1.1 DNA: code of life

pg 1-18

**What are nucleic acids?**

* Have the capacity to store the information that controls cellular activity and the 


* Specialisation of cells to form tissues
* Arrangement of tissues into organs 


* This allows organisms to perform all necessary functions for life 
* They do this by controlling protein synthesis- proteins make up a lot of the human body and enzymes, which control chemical processes

**What are the two types of nucleic acids?**

* Deoxyribonucleic acid (DNA)
* Ribonucleic acid (RNA)

**Where is DNA found?**

* Mainly in the nucleus 
* It forms an important part of the chromosomes that make up the chromatin network

**What is chromatin?**

* Chromosomal material made up of DNA, RNA and histone proteins as found in a non-dividing cell 

**What is extracellular DNA?**

* Small amounts of DNA are found outside the nucleus in mitochondria in plants and animals and in chloroplasts in plants 

**What are chromosomes?**

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* Long, thing, intertwined thread-like structures made up of a strand of DNA wound around histones (proteins) 
* The DNA molecule is coiled so that these long structures can fit inside the nucleus 
* In somatic cells, chromosomes occur in homologous pairs (1 maternal + 1 paternal)

**What are genes?**

Genes are sections of DNA molecules that control hereditary characteristics → units of heredity in living organisms

**Who discovered the structure of DNA?**

* Kings collage- Maurice Wilkins and Rosalind Franklin : x-ray crystallography/ diffraction images 
* Cambridge university- Francis Crick and James Watson : built a model out of brass plates and claps 

**Who discovered genetic replication?**

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* Watson and Crick : double helix
* Francis Crick and Sydney Brenner : triplet code was used in reading genetic material in DNA; sequence of the bases in DNA form a code by which genetic information can be stored and transmitted 

**The structure of DNA**

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Like a long, twisted ladder, forming a stable, 3D double helix

**What are nucleotides?** 

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* Doubled-stranded DNA molecules are made up of repeating units (monomers) → nucleotides to form nucleic acid chains (polymers)

**What are nucleotides made of?**

* Sugar molecule : deoxyribose (S)
* Phosphate molecules (P)
* nitrogenous bases which may be:
* Adenine (A)


* Guanine (G)
* Thymine (T)
* Cytosine (C) 


* The 4 different types of of nucleotides are bases that are the foundation of the genetic code, instructing cells on how to synthesise enzymes and other proteins 
* Same in animals and plants 

**How is the double helix made up?**

* The outer 2 strands are formed by a chain of alternating sugar/ phosphate links. The bonds between the sugar and phosphate molecules are strong 
* The pairs of bases are linked by weak hydrogen bonds
* nitrogenous base pairs are attached to sugar molecules

**What are base pairs?**

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* The shape and size of the 4 bases differ so that 


* Adenine only bonds with thymine → two hydrogen bonds
* Cytosine will only bond with guanine → three hydrogen bonds


* The hydrogen bonds are weak 

**How do organisms differ?**

* DNA sequencing: the sequence of the four bases determine the genetic code of an organism
* The sequence of in certain sections of DNA on a human is different from the same sections in every other human which results in the differences between individuals

**What is the role of DNA?**

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* Carry hereditary information in the form of genes 
* Provide a blueprint for an organism’s growth and development by coding for protein synthesis
* Can replicate: allows for genetic code to be passed on from one generation to another

**What is non-coding DNA?**

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* Less than 2% of human DNA actually codes for proteins; the rest consists of non-coding DNA 
* Protein-coding regions of a DNA molecule are called exons and they are interrupted by the non-coding region called introns 
* Non-coding regions are known to form functional RNA molecules which have regulatory functions

**DNA replication**

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* Replication is the process of making a new DNA molecule from an existing DNA molecule which is identical to the original molecule. 
* It takes place in the nucleus of a cell during interphase (in between cell division- chromatin network is visible) in the cycle of a cell 

**Why is replication necessary?**

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The DNA needs to produce another molecule exactly the same as itself to ensure that the genetic code is passed on to each daughter cell formed during cell division.

**How does replication take place?**

* The process is catalysed by the enzyme DNA polymerase
* The double helix unwinds and then:


1. The weak hydrogen bonds between the two strands break, allowing the strands to part → both exposed strand acts as a template 
2. Free DNA nucleotides in the cytoplasm bond to their matching, exposed base partners. The fact that the base pairing is complementary ensures that an exact duplicate of each DNA molecules can be made and each DNA daughter cell is exactly the same as their parents because A only bond with T and C only with G. One double helix therefore becomes two identical double helices, each containing one old and one newly synthesised strand 
3. The two daughter DNA molecules each twist to form a double helix which then winds itself around the histones, forming a chromosome. The whole process only takes a few seconds 

**DNA profiling**

* Each person has unique DNA despite the fact that 99.9% of human DNA is identical
* The differences occur in the non-coding part of DNA
* DNA profiling involves the extracting and identifying the highly variable regions of a person’s DNA  that contain repeating sequences of base pairs called STRs (short tandem repeats) 
* From 13-20 different sites on DNA molecules are investigated; enough to show that an individual’s profile is unique 
* DNA profile: an individual’s unique DNA fragments, separated by gel electrophoresis 

**INTERPRETATIONS OF DNA PROFILES**

* A DNA profile is a collection of an individual’s unique DNA fragments (STRs – Short Tandem Repeats), separated through a process called electrophoresis.
* IN A CRIME SCENE, the pattern of the DNA profile created from DNA found at the crime scene must be identical to the DNA profile created from the DNA of the suspect in order to confirm guilt.
* TO ESTABLISH PATERNITY/ MATERNITY, each part of the band in a child’s DNA profile has to match a band found in either the mother’s or father’s DNA profile.

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**How is DNA profiling used?**

* FORENSICS: DNA found in blood, skin or semen left on a victim or found at a crime scene can be matched to an individual and used to identify a criminal.
* DIAGNOSING INHERITED DISORDERS: DNA profiling can be used to accurately diagnose inherited disorders, allowing them to provide proper treatment and  allowing parents to make informed decisions regarding pregnancy.
* IDENTIFYING CASUALTIES: DNA profiles can be used to identify casualties of war.
* PATERNITY TESTING: DNA profiling can be used to determine the biological father of a child.

**What are the views against profiling?**

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* Violation of privacy
* Issues of accuracy
* Manipulation 

**RNA**

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* The different forms of RNA are made in the nucleus by DNA
* Essential for determining the structure and function of all living organisms as they are involved in protein synthesis

**What is the structure of RNA?**

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* RNA is a single-stranded molecule (polymer) made up of nucleotides
* Each nucleotide is made up of a sugar (ribose), phosphate and a nitrogenous base
* The 4 nitrogenous bases are adenine (A), uracil (U), cytosine (C) , guanine (G)

**What is the function of RNA?** 

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RNA carries instructions from DNA in the nucleus to the ribosomes in the cytoplasm of a cell where it controls protein synthesis from amino acids.

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**What are the similarities between DNA and RNA?**

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1. DNA and RNA are both made up of:

* Polymers 
* Nucleotides that are made up of a sugar (ribose), phosphate and a nitrogen base 
* Four nitrogenous bases


1. They are responsible for the synthesis of proteins

**Protein synthesis**

* All cell chemical processes are controlled by enzymes. The synthesis of these protein enzymes is determined and controlled by DNA and RNA

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**Types of RNA:**

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* Messenger RNA (**mRNA**) → functions on the ribosome
* Transfer RNA (**tRNA**) → is located in the cytoplasm 
* Ribosomal RNA (**rRNA**) → functions on the ribosomes 

**What happens in the nucleus**

* mRNA is formed in the nucleus in the same way DNA is replicated. This process is called transcription as the coded message in DNA is carried across (transcribed) into the new mRNA molecule, which carries it to the ribosomes

**Transcription**

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* A small piece of DNA which codes for a particular protein (referred to as a gene), unwinds.
* the weak hydrogen bonds between the base pairs break and the two strands separate.
* RNA nucleotides found in the nucleoplasm pair up with their complementary bases on one of the exposed DNA strands (referred to as the template strand). Adenine – Uracil Thymine – Adenine Guanine – Cytosine Cytosine – Guanine
* The RNA nucleotides join together to create a strand of mRNA.
* The completed strand of mRNA breaks away from the template DNA strand and leaves the nucleus through the nuclear pores and travels to the ribosomes.
* The DNA re-zips.
* The relatively small mRNA moves through the pres of the nuclear membrane and carries the genetic code to the ribosomes which are the site of protein synthesis

**What determines which protein is made?**

* A protein is a long chain (polymer) of small unites (monomers) called amino acids
* There are twenty different types of amino acids that are involved in protein synthesis
* These may combine in various numbers in various sequences to form thousands of different proteins 
* The shortest protein has a 50 amino acid chain 

**What is the role of mRNA**

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* The genetic code is carried as a sequence of ‘codewords’ which are transcribed to the mRNA. Each ‘codeword’ is made up of any three bases and is called a codon 
* There are 64 different codons and all except, code for one of the 20 amino acids used for proteins. Some amino acids are coded for by more than one codon
* A codon is written, using the first letter of the different bases
* The triplet code of bases is the basis of the genetic code as a gene is made up of codons that code for the synthesis of one protein 
* The order of codons in mRNA will therefore determine the sequence of amino acids which will determine which protein is made 

**What happens at the ribosome?**

* The mRNA binds to the ribosome at the start codon (first codon). The codons (3 nucleotides) of the mRNA act as the template that determines the order in which amino acids are linked

**What is the role of tRNA?**

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* There are at least 64 different tRNA molecules, made from nucleotides found in the cytoplasm of cells
* Each tRNA has 3 bases at one end called an anti-codon which picks up a specific amino acid found in the cytoplasm and transfers it to a ribosome
* The most important feature of tRNA is that it can bind to an amino acid at one end and to mRNA at the other, depositing its amino acid in the correct position to form a specific protein 

**Translation of RNA into proteins**

* mRNA binds with a ribosome.
* The mRNA nucleotides are read in groups of three (referred to as codons).
* As the codons are read, a tRNA molecule carrying a complementary anticodon and amino acid bonds with the codon.
* The amino acid is released and joins the adjacent amino acid by means of a peptide bond, eventually forming polypeptides and proteins.
* The tRNA molecule is released.
* The rRNA moves along the mRNA strand, repeating this process until a polypeptide chain is completed.

summary of translation

**Translation is the process by which a specific protein is formed from a chain of amino acids due to the sequence of codons in the mRNA, which, in turn, was coded by the DNA**

**What is the role of rRNA?**

* rRNA is the most common form of RNA in the cell and it, together with proteins, makes up the ribosomes 
* The rRNA moves from codon to codon along the mRNA, reading the code. rRNA therefore plays an important role in controlling the process of protein synthesis 

**Genetic code**

* The genetic code is stored along the length of a DNA strand as a sequence of the four nucleotides
* These nucleotides are arranged in groups of three in a specific order; each group acting as a unit called a codon
* Each codon codes for a particular amino acid 
* The codon codes for a particular amino acid
* The order in which codons are strung together, determines the order in which the amino acids for which they code,are arranged in a polypeptide chain amd finally in a protein 
* The non-coding introns are spliced out during protein synthesis and therefore do not play a part in the process. They probably go on to form functional RNA which has a regulatory function 
* The genetic code is universal