DNA

Protein synthesis and replication

DNA

Many new conservation techniques are using techniques based on genetic identification or manipulation.

Genetic strategies rely on an understanding of DNA.

The nucleus of a cell contains DNA (deoxyribonucleic acid)

DNA is also present in mitochondria and chloroplasts (plants)

DNA is arranged in structures called chromosomes, which are only visible when the cell is dividing.

The genes carry all the information for the development of the organism. Genes provide information about making proteins.

DNA is important as it controls cell activities and the appearance of individuals

How DNA works 1

Each gene is a section of DNA or sequence of bases.

The base sequences on the DNA acts as a code that controls the action of the cell.

Structure of DNA

DNA is a coiled double helix made up of nucleotides on a sugar phosphate backbone

DNA is made of nucleotides

Base pairing

Adenine always pairs with ThymineThymine always pairs with AdenineCytosine always pairs with GuanineGuanine always pairs with Cytosine

How DNA works 2

The code sequences are copied as mRNA, which is sent to the ribosomes, and used as the instructions to make proteins.

Amino acids are carried into the ribosomes by tRNA, which contain a 3 nucleotide section (anticodon) that matches with a 3 nucleotide section (codon) on the mRNA.

Each base links up with its base pair - cytosine with guanine, adenine with thymine.

This controls the order in which different amino acids are attached.

DNA codes for RNA

RNA is also made of nucleotides and is very similar to DNA except: Thymine is replaced by UracilThe sugar in the sugar-phosphate backbone is different (ribose)Strands are single not doubleThere are 3 types – mRNA, tRNA and rRNA

RNA structure

Messenger RNA

Transfer RNA

Ribosomal RNA

DNA controls protein synthesis

Definitions 1

• DNA – deoxyribonucleic acid – carries the code that controls RNA production. This is a double helix

• m-RNA - has the set of instructions for the order in which amino acids are to be assembled into proteins. This is a single strand of nucleotides

• t-RNA - carries amino acids to the ribosome. This is a single strand of nucleotides

Definitions 2• Protein - long chains of amino acids, usually folded• Amino acid - building blocks of proteins• Ribosomes – site of protein synthesis – where mRNA is

read. These are made of rRNA• Endoplasmic reticulum – membrane channels that

ribosomes are attached to. Is responsible for transport and remodelling of protein (eg refolding or attaching carbohydrate groups to make glycoproteins)

Definitions 3• Transcription - copying of DNA to

make RNA. Uses the base pair rule

Cytosine in DNA Guanine in RNAGuanine in DNA Cytosine in RNAAdenine in DNA Uracil in RNAThymine in DNA Adenine in RNA(RNA contains Uracil not Thymine)

• Translation - reading of mRNA to make proteins

Definitions 4• Codon - 3 base segment of mRNA – codes

for particular amino acid• Anticodon - 3 base segment of tRNA

(carrying a particular amino acid) that is the reverse of the codon on mRNA

• Triplet – 3 base segment of DNA – that is the reverse for mRNA – which codes for a codon

• Gene – section of DNA that controls one characteristic or protein

• Start codon – the codon (AUG) that tells the ribosome to start making a protein

Definitions 5• Coding strand – the strand of DNA that has the same sequence as he

RNA (but is not used to make the RNA)• Template strand – the strand of DNA that is the complement of the

coding strand (the side that is used to make the RNA)• RNA polymerase – enzyme that makes the new strand of RNA

Definitions 6

• Introns – nonsense sections in mRNA that are removed before it leaves the nucleus

• Exons – the sections that code for protein sequence. When the mRNA leaves the nucleus it will have only exons

Why are proteins important?

Roles of proteins in the body include

• Structural proteins eg collagen, keratin

• Enzymes (organic catalysts) eg digestive enzymes

• Transport proteins eg haemoglobin

• Regulatory proteins eg hormones

• Protective proteins eg antibodies, clotting factors

Gene expressionEach cell contains many genes that carry the

information for making many proteins.

But not all of these genes are expressed in all cells in the body eg skin cells produce pigment (melanin) but do not make contractile proteins like muscle cells

Gene expression

Gene expression is controlled by a number of factors.

• Regulator genes produce proteins that bind to an operator gene and inhibit transcription

• Operator genes is the start of a structural gene

• Promoter genes indicate the structural genes that should be used to make particular mRNA at any given time

• Environmental factors may turn genes on or off (epigenetics) by affecting how the DNA is coiled around the histones (and so whether it can be read easily)

DNA replication

DNA is capable of replication to produce identical copies

This occurs in interphase – before mitosis starts

One set of enzymes split the strands

Another set of enzymes join new nucleotides to each strand

Nucleotides match up by the base pairing rule (C – G, A - T)

The end result is two identical strands, joined at a point called the centromere

DNA replication 2

Comparing mitosis and meiosis

Mitosis Meiosis

Where it occurs

Why it occurs

Number of cells produced

Number of divisions

Number of chromosomes in daughter cells

Amount of variation in daughter cells

Comparing mitosis and meiosis

Mitosis Meiosis

Where it occurs Body cells Gonads (reproductive organs)

Why it occurs Cell repair, growth, asexual division

Sexual reproduction

Number of cells produced 2 4

Number of divisions 1 2

Number of chromosomes in daughter cells

Same as parent (diploid)

Half that of parent (haploid)

Amount of variation in daughter cells

None Lots