclear ribonucleoproteins)

complexes of proteins and small nuclear RNAs (snRNAs)

Spliceosomes

Carries out splicing, snRNPs binded to pre-mRNA

Five Carbon Sugar

Ribose and Deoxyribose

Ribose

Backbone of RNA, 2’ and 3’ OH groups

Deoxyribose

Backbone of DNA, 3’ OH group, loss of 2’ OH group increases stability of DNA

Nitrogeneous Bases

Purines and Pyrimidines

Purines

Adenine and Guanine, two fused rings

Pyrimidines

Cytosine, Thymine, and Uracil

Thymine

In DNA only

Uracil

In RNA only

Nucleoside

Five Carbon Sugar + Nitrogeneous Base, can be phosphorylated to form nucleotides

Nucleotides

Nucleoside + Phosphate, Building block of RNA and DNA, and important source of energy for metabolic processes (Ex: ATP)

Watson-Crick Base Pairing

AT or AU has 2 hydrogen bonds, CG has 3 hydrogen bonds

Watson-Crick Model of DNA

Double helix formed between two antiparallel single-stranded DNA molecules

Single-Stranded DNA Molecule

A polymer of deoxynucleotides held together by phosphodiester bonds (sugar phosphate backbone)

How are single-stranded DNA molecules held together?

By base stacking and hydrogen bonds between the nitrogeneous bases

Hybridization

Process by which a DNA/RNA molecule binds to a complementary DNA/RNA molecule

Northern Blot

A lab technique hybridization is used in

DNA replication

Process of producing identical copies of DNA, occurs in all living organisms, basis for the inheritance of genetic material

Semi-Conservative Replication

Double-stranded DNA is separated into two pieces of single-stranded DNA molecules, each single-stranded DNA serves as a template for the creation of a new complementary strand, and results in two new identical strands composed of one old and one new strand

Origin of Replication

Specific point in DNA where DNA replication is initiated

Prokaryote Origin of Replication

One origin of replication and one circular chromosomes

Eukaryote Origin of Replication

Multiple linear chromosomes and many origins of replication

DNA Helicase

A DNA Replication enzyme, separates double-stranded DNA to form a replication fork with two single-stranded DNA templates

Topoisomerase/DNA gyrase

Relieves strain while unwinding DNA

DNA primase

Synthesizes a short fragment of RNA complementary to the single-stranded DNA

DNA polymerase (DNA pol III in prokaryotes)

Continuously adds nucleotides in the 5’ to 3’ direction

Leading Strand

Is synthesized continuously

Lagging Strand

Requires multiple RNA primers and is synthesized in fragments (Okazaki Fragments)

DNA Ligase

Seals the gaps between two DNA fragments

DNA Polymerase (DNA pol I in prokaryotes)

Replaces RNA primer with DNA

Traits of DNA Polymerase

Has proofreading activity and is a DNA dependent polymerase

Traits of DNA Primase

Is a DNA dependent Polymerase

End Replication Problem

The lagging strand cannot be fully replicated by DNA polymerase in Eukaryotes

Telomeres

In Eukaryotes, the ends of the chromosomes that have a repeating nucleotide sequence

Telomere’s Function

To protect the chromosome from degradation

Telomerase

RNA-dependent DNA polymerase (Reverse Transcriptase) that can extend telomeres

Chromosomal Proteins

Histones

Histones

Proteins that help to package and organize DNA in Eukaryotes

DNA is attracted to histone proteins by…

Electrostatic interactions

The sugar phosphate backbone in DNA gives it a…

Negative Charge

Histone proteins have…

Amino acids with positive charges

Nucleosome

DNA wrapped around octamers of a histone proteins

Chromatin

“Beads on a chain” structure

30 nm Fiber

Coiling of Chromatin

Chromosome

Super coiling of 30nm fibers

Topoisomerases

Enzymes for supercoiling DNA

Prokaryote Chromosome Organization

Do not have histones and pack their DNA by supercoiling

Single Coding DNA

Contain the majority of protein coding sequences

Repetitive DNA

Mostly non-coding but are involved in gene expression regulation, is highly variable and used in forensics testing

Tandem Repeats

Adjacent repetitions of DNA

Minisatelities

Tandem repeats between 10-60 nucleotides

Microsatelities

Tandem repeats <10 nucleotides

Hungtington’s Disease

The result of microsatelities/short tandem repeats

Euchromatin

Lightly packed form of chromatin, DNA can be actively transcribed

Heterochromatin

Densely packed form of chromatin, DNA cannot be transcribed

Constitutive Heterochromatin

Repetitive DNA with structural roles (Ex: Centromeres and Telomeres)

Facultative Heterochromatin

Coding regions of DNA that have been silenced