DNA Replication & Protein Synthesis

DNA REPLICATION

DNA Replication When cells divide, they need the instructions on how to be a particular type of cell and how to function as that cell type. These instructions are found in the DNA of the parent cell. It has its own full set of DNA instructions. When the cell divides to form two cells – it must make sure the new cell has a full set of DNA as well. If one cell is to become two cells then one set of DNA must become two sets – this is replication – it’s a part of mitosis.

Semiconservative? Semi = half Conserve = to keep Therefore…semiconservative literally means “half is kept”. This means that we will use the old DNA as a template to make two new DNA molecules. Each new DNA molecule is therefore made up of one strand from the old DNA and a newly synthesized strand of DNA that match up according to the base- pair rule.

Replication – The Process Replication is a two-phase job… 1.Separate the old strands. 2.Build the new (complementary) strands. To perform these jobs we will use a number of enzymes and free-floating DNA nucleotides that we would have gotten into the cell when we ate the cells of another organism for breakfast or lunch. Your DNA may have once been used in a dinosaur or bacterial cell – it all recycles.

DNA Replication

PROTEIN SYNTHESIS

DNA  RNA  Pn DNA  RNA  Pn is known as the central dogma of modern biology. The process it illustrates is protein synthesis. Proteins build, operate and regulate the body – pretty important stuff! In fact, the word protein means “of primary importance”! DNA (deoxyribonucleic acid) resides in the nucleus of your cells and it carries all of the instructions on how to build your proteins. This is why it is called the “blueprint of life”. DNA is the blueprint and proteins are the structures made using the blueprint. So what is the role of the RNA?

Why RNA? RNA (Ribonucleic acid) is the link between DNA and proteins. DNA lives in the nucleus – it’s the boss that works in the head office of the cell – it never leaves that office! Proteins are built by little organelles called ribosomes which live in the cytoplasm of the cell – not the nucleus. RNA is what helps connect the DNA in the nucleus to the ribosomes in the cytoplasm. Think of RNA as the messenger. It carries the instructions from the DNA, in the nucleus, out to the protein-building ribosomes, in the cytoplasm. DNA  RNA  Pn (It makes more sense now eh?)

Transcription (DNA  RNA) The first step in protein synthesis is transcription. It takes place in the nucleus of the cell. “Transcript” = copy. The DNA is going to unwind and make a copy of one small section of itself. This small section is a gene – one trait about you. This small copy of the DNA is called mRNA (messeneger RNA). It carries the code for one gene on the DNA out to the cytoplasm. Transcription is complete once the mRNA has been constructed. The mRNA will travel through one of the pores in the nucleus out to the cytoplasm in search of a ribosome.

Transcription

Translation (RNA  Pn) Translation is the second step of protein synthesis. It occurs in the cytoplasm when the mRNA is used by the ribosome to build the protein. A translator is someone who takes one language and changes it into another language. The name translation applies here because the ribosome is using a nucleic acid sequence to build a protein sequence. It is changing the sequence of one type of biomolecule into a sequence of another.

Translation (RNA  Pn) Translation begins when the ribosome (the protein builder) finds the mRNA and sits on one end of the mRNA strand. The ribosome then begins moving along the mRNA, reading the nucleotide sequence (A’s, C’s, G’s & U’s). It uses this sequence to help it gather the proper amino acids (protein building blocks) and put them in order in the newly made protein. Once the ribosome reaches the end of the mRNA – it jumps off and releases the newly made protein. The protein can now go and do some work in the cell.

Translation The ribosome (green blob) is traveling down the mRNA strand (blue line) and reading it. The code on the mRNA strand tells the ribosome which amino acids (yellow ovals) are needed to build the protein (all the yellow ovals connected together).

Polysomes A polysome occurs when a number of ribosomes are translating the same piece of mRNA at the same time. This gives you many copies of the desired protein using only one mRNA. This saves the cell energy and resources.

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