Genetics

National Biology Competition

What is DNA ?

The DNA is a double helix which looks like a twisted ladder and has two strands that form the sides. It is completely made up of nucleotides .

What are nucleotides made up of ?

• sugar ( deoyglucose)

• Phosphate group

• nitrogen base

Role of sugar ( deoxyglusoce)

• What it does: It is the backbone of the DNA chain, providing a sturdy, 5-carbon, pentagon-shaped structural support.

• Why it's there: Compared to other sugars, deoxyribose is missing one oxygen atom (hence "deoxy"). This small difference makes DNA much more stable and less prone to breaking down over time compared to RNA. 

Role of Phosphate Group

• What it does: It acts as the "linker" that connects one sugar to the next, forming the sides of the "twisted ladder" (the sugar-phosphate backbone).

• Why it's there: Phosphate is excellent at connecting units together in a stable way, and it provides a negative charge to the molecule. This negative charge allows DNA to be stable, soluble in water, and protected from damage

Role of Nitrogen Base

• What it does: These are the "letters" of the genetic code—Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). They pair up to form the "rungs" of the ladder.

• Why it's there: The specific sequence of these bases (e.g., ATCGTT) contains the instructions to make proteins. They use hydrogen bonds to pair up (A with T, C with G), which are strong enough to keep the ladder together but weak enough to "unzip" for copying.

What are antiparallel strands ?

The orientation of the two DNA backbones which run parallel to each other but in the opposite direction. For instance, ( 5’ to 3’ and 3’ to 5’)

Why are antiparallel strands like that ?

DNA strands are antiparallel because each strand has a direction, from 5′ to 3′. For the bases (A, T, C, G) to pair correctly in the center of the molecule, the two strands must run in opposite directions. This arrangement also allows DNA polymerase to copy DNA properly, since it can only build new strands in the 5′ to 3′ direction.

What are the 4 Nitrogen Bases ?

1. Adenine (A)

• Pairs with Thymine (T)

• Forms 2 hydrogen bonds

2. Thymine (T)

• Pairs with Adenine (A)

• Also forms 2 hydrogen bonds

3. Cytosine (C)

• Pairs with Guanine (G)

• Forms 3 hydrogen bonds (stronger than A–T)

4. Guanine (G)

• Pairs with Cytosine (C)

• Also forms 3 hydrogen bonds

What is Protein Synthesis ?

Protein synthesis is how cells make proteins using instructions from DNA.

First, in transcription, the DNA code is copied into mRNA in the nucleus.
Then, in translation, the mRNA goes to the ribosome, where amino acids are put together to form a protein.

How does Transcription work ?

• Initiation: RNA polymerase binds to a specific region of the DNA called the promoter, signaling the start of a gene. The DNA helix unwinds and separates.

• Elongation: RNA polymerase moves along the DNA template strand (reading in the 3' to 5' direction), adding complementary RNA nucleotides to build the mRNA strand (in the 5' to 3' direction)

  . Uracil (U) is used instead of thymine (T) in RNA.

• Termination: RNA polymerase reaches a terminator sequence, which signals the end of the gene. The enzyme detaches, and the newly formed pre-mRNA strand is released. 

How does Translation work ?

1. Initiation

The small ribosome attaches to the mRNA and finds the start codon (AUG).
A tRNA carrying methionine binds to it, and then the large ribosome joins.

2. Elongation

The ribosome moves along the mRNA reading codons.
tRNAs bring the correct amino acids, which are joined together to form a growing protein chain.
The ribosome keeps moving and repeating this process.

3. Termination

When the ribosome reaches a stop codon (UAA, UAG, or UGA), no tRNA matches it.
The protein is released, and the ribosome breaks apart.

What is tRNA ?

a small RNA molecule that acts as an adaptor during protein synthesis, translating genetic code from messenger RNA (mRNA) into a sequence of amino acids to build proteins. It carries specific amino acids to the ribosome, matching its anticodon to the mRNA codon to ensure accurate protein assembly

What is mRNA ?

mRNA stands for messenger ribonucleic acid. It is a single-stranded molecule that carries genetic instructions from DNA in the cell nucleus to the ribosomes, where it directs the synthesis of proteins necessary for bodily functions. mRNA plays a key role in protein production and is used in vaccines to teach cells how to make proteins that trigger immune responses

Gene

the basic physical and functional unit of heredity, composed of specific DNA sequences that instruct cells to produce proteins or regulate molecular processes

Genome

an organisms complete set of DNA

Allele

two or more forms of DNA made by mutation and are foudn on the same palce on the locus ( chromosome)

Chromosome

thread-like structures located inside the nucleus of animal and plant cells, made of protein and a single molecule of DNA

Locus

the specific, fixed physical position of a gene or DNA sequence on a chromosome.

Genotype

an organism's unique genetic makeup, specifically the set of genes or alleles inherited from parents, represented by letters (e.g. AA,aa)

Phenotype

the set of observable characteristics, traits, or behaviors of an organism, such as height, eye color, or blood type. It is determined by the interaction of an organism's genetic makeup (genotype) and environmental influences.

Homozygous

a genetic condition where an individual inherits identical forms (alleles) of a specific gene from both biological parents.

Heterozygous

having two different versions (alleles) of a specific gene, one inherited from each biological parent. This contrasts with homozygous, where an individual has two identical alleles. Heterozygous genotypes are often represented by one dominant and one recessive allele (e.g. Aa)

Dominant Allele

a version of a gene that expresses its associated phenotype (trait) even when only one copy is inherited from a single parent.

Recessive Allele

a version of a gene that only expresses its associated phenotype (trait) when an individual inherits two copies—one from each parent (homozygous recessive).

Polygenic Trait

characteristics influenced by two or more genes,

Condominance

a genetic inheritance pattern where two different alleles (versions of a gene) are equally expressed in a heterozygote's phenotype,

Haplotype

a group of genes or DNA variations (like SNPs) on a single chromosome that are inherited together from a single parent

Pleitrophic

a genetic phenomenon where a single gene or locus influences multiple, seemingly unrelated phenotypic traits

What are non-mendelian traits ?

Non-Mendelian traits are characteristics that do not follow simple dominant/recessive patterns, often involving multiple genes, intermediate expression, or sex linkage.

Key examples of non-mendelian traits ?

• Incomplete Dominance: The phenotype is a blend of the two parents.

• Codominance: Both alleles are expressed equally. Example: Roan cattle (having both red and white hair) or AB blood type.

• Polygenic Traits: Traits controlled by multiple genes, resulting in a gradient. Examples: Human height, skin color, and eye color.

• Multiple Alleles: A gene with more than two possible alleles. Example: The ABO blood system in humans.

• Sex-Linked Traits: Traits determined by genes on the X or Y chromosomes. Examples: Red-green colorblindness and hemophilia, which are more common in males.

• Pleiotropy: One gene influences multiple, unrelated traits. Example: Sickle cell anemia causes anemia, heart damage, and immune issues.

• Epistasis: One gene hides or modifies the expression of another. Example: Labrador retriever coat color.

What are mendelian traits ?

Mendelian traits are characteristics controlled by a single gene and passed down through dominant or recessive inheritance patterns, following principles discovered by Gregor Mendel.

Mendels Three Laws of Inheritance

1. Law of Dominance

2. Law of Segrgation

3. Law of Independent Assortment

Law of Dominance

Definition : When an organism has two different alleles (variants) for a trait, one allele (dominant) will mask the presence of the other (recessive).

Outcome: The (F1) generation (first offspring) will only show the dominant phenotype (observable trait), appearing uniform. 

Law of Segregation

Definition: During gamete formation (sex cell production), the two alleles for a inheritable trait separate (segregate), meaning each gamete carries only one allele for each gene.

Outcome: Offspring receive one allele from each parent, restoring the pair during fertilization, ensuring genetic diversity. 

Law of Independent Assortment

Definition: Genes for different traits (e.g., color and height) are sorted into gametes independently of one another.

Outcome: The inheritance of one trait does not affect the inheritance of another (e.g., a plant with purple flowers is not automatically tall). Note: This applies to genes on different chromosomes or far apart on the same chromosome. 

What are Punnet Squares ?

A Punnett square is a graphical, grid-based tool used in genetics to predict the potential genotypes and phenotypes of offspring from a cross between two parents. It illustrates all possible combinations of maternal and paternal alleles, helping determine the probability of specific inherited traits. 

Mutations

A mutation is a permanent alteration in the DNA sequence of an organism or virus, acting as a change in the genetic instruction manual. Mutations occur due to DNA replication errors during cell division or external environmental factors like UV radiation

The Types of Mutations

1. Point Mutations

2. Frameshift Mutation

3. Chromosomal Mutation

Point Mutations

• One base pair is replaced by another , they are further divided into 3 subcategories

1. Silent - no chnage in amino acid

2. Missense - results in a different amino acid

3. Nonsense - creates a permenet stop codon

Frameshift Mutation

The insertation or deletion of nucleotides not in multiples of three, shifting the reading frame , which can result in a non functional protein

Chromosomal Mutation

Large-scale chamnges affecting the chormosomal stucture of number

Causes of Mutations

1. Spontaneous : occur duriing DNA replication or due to nautral chemical breackdowns

2. Induced : caused by mutagens

Effects of Muttations

1. Somatic - can occur in non reproductive cells and are paired to offsprings

2. Germiline : occurs in gamates and are not inherited

3. Imapact on protein

4. DNA repair

Gene Regulation

the fundamental, highly controlled process used by cells to turn genes "on" or "off," controlling the timing, location, and amount of functional protein products produced.

Key aspects of gene regulation

• Transcriptional Control: The primary regulation method involves transcription factors (proteins) binding to specific DNA sites (promoters, enhancers, silencers) to increase or decrease mRNA synthesis.

• Eukaryotic vs. Prokaryotic: Eukaryotes regulate expression at epigenetic, transcriptional, translational, and post-translational levels, whereas prokaryotes primarily regulate at the transcriptional level

• Cell Differentiation: While all cells in a human hold the same DNA, gene regulation defines cell-specific functions (e.g., neurons vs. muscle cells) by determining which genes are active.

• Importance: It ensures energy efficiency in cells, prevents waste, and manages development; malfunction can lead to cancer and developmental diseases. 

Mechanisms of Gene Regulation

• Epigenetic Modification: Changes that affect gene expression without altering the DNA sequence, often through DNA methylation.

• Post-transcriptional/Translational Regulation: Controls RNA processing and stability, or how efficiently mRNA is translated into protein.

• Constitutive vs. Regulated Genes: Constitutive genes are always "on" (housekeeping), while regulated genes are switched on/off in response to signals. 

Gene Expression

the fundamental process by which information from a gene's DNA sequence is used to synthesize functional gene products, typically proteins or functional RNA molecules.

Type of Genetic Disorders

1. Monogenic (Single-Gene) Disorders

2. Chromosomal Disorders

3. MultifactoralDieseases

Monogenic (Single-Gene) Disorders

These occur when a specific mutation affects one gene. They are often inherited in specific patterns. 

• Autosomal Dominant: Only one copy of the defective gene is needed (e.g., Huntington's disease, Marfan syndrome).

• Autosomal Recessive: Two copies of the defective gene are needed; both parents are typically carriers (e.g., Cystic fibrosis, Sickle cell anemia, Tay-Sachs disease).

• X-Linked: The mutation occurs on the X chromosome. These are often X-linked recessive, affecting males more often (e.g., Duchenne muscular dystrophy, Hemophilia A). 

Chromosomal Disorder

These happen when there are structural changes or an incorrect number of chromosomes. 

• Numerical Abnormalities: Having an extra chromosome (Trisomy) or missing one (Monosomy). Examples include Down syndrome (Trisomy 21), Trisomy 18, and Turner syndrome (one X chromosome).

• Structural Abnormalities: Parts of chromosomes are deleted, inverted, or moved to another chromosome (e.g., Williams syndrome, Fragile X syndrome)

Multifactoral Dieseases

These are caused by a combination of multiple gene mutations and environmental factors like lifestyle, diet, or pollution.

What is sexual reproduction?

Reproduction where two parents combine genetic material to produce offspring.

What is asexual reproduction?

Offspring are genetically identical (clones).

Why does sexual reproduction create variation?

Because DNA from two parents mixes during fertilization.

Why does asexual reproduction create identical offspring?

Because only one parent copies its DNA directly.

Key genetic process in sexual reproduction

Meiosis, which produces haploid gametes with half the genetic information.

Advantage of genetic variation

Improves survival of a species in changing environments through natural selection.

Disadvantage of asexual reproduction

Low genetic variation, making populations vulnerable to disease or environmental change.