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Biology306Updated 13 Sept 202627 pages

SQA Higher Biology: Unit 1 - Exploring DNA and the Genome

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DNA and RNA are the fundamental molecules that store and express genetic information in all living things. Understanding how DNA replicates itself and how it's used to make proteins through RNA is essential for grasping how life works at the molecular level.
SQA Higher Biology: Unit 1 – page 1

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DNA Structure and Organisation

Ever wondered how your genetic code is actually stored? DNA exists as a double-stranded helix made up of repeating units called nucleotides. Each nucleotide contains three parts: a phosphate group, a deoxyribose sugar, and a nitrogenous base.

The structure has a specific direction - new nucleotides can only be added to the 3' end in a 5' to 3' direction. The two strands are held together by weak hydrogen bonds between complementary base pairs, making the molecule stable yet easy to separate when needed.

Prokaryotes (like bacteria) keep their DNA as large circular chromosomes in the cytoplasm, plus smaller DNA rings called plasmids. Eukaryotes (like humans) have their DNA tightly coiled with histone proteins into linear chromosomes stored safely in the nucleus.

Quick Tip: Remember that prokaryotes lack a membrane-bound nucleus, whilst eukaryotes have their DNA packaged in a proper nucleus!

SQA Higher Biology: Unit 1 – page 2

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DNA Replication

Before any cell divides, it must copy its entire DNA perfectly - this is DNA replication. Think of it as making an exact photocopy of your genetic instruction manual! The process relies heavily on two key enzymes: DNA polymerase and DNA ligase.

DNA polymerase does the heavy lifting by adding new nucleotides to build the new strand, but it needs a primer (a short starter sequence) to begin work. It can only add nucleotides to the 3' end using complementary base pairing.

Here's where it gets interesting - the two DNA strands are copied differently. The leading strand is made continuously in one smooth process, whilst the lagging strand is made in short fragments that must be joined together by DNA ligase.

Remember: DNA polymerase is like a builder who can only work in one direction and needs a foundation (primer) to start!

SQA Higher Biology: Unit 1 – page 3

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Stages of DNA Replication

The replication process follows a precise sequence that ensures accuracy. First, the DNA unwinds and hydrogen bonds between bases break, creating two template strands. Two replication forks form and move in opposite directions, exposing the bases for copying.

On the leading strand, a primer binds and DNA polymerase continuously adds nucleotides to the 3' end. The enzyme catalyses chemical bond formation between nucleotides as the strand grows smoothly.

The lagging strand works differently - primers bind as DNA is exposed, and DNA polymerase extends from each primer until it meets the previous fragment. Old primers are replaced with DNA, and DNA ligase joins all fragments together. This process repeats as the DNA continues to unzip.

Key Point: Think of the leading strand as a smooth highway and the lagging strand as a series of connecting bridges being built simultaneously!

SQA Higher Biology: Unit 1 – page 4

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PCR (Polymerase Chain Reaction)

Need millions of copies of a specific DNA sequence quickly? PCR is your answer! This technique can amplify any desired DNA sequence from viruses, bacteria, plants, or animals millions of times within hours. It's highly specific, easily automated, and works with tiny sample amounts.

PCR requires several key components to work. You need a buffer to maintain pH and supply essential ions, nucleotides as building blocks, and primers to provide starting points. The star player is TAQ polymerase, a heat-resistant enzyme extracted from thermophilic bacteria.

Don't forget your template DNA - this contains the target sequence you want to copy. The beauty of PCR is that it works in vitro (outside living organisms), unlike in vivo processes that occur within organisms.

Cool Fact: TAQ polymerase comes from bacteria that live in hot springs, which is why it can survive the high temperatures used in PCR!

SQA Higher Biology: Unit 1 – page 5

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PCR Process

PCR works through a three-step temperature cycle that repeats multiple times. Heating 92°−98℃92°-98℃ splits the double-stranded template into single strands by breaking hydrogen bonds. This high temperature would destroy normal enzymes, but TAQ polymerase thrives!

Annealing 50°−65℃50°-65℃ allows primers to bind to complementary sites on the single-stranded template DNA. The temperature drops just enough for these specific bonds to form whilst preventing unwanted binding.

Extension 70°−80℃70°-80℃ sees DNA polymerase bind to the annealed primers and extend them in the 5' to 3' direction. After this step, the DNA has doubled! Each complete cycle doubles the amount of DNA, leading to exponential amplification.

Memory Tip: Think "Hot-Cool-Warm" for the three temperature stages - separate, bind, build!

SQA Higher Biology: Unit 1 – page 6

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RNA Structure

RNA (ribonucleic acid) serves as the crucial bridge between DNA and protein synthesis. Like DNA, RNA consists of nucleotides containing phosphate, sugar, and a base, but with key differences that make it perfect for its job.

Instead of deoxyribose, RNA uses ribose sugar. The four bases are adenine, cytosine, guanine, and uracil - notice that uracil replaces thymine found in DNA. Base pairing follows the rule: A pairs with U, and C pairs with G.

Unlike DNA's double helix, RNA molecules are usually single-stranded. This allows them to fold into complex shapes and perform various functions within the cell, from carrying messages to catalyzing reactions.

Quick Note: Remember "U" for uracil in RNA - it's the key difference from DNA's thymine!

SQA Higher Biology: Unit 1 – page 7

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The Structure and Functions of RNA

RNA comes in three main forms, each with a specific job in protein synthesis. mRNA (messenger RNA) carries a copy of genetic information from DNA in the nucleus to ribosomes in the cytoplasm. Each triplet of bases on mRNA is called a codon and codes for a specific amino acid.

tRNA (transfer RNA) acts like a molecular taxi service, transporting specific amino acids to ribosomes during protein synthesis. These molecules recognise the genetic code presented by mRNA through their anticodon sequences.

rRNA (ribosomal RNA) combines with proteins to form ribosomes, the cellular factories where proteins are actually made. Without rRNA, protein synthesis simply couldn't happen.

Think of it this way: mRNA is the recipe, tRNA delivers the ingredients, and rRNA provides the kitchen!

SQA Higher Biology: Unit 1 – page 8

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tRNA, rRNA, and Transcription

tRNA molecules have a distinctive structure with an anticodon at one end (complementary to mRNA codons) and an amino acid binding site at the other. This design allows precise matching between genetic code and amino acids during protein synthesis.

rRNA binds with structural proteins to form ribosomes, the essential protein-making machinery. These ribosomes use both mRNA templates and tRNA carriers to build proteins according to genetic instructions.

Transcription is the process of producing mRNA from a DNA template, and it occurs in the nucleus. The DNA strand used as a template is called the template strand, whilst the non-coding strand is the coding strand.

Key Insight: Think of transcription as making a working copy of your genetic blueprint that can leave the nucleus safely!

SQA Higher Biology: Unit 1 – page 9

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Stages of RNA Transcription

Transcription follows a systematic process to ensure accurate mRNA production. RNA polymerase unwinds the DNA and breaks hydrogen bonds between bases, causing the strands to separate and expose their bases for copying.

Free RNA nucleotides find and align with complementary DNA nucleotides through hydrogen bonding. Strong chemical bonds then form between the sugar of one RNA nucleotide and the phosphate of the next, building the growing mRNA chain.

The process continues along the entire length of the gene. Finally, weak hydrogen bonds between DNA and RNA break, allowing the completed mRNA to separate from the DNA template. The DNA strands reunite and wind back into their double-helix structure.

Remember: Transcription is like making a photocopy - the original DNA stays intact whilst creating a working copy!

SQA Higher Biology: Unit 1 – page 10

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RNA Splicing

In eukaryotic cells, the initial RNA transcript (called the primary transcript) needs processing before it can make proteins. RNA splicing is the crucial editing process that removes unwanted sections and joins the useful parts together.

Introns are non-coding regions that get removed during splicing - think of them as unnecessary paragraphs in a text. Exons are the coding regions that contain actual protein-making instructions and get joined together to form the mature mRNA transcript.

This splicing process is essential because it allows one gene to potentially make multiple different proteins by keeping different combinations of exons. It's like having one recipe that can make several different dishes depending on which ingredients you include!

Memory Trick: "EXons EXit the nucleus" - they're the parts that actually get expressed as proteins!

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