Protein Synthesis Chapter 12 p. 300 Transcription and Translation.

Protein SynthesisChapter 12 p. 300

Transcription and Translation

Protein Synthesis

In molecular terms, genes are coded DNA instructions that control the production of proteins in the cell.

In order for the cell to synthesize proteins, it must get instructions from DNA, but the DNA never leaves the nucleus and proteins are synthesized by ribosomes on the RER or in the cytoplasm.

RNA to the rescue…

Protein synthesis takes place in two steps:

Transcription - occurs in the nucleus- mRNA is made from a strand of DNAand

Translation- occurs in the cytoplasm- a protein is put together (synthesized) by a

ribosome using a strand of mRNA

Three types of RNA take part in the synthesizing of proteins from the code found on a gene.

• mRNA – messenger

• rRNA– ribosomal

• tRNA– transfer

A dazzling display of choreography:

• http://www.youtube.com/watch?v=41_Ne5mS2ls

Transcription

• Occurs in the nucleus

• Is the process by which part of the DNA sequence is copied into a complementary sequence of RNA

During transcription,

• An enzyme called RNA polymerase

– binds to DNA and separates the strands

– then uses one strand of DNA, the template strand, as a template from which nucleotides are assembled to create a strand of RNA.

• RNA polymerase will only bind to a strand of DNA in regions called promoters.

• In genetics, a promoter is a region of DNA where transcription of a gene is initiated (started). Promoters are located near the genes they transcribe, on the same strand and upstream on the DNA (towards the 3’region of the template strand.) Promoters can be about 100–1000 base pairs long.

• A gene on DNA includes introns and exons. When the RNA is copied it also includes introns and exons.

• Introns are those sequences on a gene which are not used in the synthesis of proteins. Exons are those sequences of the gene which are used in the synthesis of proteins.

• After the RNA is synthesized and while it is still in the nucleus, the introns are cut out and the exons are spliced together. This makes mRNA.

Introns and Exons

• What actually does the cutting and splicing varies depending on the gene and our knowledge of this process is far from complete.

• An excerpt from Wikipedia:At least four distinct classes of introns have been

identified.[1]

• Introns in nuclear protein-coding genes that are removed by spliceosomes

• Introns in nuclear and archaeal transfer RNA genes that are removed by proteins (tRNA splicing enzymes)

• Self-splicing group I introns that are removed by RNA catalysis.

• Self-splicing group II introns that are removed by RNA catalysis

mRNA

mRNA

The “language” of mRNA is called the genetic code.

Remember that mRNA contains 4 bases: A, U, C, and G.

Transcription (review) and Translation Introduction

• http://www.youtube.com/watch?v=LY0hZLDOb00

Translation

• Is the process by which the code on mRNA is translated into a protein (a polypeptide chain).

• rRNA reads the code in groups of three bases.

• Each group of 3 letters is a codon.

• Each codon represents a specific amino acid (AA).

Amino Acids

Remember that proteins are made of amino acids joined by peptide bonds.

• Translation begins when an rRNA attaches to mRNA in the cytoplasm.

http://www.youtube.com/watch?v=-zb6r1MMTkc&feature=related

Translation

Translation

• Initiator (start) codons: The point along a strand of mRNA where a rRNA molecule begins to translate a sequence of mRNA into amino acids. AUG

• Terminator or stop codons: The point where rRNA stops translating the sequence and releases the amino acid chain. UAA UAG UGA

• The proteins produced may vary from 2 codons in length to several thousand codons in length.

– Ex. 60 amino acids• How many nucleotides?• How long is the gene In codons?

• Once the mRNA leaves the nucleus to go to a ribosome, the next step takes place.

• Other genes in the DNA produce a transfer RNA (tRNA), shaped loosely like a cloverleaf.

• One end of tRNA is attached to a specific AA (1 of 20) and the other end has a set of 3 exposed bases called an anticodon.

Ex. Codon from mRNA = UCG Anticodon from tRNA = AGC

• The rRNA is the part of the ribosome that reads the code on the mRNA and helps the amino acids to bond.

• rRNA pulls the mRNA through the ribosome and directs a tRNA (whose anti-codon is complimentary to the mRNA codon) into the codon-anticodon slot.

• Once the codon and anti-codon are connected, the tRNA releases its amino acid which is added to the chain of amino acids growing from the ribosome.

• The amino acids are joined by peptide bonds. As each is added, a water molecule is released. (Dehydration hydrolysis)

• The chain will continue to grow until the rRNA reads the mRNA stop codon. (UAG, UAA or UGA)

• Once the tRNA has given up its amino acid, it can get another one.

• The process of decoding an mRNA message to build a polypeptide chain (protein) is called translation.

• Many ribosomes can be working on a single strand of mRNA.

Mutations (page 307)

• Mutations are changes in the genetic material.

Mutations that produce changes in a gene are called genetic mutations.

Mutations that produce changes in the whole chromosome or a portion of the chromosome are called chromosomal mutations.

Gene Mutations:

1. Point mutations: –Can include– substitution (one base changed to another)

which usually only affects the codon it is found in, therefore, only a single amino acid. This can still have devastating effects.

EX. THE FAT CAT ATE THE RAT

THE TAT CAT ATE THE RAT

Gene mutations cont’d

2. Frameshift mutations include: deletions and insertions which can be quite dramatic because the bases are read in groups of 3 and an insertion or deletion will shift everything.

Deletion: THE FAT CAT ATE THE RAT

THE ATC ATA TET HER AT

Insertion:THE FAT CAT ATE THE RATTHE FFA TCA TAT ETH ETA T

• By altering the code, every amino acid following the insertion or deletion could be different that what it was supposed to be.

• Frameshift mutations can, therefore, alter a protein so much that it is unable to do its job.

Chromosomal Mutations• Involve changes in the number or structure of chromosomes.

EX. 123-456 Normal

134-56 Deletion

122-345-6 Insertion

165-432 Inversion

123-789 Translocation (Part of one

78-9456 chromosome breaks off and attaches to another.)

Significance of Mutations

• Most mutations are neutral (have little or no effect).

• They can cause dramatic changes in the structure of protein or gene activity which can cause:

cause disruptions in normal biological activities or genetic disorders or

be beneficial and account for evolution. (Organisms in changing environments/survival of the fittest)

• Mutations in the cells that make gametes can be passed along to offspring.

• A condition in which an organism has an extra set of chromosomes is called polyploidy, caused when a complete set of chromosomes fails to separate. Polyploidy plants are often larger and stronger than diploid plants so are bred that way. (bananas, citrus fruit)

• http://www.youtube.com/watch?v=LuJuJC3ZHwA

Homework

• Page 305 Questions 1 – 5• Page 308 Questons 1 -5