DNA: The code of Life

DNA: The code of Life

(DNA) Deoxyribonucleic acid

(RNA) Ribonucleic acid

Location of DNA in the cell

Discovery of the structure of DNA by Watson, Crick, Franklin, and Wilkins

The structure of DNA

The role of DNA

DNA replication

DNA profiling

Ribonucleic acid (RNA)

The types of RNA

The structure of RNA

Protein synthesis

The genetic code

Chromosomes

Genes

Extra-nuclear DNA

Extracting DNA from cells

Nucleotides

A strand of DNA

The double helix

How DNA was discovered to be a double helix

Carrying genetic instructions (codes)

Maintaining structure & regulation

Protein synthesis

Passing on hereditary material

Replication

The cell cycle

The process of DNA replication

The need for an exact copy of DNA

Uses of DNA replication in biotechnology

The technique of DNA profiling

The ethics of DNA profiling

Messenger RNA (mRNA)

Transfer RNA (tRNA)

Ribosomal RNA (rRNA)

2 processes are involved in producing proteins from DNA

Translation

Translation from mRNA to protein

In prokaryotic organisms, DNA is found in the cytoplasm of the cell; It's not enclosed by any membrane

In eukaryotic organisms, most of the cell's DNA is located in the nucleus; there is also a small amount of DNA outside the nucleus, called extra-nuclear DNA

Chromatin

DNA is tightly wound around proteins to form a dense network of fibres called chromatin

DNA is packaged to protect it & to control which parts are active at a given time

During prophase of the cell cycle, the chromatin becomes more condensed & takes the form of chromosomes

Chromosomes are sausage-shaped structures, rather than the loose network of chromatin

Chromosomes become visible during mitosis & meiosis during prophase

Biologists cut out images of the chromosomes & arrange them in order of size, to form a karyotype

The human karyotype shows we have 46 chromosomes that occur in 23 pairs

There are 22 pairs of autosomes & 1 pair of sex chromosomes (gonosomes) that can be XX (female) or XY (male)

Karyotypes help diagnose genetic disorders

A gene is a small portion of DNA with a specific sequence that determines a particular characteristic of an organism

Genes determine what an organism looks like and how it behaves

If you unwound the DNA of a single minute cell, it would be over 2m long

Located in these organelles in the cytoplasm of the cell:

Mitochondria in animal & plant cells called Mitochondrial DNA

Chloroplasts in plant cells

Non-coding DNA

Genes are the functional units that pass the characteristics down through generations

Organisms don't inherit whole characteristics as unit; they inherit individual genes

Genes work by coding for protein molecules, which carry out functions in the cell & outside the cell

The rest 99% are called non-coding DNA or nonsense DNA

Not all DNA forms genes; Only 1% of DNA codes for proteins

Some of these DNA regulates gene copying by responding to signals from the cell & turning gene copying 'on' or 'off'

Isolating nucleic acids is the 1st step to be taken when studying DNA & RNA

The basic method can be used for extracting both DNA & RNA

There is much more DNA than RNA in cells

Start by collecting the sample from which to extract DNA

You need to get the maximum DNA yield from the sample

DNA is made up of a large number of repeated units called Nucleotides

Each is composed of 3 different organic molecules:

There are 4 different nitrogenous bases:

Guanine(G)

Smaller molecules called Pyrimidines:

Adenine(A)

Thymine(T)

Cytosine(C)

DNA molecule consists of nucleotides joined together in long chains

Chemical bonds form between the sugar group of 1 nucleotide & the phosphate group of the next nucleotide

A DNA strand is a polymer

In double-stranded DNA, there are 2 sugar-phosphate chains running in opposite directions, & complementary nitrogenous bases form hydrogen bonds between them

Double-stranded DNA creates the appearance of a ladder

The sugar & phosphate molecules are water soluble, while the bases are hydrophobic

The bases stack themselves in centre of DNA spiral to avoid water

The bases form a twisted ladder, keeping the sugar & phosphates on the outside

Read paragraph on page 21

Scientific Method

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Genes contain the genetic instructions needed to make other components of cells

Like the proteins & RNA molecules used in the development & functioning of all known living organisms & some viruses

Large sections of DNA don't code for genes but maintain the structure of the chromosome & regulate the function of genes that contain the coded genetic info

These sections are non-coding or nonsense DNA

Proteins, which are coded by DNA, are both structural & functional

Proteins are important components of all cells & molecules like enzymes & hormones etc.

DNA contains the hereditary info that is passed on from parents to offspring

Natural selection takes place to ensure useful genetic changes are passed on to next generation

The ability of DNA to replicate itself during mitosis means cells can divide into new cells with an exact copy of the DNA

In living organisms is Important:

In cell division to produce new cells & tissues

To maintain chromosome number & ensure that the same genes are in every cell

To help with growth through protein synthesis

To regenerate damaged or ageing tissues through mitosis

For production of spores & vegetative growth in asexual reproduction

To produce gametes through meiosis in sexual reproduction

For creating new cells for growth, repair & replacement of worn or damaged cells

Starts when the cell forms & Ends when it divides into 2 daughter cells

Cell cycle has 2 parts:

Second is Mitosis - which is cell Division

Typical human cell has 46 chromosomes

When cell divides by mitosis, 2 daughter cells are formed, each with 46 chromosomes.

Meaning the new chromosomes have been formed to produce 2 sets of identical DNA

First is Interphase - which is cell Growth

1st gap phase (G1)

A cell performs its normal metabolic functions & grows until its appropriate surface area to volume ratio (SVA) is reached

Synthesis phase (S)

The DNA content of the nucleus duplicates into 2 parts

2nd gap phase (G2)

The cell produces microtubules to separate the chromosomes during mitosis

Mitosis

The chromosomes separate into 2 sets.

(Cytokinesis): The cytoplasm forms 2 daughter cells that now enter the G1 phase of growth - (Restart cycle)

DNA replicates in Prokaryotic & Eukaryotic cells

A single double-stranded DNA molecule will form 2 identical DNA molecules

When human cell divides & its DNA replicates, it has to copy the exact sequence of 3 billion nucleotides

To ensure daughter cells will inherit correct characteristics & functions

There can be copying errors in which the wrong nucleotide or too many or too few nucleotides are inserted into a sequence

Most replication mistakes are corrected through DNA repair process by repair enzymes

Some errors make it past the repair processes and become permanent mutations

Some mutations are of no consequence

Some can lead to genetic problems or cancer

Some are useful & help organisms survive

Some are natural & important to allow species to adapt

DNA replication in biotechnology & genetic engineering is important for:

Cloning cells in tissue culture

Gene splicing to produce resistance to certain diseases & to manufacture antibiotics, insulin, growth hormone & genetically modified organisms

DNA profiling & forensics

Paternity & maternity testing

Genetic counselling of parents to inform them about inheritable disorders & conditions

Scientists cannot conclusively identify someone with a DNA sample by simply looking at a few genes, because there's a chance of another person having the same genes

Scientists use parts of the DNA sequence they know are unique

1 Sample DNA is extracted & put in test tube

2 The DNA is cut into fragments using a restriction enzyme. This is an enzyme that recognizes specific DNA sequences & cuts the DNA at that point

3 DNA fragments of different sizes are produced, depending on how far apart the sequences are

Electrophoresis is used to show differences in size

In this technique an electric current runs DNA through a gel

Tiny space in gel let smaller fragments move faster through it

The pieces of DNA are separated by size

4 The DNA is mixed with a fluorescent substance to show up the bands under ultraviolet light

5 The band pattern can be compared with known DNA sample.

DNA profiling is controversial

Arguments for it are:

It can be used to solve crimes

It can determine paternity

It can help people find missing relatives

It can identify people

Arguments against it are:

It invades the privacy of the victim & suspect

Tests can be tampered with

The technology can be misused

Another kind of nucleic acid, like DNA

It carries genetic info, but in a temporary form

Plays central role in gene expression

Made of a chain of nucleotides

Each nucleotide consists of ribose sugar (rather than deoxyribose), a phosphate group & nitrogenous base

Usually single-stranded

There are 4 different nitrogenous bases:

Adenine(A)

Cytosine(C)

Guanine(G)

Uracil (U)

RNA is much less stable than DNA & degrades (breaks down) easily

This is produced from DNA in the nucleus & is used in cytoplasm to manufacture protein

This is copied from sections of DNA in nucleus & is used in cytoplasm to manufacture protein

Made of proteins & rRNA

The nucleolus, in nucleus of eukaryotic cells, is the site of ribosome manufacture

Ribosomes are essential for protein synthesis

Ribosomes are found free in cytoplasm or attached to tubules of rough endoplasmic reticulum

Many genes are found in DNA molecules

Each gene codes for 1 protein

Meaning the gene carries the instructions for the formation of RNA from RNA nucleotides

Proteins are made up of amino acids that must be arranged in specific sequence

Sequence determines the type of protein

Body constantly needs new proteins, which are formed during protein synthesis

Transcription

Happens in nucleus

mRNA strands are transcribed from DNA by enzyme named RNA polymerase

New mRNA sequences are complementary to their DNA template, not identical copies

Transcription happens in nucleus as follows:

DNA strands unwind & unzip

Hydrogen bonds break

One strand is used as template to form mRNA using free nucleotides from nucleoplasm

The coded message for protein synthesis is copied as the mRNA

mRNA moves out of nucleus through a nucleopore

mRNA carries triplet code from DNA also called Codon

mRNA attaches itself to ribosome

Happens in cytoplasm

RNA translated into proteins by structures attached to rough endoplasmic reticulum, called ribosomes

In cytoplasm tRNA are anticodons

tRNA collect free-floating amino acids in cytoplasm

tRNA moves to ribosome where mRNA strands have attached

anticodons on tRNA match the bases on codons of mRNA

Amino acids carried by tRNA become attached by peptide bonds to form required protein

Each tRNA is released to pick up more amino acids

mRNA strand attaches itself to any ribosomes in cell

Process is controlled by enzymes

A tRNA molecule transfers amino acid to ribosome in sequence determined by mRNA codons

mRNA codon carries code for specific amino acid

An anticodon is sequence of 3 bases found on tRNA

Each new amino acid links with previous by forming peptide bond

Amino acids chain lengthens into polypeptide in process called elongation

After polypeptide chain is synthesised, the protein is formed

Physical structure of proteins determines how they interact with other molecules

A set of instructions in mRNA for amino acid sequences of proteins

Its contained in mRNA codons

64 possible nucleotide combinations of mRNA & amino acid they code for

More than 1 codon can code for a single amino acid

No codon codes for more than 1 amino acid