Genetics and DNA Replication: Key Concepts for Biology Students

Monohybrid cross

A genetic cross that examines one trait. Example: Aa crossed with Aa.

Dihybrid cross

A genetic cross that examines two traits at the same time. Example: AaBb crossed with AaBb.

Monohybrid cross genotype ratio

The genotype ratio for Aa crossed with Aa is one AA to two Aa to one aa. 1:2:1

Monohybrid cross phenotype ratio

The phenotype ratio for Aa crossed with Aa is three dominant to one recessive. 3:1

Dihybrid cross phenotype ratio

The phenotype ratio for AaBb crossed with AaBb is nine to three to three to one. 9:3:3:1

Test cross

A cross between an individual with an unknown genotype and a homozygous recessive individual to determine the unknown genotype.

Heterozygous

Having two different alleles for a gene, such as Aa.

Homozygous

Having two identical alleles for a gene, such as AA or aa.

Locus

The physical location of a gene on a chromosome.

Dominant allele

An allele that is expressed when only one copy is present.

Recessive allele

An allele that is expressed only when two copies are present.

Genotype

The genetic makeup of an organism, such as AA, Aa, or aa.

Phenotype

The physical expression of a gene.

Alleles

Different versions of the same gene.

Y chromosome inheritance

The Y chromosome is passed directly from father to son.

SRY gene

A gene located on the Y chromosome that triggers male development.

Pleiotropy

A single gene that affects multiple traits. Example: sickle cell anemia.

Incomplete dominance

A pattern of inheritance where the heterozygous phenotype is a blend of both alleles. Example: red and white flowers producing pink flowers.

Codominance

A pattern of inheritance where both alleles are fully expressed. Example: AB blood type.

Multifactorial traits

Traits influenced by both genetics and environmental factors. Example: height or diabetes risk.

Multiple alleles

A gene that has more than two possible alleles. Example: ABO blood group.

Blood type alleles

The three alleles involved in human blood type are IA, IB, and i.

Blood type inheritance

IA and IB are codominant, and i is recessive.

X linked traits

Traits carried on the X chromosome and more common in males.

Hemizygous

Having only one copy of a gene, as seen in males for X linked traits.

X linked inheritance example

If a carrier mother has the genotype XN Xc and the father has the genotype XN Y, fifty percent of sons will be color blind.

Map unit

One map unit equals one percent recombination frequency.

Recombination frequency meaning

Higher recombination frequency means genes are farther apart, and lower recombination frequency means genes are closer together.

Meiosis and Punnett squares connection

Alleles separate during Anaphase One of meiosis, and Punnett squares predict how those alleles appear in offspring.

Chargaff's rule

In DNA, the amount of adenine equals thymine and the amount of guanine equals cytosine.

Chargaff's rule example

If adenine makes up thirty percent of DNA, then thymine is thirty percent, and guanine and cytosine are each twenty percent.

Incorrect base pairing consequence

Pairing adenine with guanine or cytosine with thymine would distort and destabilize the DNA molecule.

Prokaryotic DNA replication

Prokaryotes have one origin of replication, replicate DNA in the cytoplasm, have circular DNA, replicate faster, and use fewer enzymes.

Eukaryotic DNA replication

Eukaryotes have multiple origins of replication, replicate DNA in the nucleus, have linear DNA, replicate more slowly, and use many enzymes.

DNA structure

DNA has a double helix structure.

Antiparallel strands

The two DNA strands run in opposite directions, one from five prime to three prime and the other from three prime to five prime.

Base pairing in DNA

Adenine pairs with thymine using two hydrogen bonds, and guanine pairs with cytosine using three hydrogen bonds.

DNA polymerase requirements

DNA polymerase requires a template strand and a primer with a free three prime hydroxyl group.

DNA replication limitation

DNA polymerase cannot begin without a primer and chromosomes shorten with each round of replication.

RNA primer

The primer used in DNA replication is RNA and contains uracil instead of thymine.

Semiconservative replication

Each new DNA molecule contains one original strand and one newly synthesized strand.

Why prokaryotes have higher mutation rates

Prokaryotes replicate faster, have fewer DNA repair mechanisms, and lack chromatin protection.

Helicase function

Helicase separates DNA strands by breaking hydrogen bonds.

Topoisomerase function

Topoisomerase prevents excessive twisting and supercoiling of DNA.

Single strand binding protein function

Single strand binding proteins stabilize separated DNA strands.

Primase function

Primase synthesizes RNA primers.

DNA polymerase Three function

DNA polymerase Three adds new DNA nucleotides in the five prime to three prime direction.

DNA polymerase One function

DNA polymerase One removes RNA primers and replaces them with DNA.

Ligase function

Ligase joins DNA fragments together.

Telomerase function

Telomerase extends telomeres in germ cells and stem cells.

Okazaki fragments

Short DNA fragments synthesized on the lagging strand.

Reason Okazaki fragments are formed

DNA polymerase can only synthesize DNA in the five prime to three prime direction.

Leading strand

The leading strand is synthesized continuously toward the replication fork.

Lagging strand

The lagging strand is synthesized discontinuously away from the replication fork.

Chromatin

Chromatin is DNA combined with histone proteins.

Chromatin in prokaryotes

Prokaryotes do not have chromatin.

Histone function

Histones package DNA and help regulate gene expression.

Histone DNA attraction

Histones are positively charged and DNA is negatively charged, allowing them to bind tightly.

Telomeres

Telomeres are repetitive DNA sequences at the ends of chromosomes.

Telomere function

Telomeres protect genes and prevent chromosomes from fusing together.

DNA repair enzymes

DNA repair uses nucleases, DNA polymerase, and ligase.

DNA repair types

DNA repair includes mismatch repair, base excision repair, and nucleotide excision repair.

Telomerase location

Telomerase is found in germ cells, stem cells, and cancer cells.

Levels of DNA packing

DNA is packed as a double helix, then nucleosomes, then chromatin fiber, then looped domains, and finally metaphase chromosomes.

Protein synthesis steps

Protein synthesis occurs in two steps called transcription and translation.

Promoter region

The promoter is a DNA sequence where RNA polymerase binds to start transcription.

TATA box

The TATA box is a common promoter sequence in eukaryotes.

Product of transcription

The product of transcription in eukaryotes is pre messenger RNA.

Transcription initiation complex

The transcription initiation complex includes RNA polymerase Two, the promoter, and transcription factors.

Messenger RNA processing

Messenger RNA is processed by adding a five prime cap, adding a poly A tail, removing introns, and joining exons.

Mature messenger RNA

Mature messenger RNA is shorter than pre messenger RNA and contains no introns.

Product of translation

The product of translation is a polypeptide chain.

Eukaryotic gene expression

In eukaryotes, transcription occurs in the nucleus and translation occurs in the cytoplasm.

Prokaryotic gene expression

In prokaryotes, transcription and translation occur at the same time.

Enzyme that makes messenger RNA

Messenger RNA is made by RNA polymerase.

Codon

A codon is a sequence of three messenger RNA bases that codes for one amino acid.

Start codon

The start codon is AUG, which codes for methionine.

Stop codons

The stop codons are UAA, UAG, and UGA.

DNA to messenger RNA complementarity example

A DNA template sequence of TAC produces a messenger RNA sequence of AUG.

Messenger RNA to transfer RNA complementarity

A messenger RNA codon of AUG pairs with a transfer RNA anticodon of UAC.

Transcription factor role

Transcription factors help RNA polymerase bind to DNA and regulate gene expression.

Transfer RNA role

Transfer RNA brings the correct amino acid to the ribosome.

Universal genetic code

The genetic code is nearly the same in all living organisms.

Alternative RNA splicing

Alternative RNA splicing allows one gene to produce multiple proteins.

Eukaryotic ribosome size

Eukaryotic ribosomes are eighty S.

Prokaryotic ribosome size

Prokaryotic ribosomes are seventy S.

Medical importance of ribosome differences

Antibiotics target bacterial ribosomes without harming human cells.

Point mutation

A point mutation affects a single nucleotide.

Silent mutation

A silent mutation does not change the amino acid.

Missense mutation

A missense mutation changes one amino acid.

Nonsense mutation

A nonsense mutation creates a stop codon.

Insertion mutation

An insertion mutation adds a nucleotide and causes a frameshift.

Deletion mutation

A deletion mutation removes a nucleotide and causes a frameshift.

Signal peptide

A signal peptide is a short amino acid sequence that directs a protein to its correct destination.