Term 3 Biology
DNA
The Structure of DNA:
DNA is the molecule that carries genetic information for the development and functioning of an organism
DNA is located in a cell in the cell nucleus (called nuclear DNA)
DNA can also be found in the mitochondria
Unicellular and multicellular organisms rely on cells to perform all necessary life functions
Cells contain membrane “organelles”
Compartmentalisation is the process in which various cells’ functions are separated within different areas of the cell
DNA contains the instructions that determine the characteristics of all living things
DNA is a double-stranded molecule, in a double helix structure, made up of four types of nucleotides
Organelles:
Nucleus: Stores genetic information
Mitochondria: produce chemical energy
Ribosomes: Assemble proteins
Nucleus:
Controls and regulates the activities of the cell (e.g. growth and metabolism)
Located in the DNA of cells
Nucleotides:
Each DNA strand is a string of Nucleotides
There are 4 types of nucleotides
Adenine, thymine, cytosine and guanine
When nucleotides join together, they form a long chain called nucleic acid
Nucleotides are joined together by their sugar and phosphate groups, forming a sugar-phosphate backbone
Weak chemical bonds called hydrogen bonds hold bases together as the rungs of the ladder
Sugar-phosphate backbones and Double helix:
The side of the ladder
Bases are the rungs of the ladder
Two nucleic acid strands wind into a double helix structure
Complementary base pairing:
Thymine (T) always pairs with adenine (A)
T-A
Guanine (G) always pairs with cytosine (C)
G-C
Memory trick: Take away (T-A) on the Gold Coast (G-C)
Base pairs (T-A) and (G-C) are called complementary base pairs
One nucleic acid strand will be complementary to the other strand
Discovery of the structure of DNA:
Rosalind Franklin was an X-ray crystallography, which she used to photograph molecules
She captured Photo 51, which showed a clear X-shaped diffraction pattern —> which indicated DNA had a helical (spiral) structure.
She produced detailed measurements of the DNA molecule, including the spacing of the helix and dimensions of its repeating units
James Watson and Francis Crick tried to build a physical model of DNA
They were shown Photo 51 and used it to build a model of DNA
In 1953, a famous paper proposing the double-helix structure —> Two strands winding around each other with paired bases (A with T, C with G) on the inside
Watson, Crick and Wilkins got a 1962 Nobel Prize in Physiology or Medicine for their work
Franklin did not receive a Nobel Prize because she died of Ovarian cancer at aged 37
Chromosomes: higher-order structure of DNA:
Individual DNA strands are twisted
DNA is wrapped around proteins (called histones) and forms structures called chromosomes
Chromosomes appear as long, thin thread-like structures which are located in the nucleus of cells
A single chromosome = a molecule of DNA (a DNA helix)
Chromosomes are composed of wound-up DNA
Genes:
Sections of DNA are called genes
Each gene is the instruction for a specific protein
It is a specific section of DNA consisting of a specific sequence of bases
The order of the bases (C, G, T AND a) along a DNA strand is the genetic code
Cells “read” the genetic code and produce proteins that can make amino acids
The sequence of bases determines the sequence of amino acids and the type of protein that is made by a cell
A typical human gene varies in size from about 300 to over 1 000 000 bases
Positioning of a gene on a particular chromosome is called locus
A particular gene is located in the same place in every member of a species
Different genes vary in —> order of bases along the DNA strand and number of bases in that section of DNA
Proteins:
They create structures and perform the actions needed for your cells to survive, grow and function
Examples are enzymes and proteins in membranes
They determine the characteristics of an organism like skin colour, hair, eyes, etc
Non-Coding DNA and Genome:
Genes are segments of “non-coding” DNA
Non-coding DNA are not genes and does not “code” proteins
Have various functions, some of which are not yet understood.
Genome is an organism’s complete set of DNA, including all genes (coding DNA)
Parts of the genome have no known function (non-coding DNA)
DNA, Genes and Chromosomes:
DNA stands for deoxyribonucleic acid
DNA is made up of very long chains of chemical ‘letters’: Adenine (A), Guanine (G), Thymine (T) and Cytosine (C)
DNA instructs our genes
Genes are the instructions for making proteins
Proteins do the work within our cells and body
Most genes are arranged on Chromosomes that are found in the nucleus of cells
Chromosomes come from both parents:
Organisms inherit their genetic sequences from both parents
Organisms possess two copies of each Chromosome:
→ one maternal chromosome (female)
→ one paternal chromosome (male)
The maternal and paternal chromosome pairs in a cell are called homologous chromosomes
Homologous chromosomes share the same structural features (same size, centromere positions) and genes at the same loci (position on the chromosomes)
Diploid and haploid chromosome number:
The majority of cells in our body (somatic cells) have a homologous pair of each chromosome
Cells that have pairs of homologous chromosomes are diploid (symbolised by 2n)
Diploid (2n) cells have two copies of each chromosome
Diploid has three pairs of homologous chromosomes (of maternal and paternal origin)
Some cells have one set of chromosomes, and these cells are called haploid cells
These cells are gametes (e.g. sperm and egg cells)
Haploid (n) cells have one copy of each chromosome, and have three non-homologous chromosomes
Cells with only a single copy of each chromosome are known as haploid (symbolised by n)
The haploid chromosome number in gametes is half the diploid number in somatic cells
Human Chromosomes:
Humans have 23 pairs of Chromosomes in each diploid cell (46 individual chromosomes)
22 pairs (44 chromosomes) are called autosomes
1 pair (2 chromosomes) are called sex chromosomes - this chrosome pair determines male or female gender
In females, the sex chromosomes are a pair of X chromosomes (XX)
In males, the sex chromosomes are one X and one Y chromosome (XY)
In human haploid gametes (i.e. egg and sperm cells), there are 23 total chromosomes (no pairs)
Karyotypes:
A karyotype is a representation of a set of chromosomes, grouped in homologous pairs according to shape and size
A karyotype can be analysed for abnormal chromosome structure or number
Used to find genetic disorders
Different species have different numbers of chromosomes
Karyotypes can be used to determine the species of a DNA sample
DNA Composition and Structure Notes:
Genes are made of DNA (Deoxyribonucleic Acid)
DNA contains the genetic instructions for living organisms
DNA is built from nucleotides
Nucleotides = Phosphate + Sugar + Nitrogen Base
Bases:
→ A = Adenine
→ T = Thymine
→ C = Cytosine
→ G = Guanine
Base- pairing rules:
A → T (2 hydrogen bonds)
C → ( 3 hydrogen bonds)
DNA stores and passes on genetic information
Complementary strands are formed using the base-pairing rules
Chromosomes Carry Genetic Information in the Form of Genes:
DNA = the molecule that contains genetic information, found in the nucleus of cells and made of nucleotides
Gene = A section of DNA that contains instructions for a specific characteristic or protein, and humans have about 20,000 genes
Chromosome = A tightly coiled structure made of DNA, contains many genes and is found in the nucleus
Karyotype = a picture showing all the chromosomes of an organism arranged in pairs
Phosphate → Nucleotide → Gene → Chromosome
Humans have 46 chromosomes in each body cell and are arranged into 23 pairs
23 chromosomes are from the mother and 23 chromosomes are from the father
Autosomes are chromosomes from 1-22
44 autosomal chromosomes in total
Sex Chromosomes are the 23rd pair and determine biological sex
Females have the sex chromosomes XX
Two chromosomes of similar size
Males have the sex chromosomes XY
One large X chromosome and one much smaller Y chromosome
A single human cell contains about 2 metres of DNA
DNA fits into the nucleus because it wraps around proteins called histones, coils tightly into chromatin, and Condenses further into chromosomes
Genes differ because they contain different sequences of DNA bases and lengths of DNA
Different base sequences code for different proteins, leading to different characteristics
DNA is the code for proteins:
Nucleotides on the DNA strands are the genetic code for an organism
The genetic code has the instructions to make a protein
The main role of DNA is to carry genetic information and dictate the process of protein synthesis
DNA provides the set of instructions to direct the cell to produce proteins vital for cell function
Some proteins (e.g. collagen) support the cells in the body
Other proteins are enzymes that help digest and speed up the chemical reactions of our metabolism
Proteins are made up of smaller units called amino acids
Amino acids form chains called peptides (long chains are called ‘polypeptides’)
A chain of amino acids is the primary structure of a protein
The amino acid chain is then folded, which creates the 3-dimensional secondary protein structure
The genetic code in DNA ( the sequence of bases) determines the order of amino acids in a protein
How genes make proteins:
Step 1 → make proteins, the DNA molecule unwinds and one strand acts as a template and is copied to form a single- stranded molecule of messenger RNA (mRNA, messenger ribonucleic acid)
Messenger RNA (mRNA) plays a key role in protein synthesis
mRNA acts like a photocopy of the original DNA blueprint
The process of making an mRNA copy from a DNA strand is called transcription
Transcription of DNA into mRNA takes place in the nucleus
Step 2 → the next process of forming a protein from RNA is called translation
mRNA can leave the nucleus
mRNA strand attaches to a ribosome in the cytoplasm
The ribosome “translates” the order of the mRNA bases into the order of amino acids that will make up the protein
Transcribe DNA into mRNA:
mRNA copy of the gene
Transcribed:
An enzyme temporarily separates the double-stranded DNA
One strand of DNA, called the template strand, acts as a template
An enzyme moves along the template strand, and the bases are ‘read’ one at a time, and a short, single-stranded mRNA molecule is built using complementary nucleotides
mRNA is made complimentary base pairing rules apply
Nitrogen bases of RNA are guanine, cytosine, adenine and uracil (not thymine)
Adenine on the DNA template is paired with a uracil on the mRNA strand
Guanine (DNA) → cytosine (mRNA)
Cytosine (DNA) → guanine (mRNA)
Thymine (DNA) → adenine (mRNA)
Adenine (DNA) → uracil (mRNA)
Translate mRNA into amino acids:
The mRNA strand leaves the nucleus and attaches to a ribosome in the cytoplasm
The nitrogen bases of the mRNA are translated by a ribosome in groups of three bases
Each group of three bases is called a codon
Each codon corresponds to a single amino acid
Amino acids are brought to the ribosome by another type of RNA called transfer RNA, or tRNA
The amino acids join in a chain according to the order specified by the sequence of codons in the mRNA
Eventually, the amino acids form a long chain, which becomes the final protein
An RNA codon table can decode the codons into the corresponding amino acids
There are 20 amino acids in nature but 64 possible codon combinations
Cell Division
Cell Division:
Cell division is the process whereby one cell produces two new cells
Occurs in all living things
Some cells die or become damaged and need replacements
Cell division is their only form of reproduction
Asexual reproduction occurs in single-celled organisms such as bacteria and plants
Cell division allows an individual to grow and change by expanding the number of total cells
Cell division allows sexually reproducing organisms to produce sperm and egg cells
Cell division produces a new offspring of the two parents
Cell division in multicellular and unicellular organisms:
Unicellular organisms reproduce asexually by dividing into two
They copy their DNA before cell division; each of the daughter cells contains the same DNA as the original parent cell
Asexual reproduction involves one parent, resulting in offspring that are genetically identical to each other, and to the parent
Binary fission is when an organism duplicates its genetic material and divides into two parts → with each new organism receiving one copy of DNA
Cells in multicellular organisms divide for growth, development and repair
Body cells divide throughout life and form new cells to replace damaged or dead cells
Cells copy their DNA before cell division → each daughter cell contains the same DNA as the original parent cell
Some cells in multicellular organisms copy their DNA and divide to produce gametes (e.g. sperm and egg cells)
Before cell division, each daughter cell has half of the DNA of the parent cell → genetic variation in the offspring after fertilisation in sexual reproduction
When two gametes (an egg and a sperm) fuse → fertilisation has taken place, and a zygote is formed
The DNA forms the zygote’s genome from each gamete and all genetic information necessary to form a new individual
Two types of cell division:
There are two distinct types of cell division → mitosis and meiosis, and both require DNA to replicate (copy)
Mitosis → cell division for growth, development, repair and asexual reproduction
Meiosis → cell division for making gametes for sexual reproduction
DNA replication:
DNA in a cell must first be copied before a cell can divide → DNA replication
Two daughter cells must receive a complete set of DNA from the parent cell
DNA replication takes place in the nucleus
Base sequences on the new DNA molecules must be identical with those on the original set
DNA replication is in three steps
Step 1:
The strands of the double helix must separate from each other
Enzymes are responsible for separating the two strands
It is like a zip
Nucleotide bases are exposed on each side of the DNA ladder
Step 2:
In every cell nucleus, there are free nucleotides available → used to make new DNA (following the rules of complementary base pairing)
Match with exposed nucleotides on the separated strands of the DNA
Bonds form between the complementary bases → creating new rungs of the ladder
The sugar and phosphate molecules that form the sides of the DNA ladder join with neighbouring nucleotides
Step 3:
Two new identical DNA molecules result
Each one a double helix
Each one identical to the original parent DNA
Semi-conservative → as each strand retains half of the original DNA material
DNA replication and chromosomes:
Karyotypes → two copies of each chromosome ( one from mother and one from father) → also known as homologous pairs of chromosomes
When DNA is replicated before the process of cell division, each of the chromosomes that make up a homologous pair makes a copy of itself (the DNA is replicated)
The original and the copy become attached at the centromere
The original and the copied chromosome are individually called sister chromatids
Distinguish between Chromosomes and chromatids:
Chromosomes may be in the form of a single strand → a DNA molecule + histones; they occur in homologous pairs
Chromosomes may be two replicated chromosomes, joined at the centromere; they still occur in homologous pairs
Chromatids are the two identical strands of a chromosome after DNA replication, often called sister chromatids
Having made copies of all the chromosomes, the cell is ready to divide.
Mitosis:
Mitosis is the type of cell division in which the parent cell divides into two identical daughter cells
It occurs in the somatic cells
Somatic cells include the body cells except the gametes (egg and sperm)
Somatic cells are → Skeletal and muscle cells, blood cells, stem cells, all other cells, organ and tissue cells, fat cells and Neuron cells
Mitosis is for growth, wound and tissue repair
It also occurs in asexual reproduction
Mitosis as part of the cell Cycle:
Mitosis is a continuous process
There are several distinct stages:
→ Interphase → cell is not experiencing any form of cell division
→ Prophase → Double-stranded chromosomes appear, nuclear membrane disappears, and spindle forms
This cell normally has 4 chromosomes in two homologous pairs; at this stage, it has 8 chromosomes that are attached in pairs of sister chromatids at the centromere, and it still has two homologous pairs
→ Metaphase → Chromosomes line up in a single line across the centre of the cell
A network of fibres (spindle fibres) appears, extending from the poles (ends) of the cell to each chromosome
Double-stranded chromosomes line up along the equator (middle) of the cell
→ Anaphase → Each pair of chromatids separates at the centromere, and each chromatid (now called a chromosome) moves to the opposite pole
The network of fibres contracts, pulling the chromatids apart
The sister chromatids separate and the single chromosomes move to the opposite sides of the cell
→ Telophase → Nuclear membranes re-form
Two nuclei form, each with the correct diploid number of chromosomes
A new nuclear membrane encloses the chromosomes at each pole
Remember the order of the stages of mitosis: IPMAT
Cells spend most of their time in interphase
When a cell is ready to divide, it goes through stages of prophase, metaphase, anaphase and telophase
The cytoplasm divides, and the result is two identical daughter cells
In the period between cell divisions, the DNA is unwound and not visible as individual chromosomes
The final stage of cell division → where the parent cell physically divides in two is called cytokinesis
Cytokinesis → Cytoplasm divides; two daughter cells are produced