Micro Exam #2

Genetics

Study of heredity and variation.

Gene

Segment of DNA (or RNA in viruses) coding for a trait or function.

Genome

Complete set of genetic material in an organism.

Bacteria (Prokaryotes)

DNA, circular chromosome.

Eukaryotes

DNA, linear chromosomes in nucleus.

Viruses

DNA or RNA (single- or double-stranded).

Central Dogma of Genetics

Flow of information: DNA → RNA → Protein.

Prokaryotes (Chromosome Differences)

Single circular chromosome, no nucleus, plasmids common.

Eukaryotes (Chromosome Differences)

Multiple linear chromosomes, in nucleus, wrapped around histones.

Nucleotide Structure

3 parts: phosphate group, sugar, nitrogenous base.

DNA

deoxyribose sugar, bases = A, T, G, C.

RNA

ribose sugar, bases = A, U, G, C.

Base Pairing

Purines (A, G) pair with pyrimidines (T, C, U).

DNA Structure

Double helix shape.

RNA Structure & Differences from DNA

Single-stranded, ribose sugar (extra -OH group), uracil replaces thymine.

Purpose of DNA Replication

Occurs before binary fission in prokaryotes and before cell division in eukaryotes.

Helicase

Unwinds the double helix.

Primase

Lays down RNA primers.

DNA Polymerase III

Synthesizes new DNA strand (5′→3′ direction).

DNA Polymerase I

Removes RNA primers, replaces with DNA.

Ligase

Seals nicks, joins Okazaki fragments on lagging strand.

Topoisomerase/Gyrase

Relieves supercoiling ahead of replication fork.

Single-stranded binding proteins (SSBPs)

Stabilize unwound DNA.

Initiation of Replication

Starts at the origin of replication (ori site).

Directionality & Strand Synthesis

DNA strands are antiparallel → replication is not identical on both sides.

Leading strand

Synthesized continuously (toward replication fork).

Lagging strand

Synthesized discontinuously as Okazaki fragments (away from fork).

Semiconservative Replication

Each new DNA molecule has 1 original (parental) strand and 1 newly synthesized strand.

Genetics

The scientific study of heredity and variation in organisms.

Gene

A segment of DNA (or RNA in viruses) that codes for a functional product, usually a protein or RNA molecule.

Genome

The entire set of genetic material of an organism, including all genes and noncoding sequences.

Bacterial Genome

DNA, typically as a single circular chromosome.

Eukaryotic Genome

DNA, organized into multiple linear chromosomes inside the nucleus.

Viral Genome

Either DNA or RNA, which may be single- or double-stranded.

Central Dogma of Molecular Biology

DNA is transcribed into RNA, and RNA is translated into proteins.

Prokaryotic Chromosomes

Usually have a single circular chromosome without a nucleus.

Eukaryotic Chromosomes

Have multiple linear chromosomes housed in a nucleus.

Plasmids

Small extrachromosomal DNA molecules that may be present in prokaryotes.

Nucleotide

Composed of a phosphate group, a five-carbon sugar, and a nitrogenous base.

DNA Sugar

Deoxyribose.

RNA Sugar

Ribose.

DNA Bases

Adenine (A), thymine (T), guanine (G), and cytosine (C).

RNA Bases

Adenine (A), uracil (U), guanine (G), and cytosine (C).

Adenine Pairing in DNA

Adenine pairs with thymine by two hydrogen bonds.

Guanine Pairing in DNA

Guanine pairs with cytosine by three hydrogen bonds.

Double Helix Structure of DNA

DNA consists of two strands twisted around each other in a spiral.

Complementary in DNA Structure

Each base pairs with its specific partner: A with T, G with C.

Antiparallel in DNA Structure

The two DNA strands run in opposite directions: one 5′→3′ and the other 3′→5′.

RNA Structural Difference from DNA

RNA is usually single-stranded, contains ribose instead of deoxyribose, and uses uracil instead of thymine.

Role of DNA Replication before Binary Fission

To ensure each daughter cell receives a complete and identical copy of the genome.

Function of Helicase in DNA Replication

Helicase unwinds the DNA double helix at the replication fork.

Function of Primase in DNA Replication

Primase synthesizes short RNA primers for DNA polymerases to begin replication.

Function of DNA Polymerase III

DNA polymerase III synthesizes new DNA strands in the 5′→3′ direction.

Function of DNA Polymerase I

DNA polymerase I removes RNA primers and replaces them with DNA nucleotides.

Function of Ligase

Ligase seals nicks in the sugar-phosphate backbone, joining Okazaki fragments.

Function of Topoisomerase

Topoisomerase (or gyrase in prokaryotes) helps relieve strain while DNA is being unwound.

Topoisomerase (or gyrase in prokaryotes)

It relieves supercoiling ahead of the replication fork.

Single-stranded binding proteins (SSBPs)

They stabilize separated DNA strands during replication.

Origins of replication

Where DNA replication begins on the DNA molecule.

Leading and lagging strands

Needed because DNA polymerases can only synthesize 5′→3′; replication is continuous on the leading strand but discontinuous on the lagging strand.

Leading strand synthesis

Continuously in the direction of the replication fork.

Lagging strand synthesis

Discontinuously in short Okazaki fragments away from the fork.

Semiconservative DNA replication

Each new DNA molecule has one parental strand and one newly synthesized strand.

Transcription enzyme

RNA polymerase.

Transcription process

RNA polymerase binds to the promoter, unwinds DNA, and synthesizes a complementary RNA strand using the DNA template strand.

Reverse transcription

The synthesis of DNA from an RNA template, carried out by retroviruses using reverse transcriptase.

Function of messenger RNA (mRNA)

To carry genetic instructions from DNA to ribosomes for protein synthesis.

Function of transfer RNA (tRNA)

To bring specific amino acids to the ribosome during translation.

Function of ribosomal RNA (rRNA)

To form part of ribosome structure and catalyze peptide bond formation.

Role of mRNA in translation

mRNA contains codons that specify the sequence of amino acids in a protein.

Role of the ribosome in translation

The ribosome reads mRNA codons and facilitates peptide bond formation between amino acids.

Role of tRNA in translation

tRNA carries amino acids and pairs its anticodon with mRNA codons.

Importance of codon-anticodon pairing

It ensures that the correct amino acid is inserted into the growing protein chain.

Codon

A three-nucleotide sequence on mRNA that specifies a particular amino acid or stop signal.

Start codon

AUG, which codes for methionine.

Stop codons

UAA, UAG, and UGA.

Promoter in an operon

The site where RNA polymerase binds to initiate transcription.

Operator in an operon

The site where regulatory proteins bind to control transcription.

Structural genes in an operon

Genes that code for the proteins or enzymes of the operon's function.

Inducible operon

An operon that is normally off but can be activated in the presence of an inducer (e.g., lac operon).

Repressible operon

An operon that is normally on but can be turned off in the presence of a corepressor (e.g., trp operon).

Substitution mutation

A mutation where one nucleotide is replaced by another.

Insertion mutation

A mutation where one or more nucleotides are added into the sequence.

Deletion mutation

A mutation where one or more nucleotides are removed from the sequence.

Silent mutation

A mutation that changes a nucleotide but does not change the amino acid coded for.

Missense mutation

A mutation that changes a codon so that a different amino acid is produced.

Nonsense mutation

A mutation that changes a codon into a stop codon, causing premature termination of translation.

Frameshift mutation

A mutation caused by insertions or deletions that are not in multiples of three, shifting the reading frame and altering all downstream codons.

Spontaneous mutation

A mutation that arises naturally, often due to errors in DNA replication.

Induced mutation

A mutation caused by exposure to mutagens such as radiation or chemicals.

Vertical gene transfer

The transfer of genetic material from parent to offspring.

Horizontal gene transfer

The transfer of genetic material between organisms of the same generation.

Conjugation

The transfer of plasmid DNA between bacterial cells through a pilus.

Transformation

The uptake of naked DNA from the environment by a bacterial cell.

Transduction

The transfer of bacterial DNA from one cell to another via a bacteriophage.

Nonliving microbes

Viruses are considered nonliving microbes because they cannot reproduce independently, lack metabolism, and require a host cell to replicate.

Viral capsid

A protein shell that protects the viral genome.

Viral envelope

A lipid membrane derived from the host cell that surrounds some viruses.

Viral spikes

Glycoprotein projections used for recognition and attachment to host cells.

Host range in viruses

The spectrum of organisms a virus can infect.