The process by which a complementary RNA strand is produced based on the DNA template.
Key features of RNA synthesis:
In DNA, thymine (T) is replaced by uracil (U) in RNA.
Messenger RNA (mRNA) plays a vital role in directing the synthesis of polypeptides.
Translation
Defined as the synthesis of polypeptides, the process occurs at the ribosome.
Translation requires several kinds of RNA which are essential for the process.
Basic Process of Transcription
Initiation:
RNA polymerase locates and binds to the promoter region on DNA.
Elongation:
RNA polymerase adds adjacent ribonucleotides that are complementary to the DNA bases.
Termination:
RNA polymerase reaches a termination sequence in DNA at the gene's end, leading to its dissociation from DNA.
Promoter Function
The promoter is a specific site in DNA where gene transcription begins.
It serves as a recognition/binding site for RNA polymerase and is located upstream of the start site, not transcribed into RNA.
Eukaryotic RNA Polymerases
There are three distinct RNA polymerases in eukaryotes:
RNA Polymerase I:
Transcribes ribosomal RNA (rRNA).
RNA Polymerase II:
Transcribes messenger RNA (mRNA) and some small nuclear RNA (snRNA).
RNA Polymerase III:
Transcribes transfer RNA (tRNA) and some small RNAs.
Each polymerase recognizes its own specific promoter.
Differences Between Prokaryotic and Eukaryotic Genes
Prokaryotic Genes:
Entire gene is a coding sequence, containing information for amino acid placement.
Eukaryotic Genes:
Contain coding sequences (exons) interspersed with non-coding sequences (introns).
Introns must be removed from mRNA during processing.
Events of Messenger RNA Modification in Eukaryotes
Due to the structure of eukaryotic genes, the first RNA synthesized must be modified to become mature mRNA:
Addition of a 5' Cap:
Protects from degradation and is involved in translation initiation.
Addition of a 3' Poly-A Tail:
Created by poly-A polymerase; protects from degradation.
Removal of Non-Coding Sequences (Introns):
Pre-mRNA splicing is performed by the spliceosome.
Roles of tRNA, rRNA, and mRNA in Polypeptide Formation
mRNA:
Provides information for the correct placement of amino acids in a polypeptide's primary structure.
tRNA:
Carries amino acids to the appropriate codon in the mRNA at the ribosome for incorporation into the polypeptide chain.
rRNA:
Decodes the mRNA and forms peptide bonds between amino acids.
Genetic Code
Describes the relationship between nucleotide sequences in DNA and/or RNA and amino acid sequences in polypeptides.
Codon:
A sequence of three DNA or RNA nucleotides corresponding to a specific amino acid.
Stop Codons:
Three codons (UAA, UGA, UAG) signal the termination of translation.
Start Codon:
The codon (AUG) marks the beginning of translation.
The code is redundant, meaning some amino acids are specified by multiple codons.
tRNA Binding Sites in the Ribosome
P Site:
Binds the tRNA attached to the growing peptide chain.
A Site:
Binds the tRNA carrying the next amino acid.
E Site:
Binds the tRNA that previously carried the last amino acid.
Aminoacyl-tRNA Synthetases
Enzymes responsible for adding amino acids to the acceptor stem of the specific tRNA.
The anticodon loop on tRNA contains three nucleotides that are complementary to mRNA codons.
Elongation Cycle of Transcription
Grows in the 5' to 3' direction as ribonucleotides are added.
Transcript Bubble:
Contains RNA polymerase, DNA template, and a growing RNA transcript.
After the transcription bubble passes, the now-transcribed DNA rewinds behind it.
Principles of Gene Regulation
Regulatory proteins access the bases of DNA at the major groove.
These proteins possess specific DNA-binding motifs that influence transcription.
Types of Regulatory Proteins
Repressor Protein:
Inhibits transcription by binding to specific DNA regions (like operators) to block RNA polymerase from starting transcription.
Activator Protein:
Increases the likelihood of transcription by binding to specific DNA sequences such as enhancers.
Induction and Repression
Induction:
The process of turning on enzymes produced in response to a substrate.
Repression:
Turning off the production of an enzyme even though it is capable of being made.
Gene Expression in the Lac Operon
Several genes are controlled together that are involved in the same metabolic pathway.
Contains genes necessary for utilizing lactose as an energy source.
The gene for the lac repressor (lacI) is linked to the rest of the lac operon.
Transcription Factors in Eukaryotic Transcription
Necessary for the assembly of the transcription apparatus and recruitment of RNA polymerase II to a promoter.
TFIID recognizes TATA box sequences.
Specific Transcription Factors
Enhance transcription levels in certain cell types or in response to environmental signals.
Chromatin Structure and Its Effect on Gene Expression in Eukaryotes
Chromatin packing affects gene expression.
Tightly packed chromatin limits or prevents transcription.
DNA is wrapped around histone proteins to form nucleosomes which may block access to the promoter.
Histone modifications can lead to condensation or relaxation of chromatin structure, influencing gene expression.
Differences in Gene Expression Between Prokaryotes and Eukaryotes
Prokaryotic Regulation: Organisms adjust gene expression based on environmental conditions.
Eukaryotic Regulation: Cells modulate gene expression to maintain homeostasis of the organism.
Purpose of Meiotic Cell Division
In Unicellular Organisms:
E.g. Bacteria, amoebas, yeast:
Asexual reproduction occurs through the division of one cell into two, increasing population size.
In Multicellular Organisms:
Cells grow by increasing their number and adding new cells to tissues and organs.
Functions in repairing damaged or dead cells and the development from a single fertilized egg into many specialized cells forming tissues and organ systems.
Function of Meiosis in Sexual Reproduction
Produces gametes:
Generates haploid cells, which contain half the number of chromosomes compared to somatic cells.
Ensures that upon fertilization, the zygote has the correct diploid number of chromosomes.
Maintains chromosome number across generations.
Prevents chromosome number from doubling with each generation.
Meiosis achieves this by reducing the chromosome number from diploid (2n) to haploid (n).
Increase Genetic Variation in Meiosis
Crossing Over: The exchange of genetic material between homologous chromosomes that increases genetic variation.
Independent Assortment: The random distribution of maternal and paternal chromosomes into gametes.
Random Fertilization: The combining of gametes during fertilization which also contributes to genetic diversity.
Germ-line vs. Somatic Cells
Germ-line Cells:
Give rise to gametes through meiosis and can pass mutations to the next generation. Found in ovaries and testes (e.g. sperm and egg production).
Somatic Cells:
Non-reproductive cells that undergo mitosis; they are not inherited by offspring. They serve functions in body tissues and organs, growth, repair, and everyday activities (e.g. skin, nerve, liver cells).
Homologous Chromosomes Pairing During Meiosis
Events of condensing begin in early prophase I where chromosomes coil and become visible.
Recognizing homologous partners occurs through molecular recognition facilitated by DNA sequences.
Synapsis:
The alignment of homologous chromosomes side by side, forming a synaptonemal complex.
Formation of a Tetrad occurs when homologous chromosomes pair, containing four chromatids.
Crossing Over
Takes place at points called Chiasmata, where non-sister chromatids exchange DNA, further increasing genetic variation.
At anaphase I, homologous chromosomes (not sister chromatids) separate and move toward opposite poles.
Synapsis and Tetrads
Synapsis: Pairing of homologous chromosomes that occurs in prophase I.
A synaptonemal complex holds them together and facilitates crossing over, ensuring accurate chromosome separation.
Tetrads: A structure comprising four chromatids formed during synapsis.
Composed of two homologous chromosomes (each containing two sister chromatids).
Steps of Meiosis
Meiosis I (Reductional Division):
Phases include Prophase I, Metaphase I, Anaphase I, and Telophase I — homologous chromosomes separate.
Meiosis II:
Phases include Prophase II, Metaphase II, Anaphase II, and Telophase II — sister chromatids separate.
Behavior of Chromosomes During Meiosis
In Meiosis I:
Homologous chromosomes pair and undergo crossing over before separating, thereby reducing chromosome number.
In Meiosis II:
Sister chromatids separate; the chromosome number remains haploid, resulting in four genetically distinct gametes.
Events of Anaphase I and Anaphase II of Meiosis
Anaphase I:
Homologous chromosomes separate; the chromosome number is reduced.
Centromeres do not split, and chromatids remain together, leading to a reductional division.
Anaphase II:
Sister chromatids separate; the chromosome number remains unchanged.
Centromeres split, leading to equational division.
Distinct Features of Meiosis
Comprises two successive divisions: meiosis I and II, involving one DNA replication followed by two divisions, generating four cells.
Results in a reduction of chromosome number (from 2n to n) during meiosis I, where homologous chromosomes separate.
Pairing of homologous chromosomes (synapsis) occurs in Prophase I, resulting in the formation of tetrads.
Crossing over results in non-sister chromatids exchanging genetic material at chiasmata, enhancing genetic variation.
Independent Assortment occurs when homologous chromosomes orient randomly during Metaphase I, leading to varied combinations of maternal and paternal chromosomes.
Meiosis and Genetic Variation
Homologous chromosomes separate in Meiosis I during Anaphase I, while sister chromatids remain together to form diverse gametes.
All four haploid daughter cells produced are genetically distinct due to crossing over and independent assortment.
Differences in Cohesin Mechanisms in Meiosis and Mitosis
Mitosis:
All cohesin is removed in a single event; shugoshin's role is absent.
Sister chromatids separate during one division.
Meiosis I:
Arm cohesin is removed yet centromeres remain protected by shugoshin, allowing homologous chromosomes to separate during the first division.
Centromere cohesin is removed in Meiosis II, leading to sister chromatids' separation.
Genetic Implications of Meiotic Division
Suppression of replication between meiotic divisions prevents genetic abnormalities.
This regulation assures proper separation of chromatids and produces functional haploid gametes.
Comparative Analysis: Mitosis vs. Meiosis
Mitosis:
Single division, produces one cell, maintains chromosome number (2n), no genetic variation (no synapsis or crossing over), homologous chromosomes do not pair, sister chromatids separate once, primarily serves growth and repair functions.
Meiosis:
Two divisions, producing four cells, chromosome number is halved (n), high genetic variation (involves synapsis and crossing over), homologous chromosomes pair, sister chromatids separate in Meiosis II, functions in sexual reproduction.
Historical Context of Genetic Theories Before Mendel
Blending Theory:
Proposed that offspring are merely a mix of parental traits.
Use/Disuse Inheritance:
Suggested that traits acquired during the lifetime of an organism can be passed down to offspring.
Advantages of Mendel's Experimental System with Peas
Clear traits (tall/short, green/yellow seeds).
Available true-breeding varieties.
Short generation times for quick results.
Ease of controlled crosses and large amounts of offspring produced, providing robust data.
Definitions of Genetic Terms
Monohybrid Cross:
A cross focusing on a single trait.
Dihybrid Cross:
A cross focusing on two traits.
Dominant Trait:
A trait that appears with at least one dominant allele present.
Recessive Trait:
A trait that is hidden unless two recessive alleles are present.
Phenotype:
Physical appearance (e.g. tall).
Genotype:
Genetic makeup (e.g. TT, Tt, tt).
Homozygous:
Two identical alleles (e.g. TT or tt).
Heterozygous:
Two different alleles (e.g. Tt).
Allele:
A version of a gene.
Punnett Square Example
Given the cross T+ x T+:
Possible genotypes: TT, Tt, Tt, tt.
Phenotype ratio: 3:1 (tall to short).
Genotype ratio: 1:2:1 (TT:Tt:tt).
Mendel's Principle of Segregation
States that allele pairs separate during meiosis; gametes receive one allele from each pair, recombining at fertilization.
This segregation of alleles occurs in accordance with the behaviors of homologous chromosomes during meiosis, where homologs separate during Anaphase I and sister chromatids separate in Meiosis II.
Punnett Square Setup:
Position parental alleles on top/side and fill the genotype combinations; derive phenotype ratios.
Dihybrid Cross and Independent Assortment
Example: Crossing RrYy x RrYy:
Requires use of FOIL method to determine gametes: RY, Ry, rY, ry, leading to a phenotypic ratio of 9:3:3:1.
Mendel's Principle of Independent Assortment states that alleles of different genes segregate independently during gamete formation.
Identifying Unknown Genotypes
If all offspring exhibit the dominant trait, the unknown is likely homozygous dominant (RR).
A 1:1 ratio in traits suggests the unknown is heterozygous (Rr).
Limitations of Mendel's Model
Not all traits are governed by only two alleles.
Some genes exhibit interactions.
Dominance is not always complete.
Environmental factors can influence traits.
Genetic Explanation for Continuous Variation
Traits such as height, skin color, and weight reveal a continuous spectrum of outcomes.
These variations arise from polygenic inheritance, where multiple genes contribute to a single trait.
Alterations to Mendel's Ratios
Codominance: Both alleles are fully expressed (e.g., AB blood type).
Incomplete Dominance: The heterozygote presents an intermediate phenotype (e.g., red and white flowers producing pink flowers).
Multiple Alleles: More than two alleles for a gene exist, as seen in blood groups (A, B, O).
Pleiotropy: One gene influences multiple traits (e.g., sickle cell disease affects blood cells, organs, and immunity).
Polygenic Inheritance: Many genes collectively control a single trait, leading to continuous variation (e.g., height and skin color).
Physical Basis of Independent Assortment
Homologous chromosomes align randomly during Metaphase I of meiosis, causing each gamete to receive a random assortment of maternal and paternal chromosomes, thereby generating genetic variation.
Probability Rules in Genetics
Rule of Addition: Used when events are mutually exclusive.
Example: The probability of getting either Aa or aa (cannot happen simultaneously).
Rule of Multiplication: Used when events occur together independently.
Example: The probability of obtaining at least one A from two heterozygous parents.
Genetic Mapping with Pedigrees
Dominant Traits: Display in every generation; affected individuals typically have an affected parent.
Recessive Traits: Can skip generations; an affected individual may be born to unaffected parents.
Test Cross: Used to infer genotypes by crossing an unknown phenotype with a homozygous recessive individual.
Sex-linked Inheritance in Fruit Flies
Genes located on the X chromosome indicate sex-linked inheritance.
Example: The white eye mutation is an X-linked recessive trait.
Males (XY) exhibit the mutant trait more readily because they possess only one X chromosome.
Females (XX) require two copies of the recessive allele to express the trait.
Relationship Between Sex Chromosomes and Sex Determination
Sex determination relies on the presence or absence of the Y chromosome.
In humans: XX= Female, XY= Male.
The Y chromosome harbors the SRY gene, which triggers male development.
Sex-linked Inheritance in Human Pedigrees
X-linked Recessive Traits: More males are affected; affected males pass the trait to all daughters (who become carriers) but have no affected sons.
X-linked Dominant Traits:
Are present in affected males who pass the trait to all daughters (no affected sons); affected females have a 50% chance of having affected children.
Y-linked Traits: Only males are affected, passed from father to all sons.
How Mutations Cause Disease
Mutations alter the DNA sequence, which may:
Change protein structure or function,
Prevent protein production,
Cause a loss of function (common) or a gain of function (rare).
Consequences of Nondisjunction in Humans
Nondisjunction refers to the failure of chromosomes to separate during meiosis.
It leads to gametes with abnormal chromosome numbers.
Trisomies:
Examples include Down syndrome, Edward's syndrome, and Patau syndrome.
Sex Chromosomes Nondisjunction:
Disorders such as Turner syndrome (XO), Klinefelter syndrome (XXY), and XXX females (usually normal) are caused by nondisjunction.
XYY males are typically normal yet often taller than average.