Gene Expression

From a gene to a protein

Central Dogma

• (Crick 1958)• Determines the genetic flow of information

Central Dogma

First step in decoding a genetic message from DNA is to copy (transcribe) it into mRNA, which contains the code for making proteins

Second step in decoding, is to decode mRNA into a polypeptide chain which builds a protein.

Functions of a Protein

Functions of Protein:Functions of Protein:

• StructuralStructural– muscle, hairmuscle, hair

• ChemicalChemical– antibodies, hormones, enzymes (regulate antibodies, hormones, enzymes (regulate

all chemical reactions in cells)all chemical reactions in cells)

Proteins

• Proteins are made, from mRNA, by joining amino acids into long polypeptides (which are proteins)– There are only 20 naturally occurring amino

acids

Review of DNA

• DNA is the genetic material

• DNA codes for different genes– Genes are codes for a protein which

determines different traits

DNA is made up of nucleotidesWhich contain: - a phosphate group

- a sugar (deoxyribose)- a nitrogenous base

Structure of DNA

RNA

• Involved in protein synthesis

• Made up of nucleotides:

–A phosphate group

–A sugar (ribose)

–Nitrogenous bases (RNA only has A, U, C, G there is no T!)

Types of RNA

• There are three main types:–Messenger RNA (mRNA)–Transfer RNA (tRNA)–Ribosomal RNA (rRNA)

mRNA

• Messenger RNA (mRNA) carries copies of instructions for assembling amino acids into proteins.

rRNA

• Ribosomes are made up of proteins and ribosomal RNA (rRNA).

tRNA• During protein

construction, transfer RNA (tRNA) transfers each amino acid to the ribosome.

Transcription• Flow of info: DNA -> mRNA

• Location: Nucleus

• mRNA is produced by copying part of the DNA

• Where does the mRNA go?– Cytoplasm/Ribosomes

Transcription

• Messenger RNA is transcribed in the nucleus, and then enters the cytoplasm where it attaches to a ribosome.

The Genetic Code

• The genetic code is read from mRNA

• mRNA is only 4 letters A, U, C, and G

• The code is read 3 letters/bases at a time• Codon= three consecutive nucleotides

which are specific for an animo acid

Codons

Translation

• Flow of info: mRNA -> Proteins

• Location: Cytoplasm/Ribosomes

• Translation is decoding mRNA into a polypeptide chain(protein)

Step 1 Translation

• mRNA attaches to a ribosome

• The start codon (AUG) is located by tRNA

• The matching tRNA, containing the anitcodon UAC, will bind to AUG

• The tRNA carries the animo acid specific to the mRNA sequence AUG, which is methionine

Step 2•The ribosome binds new tRNA molecules and amino acids as it moves along the mRNA.

Step 3As each new tRNA enters the ribosome, one leaves. Before tRNA can leave the ribosome, the animo acidswill bond together to make a polypeptide chain

Step 4The process continues until the ribosome

reaches a stop codon.

Mutations• Changes in genetic material

• Many have little effect on gene expression or protein function

• A few can be harmful and then some are good– Harmful mutations can cause cancer and

genetic disorders– Good mutation can make altered proteins

which may be beneficial in different/changing environments

Point Mutations

• Mutation of 1 or more nucleotides– Substitution – changing one base, usually

only changes one amino acid– Insertion – addition of 1 or more bases,

causes frameshifts– Deletion – removal of 1 or more bases,

causes frameshifts

Point Mutations

Kinds of Mutations

•Substitutions usually affect no more than a single amino acid.

Chromosomal Mutations

• Change in number or structure of a chromosome– Deletion – loss of all or part of a chromosome– Duplication – extra copies of parts of

chromosomes– Inversion – reverse direction of parts of

chromosomes– Translocation – chromosome breaks and

attaches to another

Chromosomal Mutations

• Occur during Meiosis– Prophase I– Anaphase I or Anaphase II

Nondisjuction

• Error in meiosis in which chromosomes fail to separate.

Changes to Chromosome Number

• Monosomy 2n-1– 45 chromosomes– Turners (XO)

• Trisomy 2n+1– 47 chromosomes– Down syndrome(trisomy 21), Klinefelters

(XXY), Triple X (XXX), Jacobs (XYY)

• Polyploidy (Triploid, Tetraploid, etc)

Turners (45, XO)

• 1 in 3,000 female births• Sterile females

Down Syndrome

•Mothers in early 20s 1 in 1,500 births•Mothers over 35 1 in 70 births•Mothers over 45 1 in 25 births

Klinefelters (XXY)

• 2 in 1000 male births• Sterile males

Jacobs (XYY)

• 1 in 1000 male births

• Tall

• Lower mental ability

• Tendency for aggressiveness