Transcription and translation Gene Expression Transcription and translation

Types of RNA The three main types of RNA are messenger RNA, ribosomal RNA, and transfer RNA. Messenger RNA (mRNA) carries copies of instructions for assembling amino acids into proteins.

Types of RNA Ribosomes are made up of proteins and ribosomal RNA (rRNA). The three main types of RNA are messenger RNA, ribosomal RNA, and transfer RNA. Ribosomal RNA is combined with proteins to form ribosomes.

Types of RNA During protein construction, transfer RNA (tRNA) transfers each amino acid to the ribosome. The three main types of RNA are messenger RNA, ribosomal RNA, and transfer RNA.

Gene Expression: How Proteins are Made Directing the production of PROTEINS is the job of DNA. Your genes contain codes for proteins, which are responsible for your “TRAITS”. Some protein products can be seen directly. MELANIN, the protein that gives your skin color is an example. Other proteins regulate your DEVELOPMENT or body processes. The process of protein synthesis occurs in TWO stages, and at two different LOCATIONS in the cell.

Important things to remember: DNA holds the code for all proteins made by a living thing. DNA is found in the NUCLEUS of a cell and CANNOT leave the nucleus. Proteins are constructed by organelles called RIBOSOMES, which are in the CYTOPLASM of a cell. Proteins are made of hundreds or thousands of AMINO ACIDS bonded together.

Step One: Transcription Occurs in the NUCLEUS of the cell. The information in a gene is copied into MESSENGER RNA (mRNA). This copy will be able to leave the nucleus, the DNA cannot. mRNA is the equivalent of a PHOTOCOPY of DNA. It is only single stranded, but carries all of the information in the DNA code for a particular gene. There are no THYMINES in RNA. All ADENINES in DNA correspond to a base called URACIL (U) in RNA.

Trascription

RNA leaving the nucleus

The Genetic Code The Genetic Code The genetic code is the “language” of mRNA instructions. The code is written using four “letters” (the bases: A, U, C, and G).

The Genetic Code A codon consists of three consecutive nucleotides on mRNA that specify a particular amino acid. A codon is a group of three nucleotides on messenger RNA that specify a particular amino acid.

The Genetic Code Each codon specifies a particular amino acid that is to be placed on the polypeptide chain. Some amino acids can be specified by more than one codon.

The genetic code shows the amino acid to which each of the 64 possible codons corresponds. To decode a codon, start at the middle of the circle and move outward.

The Genetic Code There is one codon AUG that can either specify the amino acid methionine or serve as a “start” codon for protein synthesis. There are three “stop” codons that do not code for any amino acid. These “stop” codons signify the end of a polypeptide.

Step Two: Translation The mRNA leaves the nucleus through a NUCLEAR PORE and enters the cytoplasm. The RIBOSOMES of a cell are the organelles that actually assemble the proteins and they are in the cytoplasm. Ribosomes attach to the mRNA and read it 3 bases at a time. A unit of 3 bases is called a CODON. Each codon codes for one AMINO ACID (the building blocks of proteins). The codons on the mRNA base pair with ANTI-CODONS on tRNA (transfer RNA) molecules.

Step Two: Translation continued The tRNAs bring amino acids to the ribosome one at a time. The message in a gene begins at an INITIATOR or “START” codon on the mRNA. This codon is AUG. If you are asked to translate a gene and the mRNA doesn’t begin with AUG, you must search and begin the process at the location of the AUG codon. The message in a gene ends at a TERMINATION or “STOP” codon. There are THREE different stop codons. The stop codon signals the RIBOSOME to disassemble. The mRNA and the finished POLYPEPTIDE are released.

Translation:

Protein Synthesis Odds and Ends: A ribosome is made of rRNA and PROTEINS. A ribosome is made of TWO subunits, the LARGE ribosomal subunit and the SMALL ribosomal subunit. MULTIPLE ribosomes may read a single piece of mRNA at the same time. TWO tRNA molecules can be in the ribosome at the same time. The bonds that form between amino acids are called PEPTIDE BONDS formed by dehydration synthesis. The start codon, AUG codes for the amino acid METHIONINE. In most cases, finished proteins do not begin with methionine because it is removed during processing.

RNA Editing RNA Editing The DNA of eukaryotic genes contains sequences of nucleotides, called introns, that are not involved in coding for proteins. The DNA sequences that code for proteins are called exons. When RNA molecules are formed, introns and exons are copied from DNA.

RNA Editing The introns are cut out of RNA molecules. Exon Intron DNA The introns are cut out of RNA molecules. The exons are the spliced together to form mRNA. Pre-mRNA mRNA Many RNA molecules have sections, called introns, edited out of them before they become functional. The remaining pieces, called exons, are spliced together. Then, a cap and tail are added to form the final RNA molecule. Cap Tail

Chromosome 7 activity Task: transcribe and translate a gene Answer = sequence of amino acids RNA polymerase reads from 3’5’ Start at 3’ at top of page Code twists along with helix Promoter/Initiator region on DNA = ATATTAG Promoter/Initiator gets transcribed Terminator region on DNA = CCCC Intron on mRNA = UAGC

Polyribosome

Genes on a chromosome

Genes on a chromosome

Warm up: If a finished protein contains 100 amino acids and begins with methionine, what is the MINIMUM number of codons used to code for this protein? How many peptide bonds are needed to hold together the protein described above? How many bases are in the mRNA sequence for this protein? How many base pairs of DNA long is the gene for this protein?

Warm Up Answers If a finished protein contains 100 amino acids and begins with methionine, what is the MINIMUM number of codons used to code for this protein? 101 (1 for each amino acid and a STOP codon) How many peptide bonds are needed to hold together the protein described above? 99 How many bases are in the mRNA sequence for this protein? 303 (101 codons x 3 bases per codon) How many base pairs of DNA long is the gene for this protein? 303

Warm Up Two: If a finished protein contains 100 amino acids and does not begin with methionine, what is the MINIMUM number of codons used to code for this protein? How many peptide bonds are needed to hold together the protein described above? How many bases are in the mRNA sequence for this protein? How many base pairs of DNA long is the gene for this protein?

Warm Up Two: If a finished protein contains 100 amino acids and does not begin with methionine, what is the MINIMUM number of codons used to code for this protein? 102 (start codon + 1 per aa + stop codon) How many peptide bonds are needed to hold together the protein described above? 99 How many bases are in the mRNA sequence for this protein? 306 How many base pairs of DNA long is the gene for this protein?