Welcome to your Complete Guide to Module 5 of HSC Biology, where we deep dive into the concept of heredity! From breaking down reproduction to DNA replication and RNA synthesis, to understanding how to read pedigrees, this module lays the foundations for the rest of the HSC Biology course.
This guide aims to get you up to speed on the concepts and skills needed to blitz HSC Biology Module 5, covering the key concepts and skills you’ll need.
Concept #1: Reproduction
There are 2 types of reproduction - sexual and asexual reproduction.
Asexual methods of reproduction involves one parent individual, making an identical copy of itself. It typically occurs in plants, fungi, and bacteria. Methods include budding, binary fission (for bacteria), and fragmentation (for plants).
- Major advantages of this include simplicity (no mating required), efficiency since a large number of gametes (sex cells) can be produced quickly, and being less energy-demanding on the organism
- Major disadvantages include a lack of genetic variation, and potential overcrowding and competition for resources, since so many offspring are created at once
Sexual reproduction involves the fertilisation of an egg by a sperm, from two parents. It typically occurs in animals, and can occur inside or outside of the female organism.
- Major advantages of this include increased genetic variation due to a high number of possible allele combinations, and increased chance of survival
- Major disadvantages of this are that it’s time-consuming and less efficient
Concept #2: Cell division and replication
So after fertilisation, how does one cell turn into a whole organism? This is due to the process of cell division. One cell divides repeatedly to produce different types of cells, eventually reaching the complexity of a whole organism.
There are two types of cell division you should know. Mitosis is the process of cell division whereby a parent cell divides into 2 daughter cells with identical genetic material (DNA). Meiosis is the process of cell division that specifically produces gametes (sex cells) with different genetic material from the parent, and only contains half the amount of DNA. Female gametes are called eggs/ovum, and male gametes are called sperm.
Concept #3: Replication of genetic material
As mentioned above, the daughter cells produced via mitosis contain the same genetical material as the parent cells. This happens via DNA replication.
What is DNA?
DNA (deoxyribonucleic acid), can be thought of as the ‘blueprints’ of an organism. It codes for your eye colour, your height, and an even wider variety of things you wouldn’t even have thought twice about.
DNA is made of 4 building blocks called nucleotides, which consisted of three parts: a phosphate group, a deoxyribose sugar, and a nitrogenous base. The sugar-phosphate backbone forms the outer structure of the DNA, and the nitrogenous bases (adenine, thymine, cytosine, and guanine) form pairs between the two strands, creating the double helix structure which allows DNA to safely store and transmit genetic material.
How does DNA replication work?
So let’s break this down into a step-by-step understanding of DNA replication:
- Helicase enzymes unwind the DNA strand, seperating the two strands into the Leading and Lagging strand. (Helicase separates the strands by disrupting the hydrogen bonds between the nitrogenous base pairs into a Y-shape known as the replication fork).
- Initiation - The leading strand is the easiest to replicate. After the DNA strands have been separated, a short RNA segment known as a primer attaches to the 3’ end of the strand. The primer always binds as the starting point for replication. DNA Polymerase (DNAp) binds to the primer region of the DNA and begins replicating.
- Elongation - DNAp begins to create the complementary DNA strand by reading the nitrogenous base of its template strand and attaching the complementary nucleotide to the ‘daughter strand’.
- Whilst understanding that is all well and simple, there is a slight difference in the way that each side of the DNA strand is replicated (recall the terms Leading and Lagging strand) this is because DNA is actually set in TWO different directions when bonded to form a Double-Helix: The 5’ to 3’ direction and the 3’ to 5’. This point is relevant to our understanding of the lagging strands (3’ to 5’) DNA synthesis whereby the DNAp is forced to read the DNA in chunks to interpret the information in a 5’ to 3’ direction. This concept was officially understood by Reiji Okazaki in 1968, earning these chunks/ fragments to be coined Okazaki Fragments. Once as much of the Lagging strand is transcribed DNA polymerase does one last check for errors and then an enzyme known as Ligase comes to fill in the gaps and reconnect the two daughter strands.
- Termination - Once both the continuous and discontinuous strands are formed, an enzyme removes all RNA primers from the original strands, “proofreads”/ checks for errors in the newly replicated DNA then fixes them, and another enzyme known as DNA ligase joins the Okazaki fragments together forming a single unified strand.
This process is illustrated below:
Image credit: iStock
Concept #4: RNA synthesis and processing
What is RNA?
In our cells, RNA (Ribonucleic Acid) plays a crucial role in the expression of our genetic information. While DNA serves as the ‘blueprint’ for our traits and biological functions, RNA acts as the messenger that carries these blueprint instructions from DNA, to the protein-making machinery of the cell.
RNA is composed of nucleotides similar to DNA, but with some key differences:
- RNA contains a ribose sugar (instead of deoxyribose)
- Uracil replaces Thymine as a nitrogenous base
- RNA is usually single-stranded
These structural features make RNA suitable for its roles in coding, decoding, and regulating genes.
How does RNA synthesis and processing work?
At a base level, RNA Synthesis is split into TWO parts; transcription (which contains several steps) and translation.
- Initiation: RNA synthesis, or transcription, starts when an enzyme called RNA polymerase attaches to a specific DNA region known as the promoter. This promoter region marks the starting point for transcription. Upon binding, RNA polymerase unwinds the DNA double helix, revealing the template strand used for RNA synthesis.
- Elongation: During elongation, RNA polymerase travels along the DNA template strand in the 3’ to 5’ direction, synthesizing a complementary RNA strand in the 5’ to 3’ direction. As the enzyme moves forward, it reads the DNA sequence and incorporates RNA nucleotides (adenine, uracil, cytosine, and guanine) into the growing RNA molecule, creating a strand that mirrors the DNA template. This newly formed RNA strand is called the pre-mRNA.
- Termination: Transcription proceeds until RNA polymerase encounters a termination signal in the DNA, which marks the end of the gene. At this stage, RNA polymerase releases the newly synthesized RNA transcript, allowing the DNA double helix to re-form.
- RNA Processing: Before the RNA transcript can be translated into a protein, it undergoes several processing steps. First, a modified guanine nucleotide is added to the 5’ end of the RNA, forming a “5’ cap” that protects the RNA from degradation and assists with ribosome binding during translation. Then, a sequence of adenine nucleotides, known as a “poly-A tail,” is added to the 3’ end, offering further protection to the RNA and facilitating its export from the nucleus to the cytoplasm.
- Splicing: The initial RNA transcript often contains non-coding regions known as introns that must be removed. This process, called splicing, is performed by a complex known as the spliceosome, which excises the introns and links the coding regions, or exons, together. The final result is a mature mRNA molecule that carries the correct genetic instructions for protein synthesis.
- Translation: After exiting the nucleus, the mature mRNA travels through the cytoplasm to a ribosome. The ribosome binds to the mRNA’s 5’ cap and scans along the mRNA until it finds the start codon, which is a specific sequence of three nucleotides that signals the beginning of translation. The ribosome reads the mRNA sequence three nucleotides at a time, with each triplet, or codon, specifying a particular amino acid.
As the ribosome moves along the mRNA, a transfer RNA (tRNA) molecule with an anticodon complementary to the mRNA codon binds to the ribosome. This tRNA carries the amino acid corresponding to the codon it recognizes. The ribosome then catalyzes the formation of a peptide bond between this amino acid and the growing polypeptide chain. This process repeats, elongating the polypeptide until the entire mRNA sequence has been translated into a protein.
Key Skills to ace HSC Biology Module 5 exams
Reading a pedigree chart
Pedigrees are a visual way to represent and analyse the inheritance pattern of a trait. With that being said, many students struggle with questions centred around pedigrees. The good news is, it’s definitely a skill you can build, with repetition and practice.
Here are some of the fundamentals of a pedigree chart:
- Circles represent females, squares represent males
- Individuals affected by a trait are shaded in, whereas unaffected individuals have a blank shape
- Carriers of the trait are sometimes denoted by a half-shaded shape
- Roman numerals represent generations (from top to bottom), and digits represent an individual within a generation (from left to right)
- Horizontal lines represent mating or partnerships
- Vertical lines represent offspring
Here are some rules you can use to work out inheritance patterns illustrated by a pedigree:
- If a trait is in every generation, it is likely dominant
- If a trait seems to skip generations, it is likely recessive
To break down whether an inheritance pattern is autosomal (affecting male and female equally) or sex-linked (inheritance of a trait via a sex chromosome, X or Y), consider these rules:
- Sex-linked dominant (X-linked)
- Is more common in females. Females have two X chromosomes, meaning they have a higher chance of inheriting the dominant allele
- No male-to-male transmission, since fathers never pass an X chromosome to sons
- Daughters of affected fathers are always affected
- Each generation typically contains an affected individual
- Sex-linked recessive (X-linked)
- More common in males, since they only have one X chromosome and a singular recessive gene on that X chromosome can cause the disease
- No male-to-male transmission, since fathers never pass an X chromosome to sons
- Daughters of affected fathers and unaffected mothers are carriers, since they inherit one X chromosome from each parent
- Traits can skip generations
- Sex-linked (Y-linked)
- Only males are affected, as females do not have a Y chromosome. No female carriers.
- Affected fathers pass it to all sons
- Autosomal dominant
- Trait does not skip generations
- Males and females are equally affected
- Traits are passed to roughly 50% of offspring, regardless of gender
- Autosomal recessive
- Trait can can skip generations
- Males and females are equally affected
For an example of how this works:
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Always give details, and use the illusion of lines
Essentially, write more if you know more. If you’re not 100% sure whether your response hits all the criteria points and you’ve already used up the provided lines, then write more in another booklet. It’s better to lay out all the relevant knowledge during the exam, than to realise after the exam that you had forgotten a crucial component. With that being said, only include RELEVANT information.
Signpost in your answer
Place boxes around key definitions or “buzzwords” that guide the marker through your response. It makes it easier for them to compare your answer to the rubric, to see whether you deserve full marks!
Approach any exam paper with a strategy
Instead of blindly attempting a 3 hour paper from beginning to end, consider using some of these exam techniques that students often find helpful:
- Do as many questions as possible during reading time - you can often answer a lot of them in your head
- Do the harder/longer questions first - they’re more time-consuming and difficult, so you should attempt them early, when your brain hasn’t gotten tired yet
- Never spend too long on any question - 180 minutes for 100 marks means you should only spend 1.8 minutes to secure every mark in the paper. If you don’t know, skip it and come back.
Conclusion
And that’s a wrap for this HSC Biology Module 5 guide! Heredity is an awesome topic that dips your toes into the world of Year 12 Biology, setting up great foundations for the later modules. Hopefully this article gave a nice overview of what to expect from the HSC Biology course, and got you excited for what’s to come!
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