Topic: Genetics

⋆｡˚⭒°✮˙⋆ Important Definitions ⋆˙✮°⭒˚｡⋆

• Gene - A segment of  DNA on a chromosome that codes for a protein.
• Genome - A set of all DNA molecules in a cell, or an organism.
• Chromosome - A thread of DNA, made up of a string of genes.
• Genotype  - The genetic make-up of an organism in terms of the alleles present.
• Phenotype  - The observable features and traits of an organism.
• Homozygous - It means having two identical alleles of a particular gene e.g. TT, where T is tall. 
• Pure Breeding - Occurs when two identical homozygous individuals breed together.
• Heterozygous - It means having two different alleles of a particular gene e.g. Tt
• Dominant Allele - An allele that is always expressed if it is present is dominant.
• Recessive Allele - An allele that is only expressed when there is no dominant allele of the gene present is recessive.
• Inheritance - Transmission of genetic information from generation to generation.
• Haploid nucleus - A nucleus containing a single set of unpaired chromosomes present, for example, in sperm and egg cells.
• Diploid nucleus - A nucleus containing two sets of chromosomes present, for example, in body cells.

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The Laws of Mendel

°✮˙⋆ Mendel's 1st Law⋆˙✮°

1. The Law of Segregation: 

- States that when any individual produces gametes, the 2 copies of gene separate, so half of the gametes receive one copy and half receive the other copy. 

↳ Allele comb from a cross can be predicted using punnette square.

°✮˙⋆ Mendel's 2nd Law⋆˙✮°

2. The Law of Independent Assortment: 

- States that Alleles of different genes assort independently of one another during gamete formation.

↳ Homologous chromosome pairs segregate independently of one another.

°✮˙⋆ Mendel's 3rd Law⋆˙✮°

3. The Law of Dominance:  

- States that when parents with pure traits are crossed together, only one form of trait appears in the next generation.

The Structure of genes

Genes are a sequence of nucleotide bases that forms part of a DNA molecule (Chromosome) that codes for a protein. 

Most genes are around 1000 to 2000 nucleotides in length.

Basically, each chromosome is made up of a long strand of DNA. The DNA strand is tightly packaged around histone proteins. And the different sections along the DNA are what we call genes. Simple!

Levels of Gene Expression

Gene expression is the transcription and translation of the gene, resulting in the polypeptide for which it codes.

Gene expression can be controlled at the transcriptional, post-transcriptional, translational and post-translational level.

Transcriptional Control - The Lac Operon 

The Lac Operon is a length of DNA made up of structural genes and control sites, which controls the expression of β-Galactosidase (responsible for hydrolysis of lactose) in E. coli bacteria.

  The operon has:

  1. A promoter region; binding site for RNA polymerase to initiate transcription

  2. An operator region; binding site for inhibitor/transcription factor

  3. Structural genes; code for proteins involved in producing the enzyme

* Regulatory gene codes for the inhibitor/repressor protein, and is located outside the operon.

*Promoter for regulatory gene allows transcription to begin.

--When conc. of glucose is high and conc. of lactose is low, regulatory gene is expressed and repressor protein is produced and binds to operator. So RNA polymerase cannot bind to promoter and no transcription of lactase enzymes occurs.

--When conc. of glucose is low and con. of lactose is high, lactose binds repressor protein and prevents it from binding to operator. So RNA polymerase can bind to promoter and transcription of lactase enzymes occurs.

Gene expression is also controlled by transcription factors which can switch genes on and off, by interacting with promoter.

Post-Transcriptional Control

Gene expression is controlled at post transcriptional level by editing the primary mRNA transcript, where non-coding regions called introns are removed. Thus leaving only exons. 

Post-Translational Control

Gene expression is controlled at post translational level.

 e.g Proteins being activated by the help of cyclic AMP. 

  ↳ This occurs when adrenaline binds to a complementary receptor, thus activating adenylate cyclase, which converts ATP to AMP. This starts a cascade of enzyme reactions in the cell which activates the protein. 

Gene Mutations: Types and Consequences

Mutations are changes in the sequence of nucleotides in the DNA. 

These are the types of Mutations:

• Insertion/Deletion Mutations - One or more nucleotide pairs are inserted or deleted from the sequence, causing a frameshift.
• Point Mutation/Substitution - Occurs when one base pair is replaced by another.
• Nonsense Mutation - When translation is stopped early, resulting in a polypetide that's been cut short with the end missing, due to a stop codon being introduced too early.
• Missense Mutation - When there's a codon change, resulting in the production of a different amino acid, leaving us with an altered Tertiary structure of a protein. 
• Silent Mutation - When there's a codon change but it does not affect the amino acid sequence produced. 

Conditions caused by Gene Mutations

Some examples:

Albinism - Mutation in TYR gene. Mutant allele is recessive and does not allow melanin to be produced. 

Sickle Cell Anemia - Missense Mutation in gene coding for β polypeptide in a haemoglobin molecule. It causes red blood cells to become sickle shaped.

Haemophilia - Mutation in gene coding for a clotting factor, F8. Mutant allele is recessive, and the gene is found on X chromosome so it's sex-linked.

Huntington's Disease - Mutation in HTT gene coding for Huntingtin protein. It leads to degeneration of nerve cells in brain causing cognitive and movement problems.

Cystic Fibrosis - Mutation in gene coding for CFTR protein. 

Chromosomal Mutations: Human Chromosome Number Abberations

Non-disjunction mutation is when there's a problem in the separation of chromosomes during meiosis. This results in one daughter cell losing an entire chromosome, and one daughter cell gaining an extra chromosome. 

It occurs during Anaphase and Telophase.

Conditions caused by Chromosomal Mutations

Down Syndrome - Occurs when there's an extra chromosome 21. (One parent cell has 3 copies instead of just two)

Turner's Syndrome - Occurs when there's only one complete X chromosome. (The second copy of the X chromosome is incomplete or missing)

Darwin's Theory of Evolution by Natural Selection

1. Reproduction generally creates more individuals than can possibly survive.

2. As a consequence, there's competition for food, nutrients mates, nesting sites, light and other selection forces.

3.All individuals are different; there is variation in every species in traits such as size, color, strength. 

4. Those with advantageous traits have a selective advantage, meaning they're more likely to survive and reproduce.

5. Natural selection increases the frequency of Alleles conferring selective advantage. How? The alleles are passed down to offsprings. 

6. Over generations, these changes accumulate, leading to the formation of new species.

Note: Genetics isn't my strongest topic, but I've tried my best to make sure these notes are correct and accurate. If you see any mistakes, feel free to comment and I'll fix it!