DNA Flashcards
What is DNA
• DNA = Deoxyribonucleic Acid
• DNA carries the genetic info in the cell
- It carries the instructions for making all the structure and materials the body need to function
• DNA is capable of self-replication
• Most of the cells DNA is carried in the nucleus – small amount in mitochondria
• Uncoiled DNA is 2-3m long
• DNA strands are bound to special proteins called Inibones
Each DNA has…
• Coiled DNA is coiled chromatin
• When cells undergo mitosis, the chromatin will coil up farther into chromosome
• Sections of DNA molecules make up genes
• Genes contain genetic code which determines the structure of activities of that cell
• DNA is stored in the nucleus as Chromosomes, which are thin, thread-like structures.
• There are 46 chromosomes in every cell arranged in 23 pairs.
DNA Structure
• Double Helix – twisted ladder shape
• Nucleotides are the building blocks of DNA. It Consists of
- a sugar
- a phosphate
- a nitrogen base (A,T,C,G)
• Ladder model:
- Side rails (or BACKBONE) are made up of alternating Sugar and Phosphate molecules
- The RUNGS are made up of nucleotide base pairs (A-T or C-G) held together by a hydrogen bond
- The nitrogen base is attached to the sugar molecule of the backbone.
• The backbone:
- The two side rails of the backbone run in opposite directions.
- They each have a THREE BASE (3’) end and a FIVE BASE (5’) end.
- 3 & 5 refer to the carbon atoms in the sugar molecule that form a bond with the adjoining phosphate
- One strand runs from 3’ to 5’ and the other from 5’ to 3’.
• Base paring:
- Each rung is formed by two nitrogen bases joining
- ADENINE always pairs with THYMINE
- CYTOSINE always pairs with GUANINE
- A-T are joined by 2 HYDROGEN bonds
- C-G are joined by 3 HYDROGEN bonds
• The DNA strands are bound to proteins called HISTONES and wrapped around them so they can fit inside the nucleus.
• In a cell that is not dividing the DNA coiled around the histones forms a tangled network called CHROMATIN.
• Once the DNA has replicated in mitosis, the coiled chromatin condenses to become CHROMOSOMES
Function of DNA
• A chromosome consists of segments of DNA known as GENES.
• The base sequence (the order of the bases) of each gene contains instructions for construction of one particular protein. This is known as the GENETIC CODE.
• Proteins control many characteristics and functions of the body, eg. Hair and Nails, Hormones, Enzymes.
• It is estimated that there are about 20,000–25,000 genes in the human genome (i.e. about 3 billion base pairs).
Mitochondrial DNA
• Most of the cells DNA is found in the nucleus but a small amount it found in the mitochondria.
• mtDNA is only inherited from the mother through the egg cell.
- All mitochondria in a sperm are destroyed at fertilisation, so any mtDNA in the sperm will also be destroyed.
• mtDNA takes the form of small circular strands of DNA, which are not bound to histone proteins.
• There are about 5-10 in each mitochondria.
• mtDNA has 37 genes which are all essential for mitochondria to function.
- 24 also contain the code for making transfer RNA molecules.
- The other 13 contain instructions for making enzymes needed
in cellular respiration.
Introns and Exons
• Genes consist of introns and exons
• Exons are sections of coding DNA
- They contain instructions for making proteins
• Introns are sections of non-coding DNA
- They do not contain instructions for making proteins but contain other information such as when to make each protein
The Genetic Code
• The sequence of bases is the code instructing the cell how to construct a particular protein
- The number of amino acids and the order in which they are to be assembled
DNA Replication
• DNA has ability to make exact copies of itself known as SELF-REPLICATION
- This is used in cell division during interphase
• DNA replication requires three steps:
- Separating the strands
- Building the new strands
- Joining the strands back together
Separating the Strands
• An enzyme called HELICASE untwists the double helix and gradually ‘unzips’ the DNA molecule into the 2 strands.
• It works by disrupting the hydrogen bonds between the bases, which exposes the nitrogen base code.
• Each exposed strand acts as a set of instructions or a TEMPLATE for building a new ‘complimentary’ nucleotide strand.
• Single-Strand Binding Proteins (SSBs) temporarily bind to each side and keep them separated.
Building the New Strands
• Building of the new strand of complimentary DNA cannot just start from scratch, it must have a primer in place to start it off.
• A PRIMER is a short strand of DNA, complimentary to the first part of the DNA strand being copied, that starts the replication process.
• Primers are found on both strands at the 3’ end.
• Once the primer is in place, an enzyme called DNA POLYMERASE adds nucleotides onto the end of the primer.
• DNA Polymerase works its way along the template strand and positions the new complimentary nucleotides (C to G, G to C, A to T, and T to A) and binds them together in the correct order.
• DNA polymerase works in only one direction, from 5’ to 3’, building onto the 3’ end.
• The first strand is called the LEADING STRAND, it runs from 5’ to 3’, and therefore continuously adds onto the 3’ end, working towards the site where the strands are unzipping.
• The second strand is called the LAGGING STRAND, it runs from 3’ to 5’.
- This means is has to be constructed in FRAGMENTS (or segments) that run in the opposite direction (therefore working away from the unzipping).
- Each fragment has to be initiated by a primer, which are later ‘erased’ and replaced by DNA nucleotides by the polymerase.
Joining the Strands back together
• The 2 new identical DNA strands are then joined to their respective template strands by an enzyme called LIGASE.
- Ligase helps to build the hydrogen bonds between nucleotide bases
- Ligase also joins the separate fragments of the lagging strand together.
• Each new molecule consists of one old and one new nucleotide strand.
• This is referred to SEMI-CONSERVATIVE REPLICATION.
• As soon as replication ends, the strands coil around histones and forms chromatin.
- At interphase it will form the supercoiled chromatids, joined in the centre by centromeres.
Protein Synthesis
• DNA contains the instructions for the cell’s activities… What proteins to build
• It is the ribosomes that build the proteins, but the instructions are in the nucleus
• So, we need a way for the DNA to get from the nucleus to the ribosomes
Transcription (Protein Synthesis)
• A RNA polymerase runs along the open DNA and puts the matching RNA nucleotides in place
- RNA is a single strand of DNA, with a ribose sugar
• As a result the gene is copied, forming messenger RNA (mRNA)
Protein Synthesis Coding
• RNA has 3 bases the same as DNA:
- Adenine (A)
- Cytosine (C)
- Guanine (G)
• But instead of thymine (T), RNA has Uracil (U) that will bind with adenine
- C – G
- G – C
- T – A
- A – U
Translation (Protein Synthesis)
• The mRNA will pass out of the nuclear pores and attaches to a ribosome
• Amino acids are brought to the ribosome by Transfer RNA (tRNA) from the cytoplasm
• At the ribosome, the amino acids (on tRNA) are joined (with a peptide bond) in sequence determined by the mRNA codons
• 3 Major Phases:
• Initiation:
- mRNA binds to the ribosome
- tRNA binds to mRNA, bringing along an amino acid
- tRNA uses its anti-codon sequence as a template to bind the first group of amino acids to the mRNA chain
- The first 3 bases are called the start codon: AUG
• Elongation:
- The amino acids are attached one by one to the protein chain by:
- The ribosome moves along the mRNA strand to the next codon
- A new tRNA anti-codon binds to the compulsory
• Termination:
- The protein chain stops growing when the ribosome reaches a stop codon on the mRNA molecule (UAA)
- There is no corresponding tRNA for a stop codon
- The finished protein is released from the ribosome
- The protein may be used in the cell of exported to another cell
