Life Sciences Grade 12 Notes

Life Sciences Skills and Benefits

Skills Acquired:
- Evaluation and discussion of scientific issues and processes.
- Awareness of the benefits of biotechnology and Life Sciences to humankind.
- Understanding the negative human impact on the environment and organisms.
- Promotion of responsible environmental citizenship and conservation.
- Appreciation of South Africa's contribution to Life Sciences, including its biomes and scientists.

Life Sciences Strands for Grade 12

Broad Knowledge Strands:
- Knowledge Strand 1: Life at a Molecular, Cellular, and Tissue Level.
- Knowledge Strand 2: Life Processes in Plants and Animals.
- Knowledge Strand 3: Diversity, Change, and Continuity.
- Focus on discovering the links between related topics within each strand to understand the interconnectedness of life.

Purpose of Studying Life Sciences

Three Broad Aims:
- Aim 1: Knowing the content (theory).
- Aim 2: Doing practical work and investigations.
- Aim 3: Understanding Life Sciences applications in society (present indigenous and western societies) and within the context of history.

Aim 1: Knowing the Content of Life Sciences

Learning Content:
- Understanding and making meaning of scientific ideas.
- Connecting scientific ideas.
- Theory involves selecting important ideas, using different sources to learn, and describing concepts, processes, and theories.
- Skills include writing summaries, developing diagrams, reorganizing data, interpreting data, and linking data to theory.

Aim 2: Doing Practical Work and Investigations

Practical Investigations Skills:
- Following instructions safely.
- Naming, recognizing, and handling laboratory equipment.
- Making observations through drawings, descriptions, measurements, and comparisons.
- Measuring and recording observations.
- Interpreting data to find value, discuss changes, trends, and applications.
- Designing investigations and experiments, including identifying problems, hypothesizing solutions, identifying variables, controlling variables, selecting apparatus and materials, and planning repeatable experiments.

Aim 3: Understanding the History, Importance, and Modern Applications of Life Sciences

Relevance of Studying Life Sciences:
- Understanding the history of science and indigenous knowledge systems.
- Learning how knowledge was developed by scientists across ages.
- Awareness that modern science and traditional knowledge systems may differ in their approach to science, but both bring a certain dynamic.
- Exposure to possible career fields branching out of Life Sciences, such as palaeontology, horticulture, game ranch management, preservation, biotechnology, and genetic engineering.

Textbook Usage Recommendations

Pay careful attention in class.
Take note of sections you don't understand and revisit them.
Ask questions to ensure understanding.
Consider end-of-chapter summaries and build on them to create your own point-form summary notes.
Practice re-sketching the given diagrams.
Work through all the given questions and answers at the end of the chapter.

Strand: Life at a Molecular, Cellular, and Tissue Level

LO: DNA – the code of life

Introduction

All living organisms contain both DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). We focus on their location, structure and function.
We explore the discovery of DNA, its role in the human body and how it replicates.
Protein synthesis is vital for life – we examine how proteins are formed by both DNA and RNA.

Revision of cellular structure

It is important to know the location and functions of certain organelles, illustrated in Figure 1 below.
Cytoplasm is the base substance in which the organelles of the cell are suspended. It is a watery substance and allows for metabolic reactions to take place.
Ribosomes are small, round organelles which are mainly found attached to the endoplasmic reticulum or are free-floating in the cytoplasm. Ribosomes can also be found inside other organelles such as the chloroplast and mitochondria but in smaller numbers. They are the site of protein synthesis and consist of RNA and protein.
The nucleus controls all of the cell’s activities.
A nucleus has four main parts:
- the double nuclear membrane – it encloses the nucleus and contains small pores to allow for the passage of substances in and out of the nucleus
- the nucleoplasm – this is a jelly-like fluid within the nucleus
- the nucleolus – a dark body suspended in the nucleoplasm which contains free nucleotide bases and produces ribosomes
- the chromatin network – found in the nucleoplasm: contains the DNA which forms the chromosomes containing the genetic code of a person / organism

The structure of nucleic acids

Key terminology
- nucleic acid - a type of organic compound
- monomer - a building block
- nucleotide - the monomer which forms DNA and RNA
There are two types of nucleic acids in the human body – DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Together these form the basis of all life of earth. They consist of monomers (building blocks) called nucleotides.
Each nucleic acid is composed of a phosphate $(P)$ , a sugar molecule $(S)$ and a nitrogenous base $(NB)$ .

DNA – deoxyribonucleic acid

Key terminology
- DNA - deoxyribonucleic acid is made up of nucleotides, nitrogenous bases adenine, thymine, guanine and cytosine, carries the genetic code for protein synthesis
- nuclear DNA - DNA found in the nucleus
- extra-nuclear DNA - DNA found outside of the nucleus: mitochondrial and chloroplastidic DNA.
- double helix - the shape of DNA consists of two strands joined together and twisted spirally
- hereditary - genetic information passed on from parent to offspring

A brief history of the discovery of DNA

1952 – Rosalind Franklin and her assistant Maurice Wilkins researched the structure of DNA using X-ray diffraction images.
Watson and Crick did independent research on DNA. Upon seeing Franklin’s images, they proposed a 3-D double helix model for DNA in 1953.
1962 – Watson and Crick received the Nobel Prize for the discovery of the structure of DNA, and Wilkins received an award for his X-ray photography. Franklin had died of cancer.

The location of DNA

DNA is found in two locations in a cell:
- Mostly in the nucleus of a cell – this is referred to as nuclear DNA
- a small amount is found outside the nucleus – it is referred to as extra-nuclear DNA.
There are two types of extra-nuclear DNA:
- chloroplastic DNA – found in the chloroplasts of plant cells
- mitochondrial DNA – found in the mitochondria (useful for tracing ancestry)

The structure of DNA

DNA has a double helix structure, consisting of monomers called nucleotides which link to form long chains, called polymers. The sugar in DNA is deoxyribose sugar and is attached to a nitrogenous base. The phosphate and sugar molecules are attached to one another by strong bonds alternately to form the long chains.
There are four types of nitrogenous bases in DNA:
- adenine (A)
- cytosine (C)
- thymine (T)
- guanine (G)
Nitrogenous bases are complementary and always join together in a specific order:
- adenine always links to thymine
- guanine always links with cytosine
This pairing of bases means that two strands of DNA are joined together, forming a long ladder-like structure. The nitrogenous bases are held together by weak hydrogen bonds. The ladder-like structure becomes coiled and is known as a double helix structure. The DNA strands wind around proteins which are known as histones.

The role of DNA

DNA carries hereditary information in the form of genes. Genes are short sections of DNA which code for a specific trait, and determine the physical characteristics and behaviour of an organism.
Most of the DNA strands do not code for anything and are known as non-coding DNA. Scientists are still researching the importance of the non-coding DNA.
The main functions of DNA include:
- Controls the functioning of cells
- Regulate the functioning of genes
- Passes on hereditary characteristics

RNA – ribonucleic acid

Key terminology
- RNA - RNA consists of nucleotides. Nitrogenous bases found in RNA are adenine, uracil, guanine and cytosine.
- messenger RNA - mRNA carries the code for protein synthesis from DNA to the ribosome
- ribosomal RNA - rRNA forms ribosomes which are the site of protein synthesis
- transfer RNA - tRNA brings amino acids to the ribosome to form the protein
There are three types of RNA (ribonucleic acid), all formed in the nucleus by DNA. They perform different functions in different places in a cell. The types are:
- messenger RNA (mRNA)
- ribosomal RNA (rRNA)
- transfer RNA (tRNA)

The location of RNA

Messenger RNA (mRNA) is formed in the nucleus but then enters the cytoplasm where it attaches to ribosomes.
Ribosomal RNA (rRNA) is found in the ribosomes in the cytoplasm of the cell.
Transfer RNA (tRNA) is found freely in the cytoplasm of the cell.

The structure of RNA

Like DNA, RNA also consists of monomers (nucleotides) which link to form longer chains (polymers).
However, RNA is a single-stranded structure which is not coiled. The sugar in RNA is ribose and is attached to a nitrogenous base. The phosphate and sugar molecules are attached to one another alternately to form the chains.
There are four types of nitrogenous bases in RNA:
- adenine (A)
- uracil (U) – not thymine as in DNA
- cytosine (C)
- guanine (G)

The role of RNA

The three types of RNA are very important to the process of protein synthesis, with each type playing a unique role.

Comparison between DNA and RNA

DNA and RNA are similar in some respects. They both …
- contain sugar alternating with phosphate
- contain the nitrogenous bases adenine, guanine and cytosine
- play a role in protein synthesis
DNA and RNA also have significant differences, tabulated in Table 1 below.

	DNA	RNA
Sugar	contains deoxyribose sugar	contains ribose sugar
Structure	double helix and coiled	single stranded
Nitrogenous base	contains thymine	contains uracil
Location	found in the nucleus only	found in the nucleus, ribosomes and cytoplasm of cells

DNA replication

DNA replication is the process through which DNA makes an identical copy of itself. This occurs during interphase of the cell cycle in the nucleus.
The steps of DNA replication:
1. The DNA double helix unwinds
2. The weak hydrogen bonds between the nitrogenous bases are broken. The DNA strands separate (they unzip)
3. Each original DNA strand serves as a template on which its complement is built
4. Free nucleotides build a DNA strand onto each of the original DNA strands, attaching their complementary nitrogenous bases (A to T and C to G)
5. This results in two identical DNA molecules. Each molecule consists of one original strand and one new strand
DNA replication is important for cell division, particularly mitosis. It allows each chromosome to be copied so that each new identical daughter cell produced contains the same number and type of chromosomes.

Errors that occur during DNA replication

Errors that occur during DNA replication may sometimes lead to mutations (a change in the nitrogenous base sequence)
If the incorrect nitrogen base attaches to the original strand and a nitrogen base is added or deleted …
- the sequence or order of the bases changes on the new DNA molecule …
- resulting in a change in the gene structure

DNA profiling

A DNA profile is a pattern produced on X-ray film. This pattern consists of lines which are of different lengths and thicknesses and in different positions
All individuals, except identical twins, have a unique DNA profile.
DNA profiles are used to:
- identify crime suspects in forensic investigations
- prove paternity (father) and maternity (mother) (biological parents)
- determine the probability or causes of genetic defects
- establish the compatibility of tissue types for organ transplants
- identify relatives.
DNA profiling is generally accepted as being extremely reliable.
However, the interpretation and comparison of profiles should however be approached with caution, for the following reasons:
- Humans interpret the results which means mistakes could be made
- The method of profiling may be different in different laboratories producing inconsistencies
- Only a small piece of DNA is used in profiling, so the profile might not be 100% unique to a particular individual
- DNA profiling is expensive and therefore not readily accessible to those who cannot afford it, particularly in criminal cases
- DNA profiles may reveal information about a person which could be used against them in a prejudicial way. For example: being HIV positive or having genetic abnormalities may lead to insurance companies not covering a person or prejudice in the court room

Protein synthesis

Key terminology
- amino acids - monomers of proteins
- base triplet - three nitrogenous bases one after the other on DNA
- transcription - 1st stage of protein synthesis – mRNA formed from DNA carrying code for the protein to be made
- translation - 2nd stage of protein synthesis – amino acids combine to form a protein
- codon - three nitrogenous bases one after the other on mRNA – these are complementary to the triplet on DNA
- anti-codon - three nitrogenous bases one after the other on tRNA – these are complementary to the codon on mRNA
The process in which proteins are made is called protein synthesis. Proteins are made by linking various amino acids that are present in the cytoplasm of cells. There are 20 different amino acids, and they combine in a large variety of combinations.
The number of amino acids and the sequence of the amino acids determine the type of protein that is formed.
The genes found in DNA contain the code which determines which type of protein that will be formed.
- The smallest protein contains 50 amino acids linked together
- Proteins generally contain 300 or more amino acids.
Three consecutive nitrogenous bases on the DNA strand are called the base triplet. The base triplets determine which amino acid will be placed into the protein as well as the sequence in which the amino acids will be joined.

Protein synthesis occurs in two stages

Stage 1: Transcription
Stage 2: Translation

Stage 1: Transcription

The first stage of protein synthesis, called transcription, occurs in the nucleus:

A section of the DNA double helix unwinds. As a result,
- the weak hydrogen bonds between the nitrogenous bases of DNA break
- the DNA unzips (in this particular section of the DNA)
One strand acts as a template
This DNA template is used to form a complementary strand of messenger RNA (mRNA)
- This is done using free RNA nucleotides in the nucleoplasm
- The mRNA now contains the code for the protein which will be formed
- Three adjacent nitrogenous bases on the mRNA are known as codons. These code for a particular amino acid.
mRNA moves out of the nucleus through a nuclear pore into the cytoplasm, where it attaches onto a ribosome

Stage 2: Translation

The second stage of protein synthesis, called translation, occurs in the cytoplasm.

Transfer RNA (tRNA) in the cytoplasm has three adjacent nitrogenous bases known as the anti-codon
- mRNA’s codon will be complementary to a tRNA’s anti-codon
- Each tRNA will carry a specific amino acid
- According to the codons on the mRNA, the tRNA will bring the required amino acid to the ribosome
The amino acids are linked by a peptide bond to form the required protein.

Note: it is important to know the difference between base triplets (DNA), codons (mRNA) and anti-codons (tRNA).

The effect of mutation on protein structure (DNA sequence)

A mutation is a change in the nitrogenous base sequence of a DNA molecule (or a gene)
- since mRNA is copied from the DNA molecule during transcription.
- This will result in a change in the codons.
- As a result, different tRNA molecules carrying different amino acids will be required.
- The sequence of amino acids changes, resulting in the formation of a different protein.
- If the same amino acid is coded for, there will be no change in the protein structure.