Wk 4 Biomedical Science PT 1

Learning Objectives

Topic 8 – DNA and protein synthesis

• Describe the sequence of events in protein synthesis 

• Describe transcription

• Understand what is meant by RNA processing

• Describe translation

• Understand what is meant by the genetic code

Topic 9  - Genes and inheritance

• Describe what is meant by a trait and a trait variant

• Define the terms; allele, dominant, recessive, genotype, phenotype, homozygous, heterozygous, diploid, haploid, autosome, sex chromosome, karyotype, homologous chromosome, genotype, phenotype

• Explain the inheritance of dominant (including Mendelian dominant, incomplete dominant and co-dominant), recessive, complex (multifactorial) and sex-linked traits

• Use the Punnett square to determine the probabilities of different allelic combinations in simple genetic crosses

• Understand the principles described by Mendel’s Laws of Segregation and Independent Assortment and how they relate to the process of meiosis, heritability and genetic variation

• Discuss how traits can be traced through family pedigrees

Topic 10 - Cell Division and Differentiation

• Discuss the stages (Interphase, M-phase, Cytokinesis), events and significance of somatic cell division

• Explain the processes that occur during Interphase (G0, G1, S, G2) and M-phase (PMAT) and cytokinesis, that ensure accurate cell division of genetic material

• Understand the difference between mitosis and meiosis

• Appreciate that the cell cycle is regulated by many complex factors and that specific signals are required to induce somatic cell division

1. Cells Produce Proteins 

• Cells contain DNA, which serves as a blueprint for making proteins. 

• Through transcription (DNA → mRNA) and translation (mRNA → protein), cells synthesize proteins based on genetic instructions. 

• Ribosomes, found in both the cytoplasm and the rough endoplasmic reticulum, are the cellular structures responsible for protein production. 

2. Proteins Drive Cellular Functions 

• Structural Proteins: Provide support (e.g., collagen in connective tissue). 

• Enzymes: Speed up chemical reactions (e.g., DNA polymerase for DNA replication). 

• Signalling Proteins: Relay messages between and within cells (e.g., insulin regulates blood sugar). 

• Transport Proteins: Move molecules across membranes (e.g., haemoglobin carries oxygen in blood). 

• Defensive Proteins: Part of the immune response (e.g., antibodies fight infections). 

3. Cells Communicate Through Proteins 

• Cells release signaling proteins (hormones, cytokines) to regulate bodily functions. 

• Membrane receptors on cells recognize and bind proteins, triggering internal responses (e.g., neurotransmitters in the nervous system). 

4. Different Cells Produce Different Proteins 

• Not all cells make the same proteins; gene expression varies by cell type and function. 

• Muscle cells produce actin and myosin for contraction, while pancreatic cells produce insulin for glucose regulation. 

Structure of a gene 

A gene is a section of DNA that acts like an instruction manual for making proteins or functional RNA. It has different parts that help it work properly: 

1. Promoter – The "on/off switch" that tells the cell when to start reading the gene. 

2. Exons – The "important instructions" that get used to build a protein. 

3. Introns – Extra "junk" sequences that are removed before making the final protein. 

4. Stop Signal – A marker that tells the cell when to stop reading the gene. 

Why Do Different Cells Express Different Genes? 

1. Cell Specialization (Differentiation) 

   • During development, cells turn on specific genes to become a certain type (e.g., blood cells activate haemoglobin genes, but skin cells do not). 

2. Gene Regulation 

   • Special proteins (transcription factors) control which genes are turned on or off. 

   • Chemical tags (like DNA methylation) can silence or activate genes. 

3. External Signals 

   • Hormones, temperature, stress, and other environmental factors can influence which genes a cell expresses 

Transcription & Translation: How Genes Make Proteins 

Cells use two main steps to turn genetic instructions (DNA) into proteins: 

1. Transcription (DNA → mRNA) 

📍 Where does it happen? 

• In the nucleus of eukaryotic cells. 

• In the cytoplasm of prokaryotic cells (since they don’t have a nucleus). 

📌 Steps of Transcription: 

1. Initiation: 

   • An enzyme called RNA polymerase binds to the promoter (a special DNA sequence that signals the start of a gene). 

   • The DNA strands unwind, and one strand (the template strand) is used to make RNA. 

2. Elongation: 

   • RNA polymerase moves along the DNA, reading the template strand and building a complementary mRNA strand. 

   • Instead of thymine (T), RNA uses uracil (U), so A pairs with U instead of T. 

   • Example: If the DNA has TACG, the mRNA copy will be AUGC. 

3. Termination: 

   • RNA polymerase reaches a termination signal (a specific DNA sequence that tells it to stop). 

   • The new mRNA strand is released. 

📌 Extra Step in Eukaryotes: 
Before the mRNA leaves the nucleus, it undergoes processing: 

• Introns (non-coding parts) are removed. 

• A 5' cap and 3' poly-A tail are added to protect the mRNA from being broken down. 

• ✏ End result: A processed mRNA strand that carries the genetic instructions out of the nucleus and into the cytoplasm 

✏ Think of it like this: 
DNA is a recipe book, and transcription is like writing down a copy of a specific recipe (mRNA). 

2. Translation (mRNA → Protein) 

📍 Where does it happen? 

• At the ribosomes, either floating in the cytoplasm or attached to the rough endoplasmic reticulum (ER). 

📌 Steps of Translation: 

1. Initiation: 

   • The ribosome attaches to the mRNA at a start codon (AUG), which codes for methionine (the first amino acid). 

2. Elongation: 

   • Transfer RNA (tRNA) molecules bring amino acids to the ribosome. 

   • Each tRNA has an anticodon that matches a codon on the mRNA. 

   • The ribosome links the amino acids together in the correct order, forming a polypeptide chain (a growing protein). 

   🔹 Example: If mRNA has AUG GCU UAC, the ribosome reads it as: 

   • AUG (Start → Methionine) 

   • GCU (Alanine) 

   • UAC (Tyrosine) 

3. Termination: 

   • When the ribosome reaches a stop codon (UAA, UAG, or UGA), translation stops. 

   • The new protein is released and folds into its proper shape 

✏ Think of it like this: 
The ribosome is a chef, reading the mRNA recipe and using ingredients (amino acids) to make a dish (protein). 

Summary 

🧬 Transcription = DNA → mRNA (copying the instructions). 
🍳 Translation = mRNA → Protein (following the instructions to make something useful) 

Protein synthesis 

• Occurs via process called translation  

• Requires all 3 types of RNA 

• Translates mRNA to protein 

MRNA (translated by ribosomes) > linear chain of amino acids  

5. Protein Misfolding and Disease 

• Errors in protein folding can lead to diseases like Alzheimer's, Parkinson's, and cystic fibrosis. 

• Cells have quality control systems (e.g., chaperone proteins, proteasomes) to manage protein folding and degradation. 

The Cell Cycle: How Cells Grow and Divide 

The cell cycle is the series of steps a cell goes through to grow, prepare for division, and divide into two daughter cells.  

• Ensures that cells develop properly, repair damage, and replace old or dead cells. 

Phases of the Cell Cycle 

The cell cycle is divided into two main stages: 

1. Interphase (Cell Growth & Preparation) 

🕒 Takes up about 90% of the cycle 
This is when the cell grows in preparation for mitosis, makes proteins, and duplicates its DNA.  

It has three phases: 

G₁ Phase (Growth 1)  

• The cell grows and carries out its normal metabolic functions. 

• It makes proteins and organelles & duplicates organelles & cytosolic components  

• A "checkpoint" ensures the cell is ready to move to the next phase. 

• Take hours to months  

S Phase (Synthesis) 

• The cell copies its DNA so that each new cell will have a full set of genetic material => DNA & histones synthesised 

• GENERAL PRINCIPLE: The parental DNA strands are separated and serve as templates to generate new complementary DNA strands (daughter strands) 

• Duration of 6-8 hours  

G₂ Phase (Growth 2) 

• The cell continues growing and prepares for division 

• Completion of centriole replication  

• Protein synthesis  

• Duration of 2-5 hours  

• Another "checkpoint" ensures DNA was copied correctly, ready for M phase  

📌 At the end of interphase, the cell has doubled its DNA and is ready to divide. 

2. M Phase (Cell Division) 

📍 Mitosis + Cytokinesis 

• Mitotic phase (Nuclear Division) 

  • The nucleus divides, ensuring that each new cell gets an identical set of DNA. 

  • It has 4 stages:  

1. Prophase – DNA condenses into chromosomes, spindle fibres form.  

• Centrosome divides; centrioles migrate to poles.  

• Nuclear envelope disappears  

2. Metaphase – Chromosomes line up in the middle of the cell. 

3. Anaphase – Chromosomes are pulled apart to opposite sides. 

4. Telophase – Two new nuclei form. 

• Nuclear envelope reforms  

• Chromosomes uncoil  

5. Cytokinesis (Final Division of the Cell) 

• The cytoplasm splits, forming two identical daughter cells. 

• In animal cells, the membrane pinches in. 

• In plant cells, a cell plate forms between the two new cells. 

Summary of the Cell Cycle 

This cycle repeats continuously for growth, repair, and replacement of cells 

 Control of cell density 

A cell has 3 possible destines 

1. To remain alive & functioning without division 

2. To grow & divide 

3. To die  

Regulation of the Cell Cycle: Keeping Cell Division in Check 

The cell cycle is tightly regulated to ensure that cells grow, copy their DNA, and divide correctly. This regulation is crucial to prevent problems like uncontrolled cell division (cancer) or cell death. 

1. Checkpoints: "Quality Control Stops" 

At specific points in the cycle, the cell pauses to check if everything is correct before moving forward. 

🔹 Checkpoint Locations: 

• G₁ Checkpoint (Before S Phase) 
  ✅ Is the cell big enough? 
  ✅ Is the DNA undamaged? 
  ✅ Are there enough nutrients? 

• If the cell fails, it may enter G₀ phase (a resting state) or undergo apoptosis (self-destruction). 

• G₂ Checkpoint (Before M Phase) 
  ✅ Was DNA copied correctly in S phase? 
  ✅ Is the cell prepared for mitosis? 

• If mistakes are found, the cell pauses for repairs. 

• M Checkpoint (During Mitosis, Before Anaphase) 
  ✅ Are chromosomes properly attached to the spindle fibres? 

• If not, the cell stalls until it is fixed to prevent errors in division 

Summary of Cell Cycle Regulation 

🔹 Checkpoints make sure everything is correct before moving to the next phase. 
🔹 Cyclins & CDKs control the timing of the cycle. 
🔹 Tumour suppressor genes stop the cycle if something is wrong. 
🔹 Proto-oncogenes push the cell forward when needed. 
🔹 If regulation fails, uncontrolled division (cancer) or cell death can occur. 

Apoptosis vs Necrosis  

  How Heredity Works 

🧬 DNA carries genes, which determine traits. 
🧬 Chromosomes (23 pairs) are inherited from parents. 
🧬 Alleles (dominant/recessive) determine variations in traits. 
🧬 Punnett squares help predict genetic inheritance. 
🧬 Mutations can lead to genetic variation or disease. 

• Heredity => when genes (units of inheritance) are passed from parents to offspring. This happens through DNA, which carries instructions for building and maintaining the body. The key players in heredity are genes, chromosomes, & alleles. 

1. DNA & Genes: The Blueprint of Life 

🔹 DNA (Deoxyribonucleic Acid) contains all the genetic instructions for life. 
🔹 A gene is a specific section of DNA that codes for a protein. 
🔹 Each gene determines a particular trait, such as eye colour, hair texture, or blood type. 

📌 Example: 

• The gene for melanin (a pigment) influences skin colour. 

• Different versions of the gene (alleles) determine how much melanin is produced. 

2. Chromosomes: How DNA Is Organized 

🔹 DNA is packaged into chromosomes, which are found in the nucleus of cells. 
🔹 Humans have 46 chromosomes (23 pairs): 

• 22 pairs of autosomes (control most traits). 

• 1 pair of sex chromosomes (XX = female, XY = male). 
  🔹 Each parent contributes half of their chromosomes (23 from each). 

📌 Example: 

• If a child inherits an X chromosome from Dad, they will be female (XX). 

• If they inherit a Y chromosome from Dad, they will be male (XY). 

3. Alleles: Different Versions of a Gene 

🔹 A gene can have different versions, called alleles 

• Identical alleles in a gene pair – homozygous 

• Different alleles in a gene pair – heterozygous. 

 
🔹 Some alleles are dominant, while others are recessive. 

📌 Example: 

• If a person has BB (homozygous dominant) or Bb (heterozygous), they will have brown eyes. 

• If they have bb (homozygous recessive), they will have blue eyes. 

SIMPLE V COMPLEX GENETICS 

• Simple: considers a single trait influenced by a single gene with clear dominant and recessive inheritance patterns 

• Complex: considers traits influenced by either multiple genes 
  and/or the environment, eg. Spina Bifida 

VARIATIONS OF DOMINANCE  

Co-dominance: 

• both alleles in a gene pair are dominant; both are expressed in the phenotype equally E.g. ABO Blood grouping  

Incomplete dominance 

• both alleles in a gene pair contribute to the phenotype giving rise to an intermediate phenotype E.g. Tay Sachs Disease  

HEREDITARY & VARIATION 

• Sexual reproduction (meiosis) creates genetic variation in the gametes of an individual 

Mendel’s Law Of Segregation 

1. In the formation of gametes, two members of a gene pair (alleles) segregate into different haploid gametes with equal probability 

2. During meiosis the alleles of two (or more) different genes get sorted into gametes independently of one another 
- the allele a gamete receives for one gene does not influence the allele received for another gene 
- as long as they are on different chromosomes! 

4. Punnett Squares: Predicting Inheritance 

A Punnett square is used to predict the likelihood of inheriting traits. 

📌 Example: Brown (B) vs. Blue (b) Eyes 

🔹 75% chance of Brown Eyes (BB or Bb) 
🔹 25% chance of Blue Eyes (bb) 

Example 1: Male AA (homozygous dominant) with a Female aa (homozygous recessive) 

Example 2. Male Aa (heterozygous) with a Female aa (homozygous recessive) 

Example 3: Male Aa (heterozygous) with a Female Aa (heterozygous) 

FAMILY PEDIGREES 

• A genetic 'family tree' 

• Can identify inheritance pattern of a trait or disease 

5. Types of Inheritance 

There are different ways traits are passed down: 

Example 4. X-linked recessive condition Male XY with a female Xa X (carrier of recessive allele - Xa ) 

50 % male => 50% disease 

50% female => 50% carrier, 

50% not carriers  

Example 5. X-linked recessive condition Male Xa Y (affected) with a female XX (normal phenotype) 

Both male offspring have 

The normal phenotype = 

None of the offspring are  

going to display the disease  

Condition, but both female  

offspring are carries of the c 

condition  

The genotype ratio of offspring from a mating between a homozygous dominant man and a heterozygous dominant female would be 
1. 1:2:1, AA:Aa:aa 
2. 1:3, aa:AA 
3. 1:1, AA:Aa 
4. All AA 

6. Mutations: Changes in DNA 

🔹 Mutations are changes in DNA that can affect traits. 
🔹 Some mutations cause diseases, while others have no effect or can be beneficial. 

📌 Example: 

• Sickle Cell Anaemia – A mutation in the haemoglobin gene causes red blood cells to become misshapen. 

• Lactose Tolerance – A beneficial mutation allows some adults to digest milk