Chapter 13 The Structure of RNA - PowerPoint Presentation

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Chapter 13

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The Structure of RNA

RNA is made of a long chain of nucleotides.

Differences between DNA and RNA:

1 – the sugar on RNA is ribose instead of deoxyribose

2 – RNA is generally single-stranded

3 – RNA contains uracil in place of thymine

An RNA molecule is like a disposable copy of a segment of DNA.  IT is a working copy of a single gene.

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Types of RNA

Most RNA is involved in protein synthesis.

There are three main types of RNA:  messenger RNA, ribosomal RNA, and transfer RNA

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Messenger RNA (mRNA) – RNA molecules that carry copies of instructions for assembling amino acids into proteins

Ribosomal RNA (rRNA)

– RNA that makes up the major part of ribosomes

Transfer RNA (tRNA)

– transfers each amino acid to the ribosome as it is specified by coded messages in mRNA

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Transcription

Transcription is producing RNA molecules by copying part of the nucleotide sequence of DNA into a complementary sequence in RNA

Transcription requires an enzyme called RNA polymerase

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During transcription, RNA polymerase binds to DNA and separates the DNA strands.  RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA

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RNA polymerase binds only to regions of DNA known as

promoters. 

Promoters are signals in DNA that indicate to the enzyme where to bind to make RNA.  Other signals in DNA cause transcription to stop when the new RNA molecule is finished.

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RNA Editing

Some of the RNA molecules are produced from larger RNA molecules that are cut and trimmed to their final sizes.  Large pieces are removed from the RNA molecules transcribed from many eukaryotic genes before they become functional.  These pieces are

introns

.  Introns are cut out of RNA molecules while they are still in the cell nucleus.

The remaining portions are

exons

.  Exons are spliced back together to form the final mRNA.

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The Genetic Code

Proteins are made by joining amino acids into long chains called polypeptides.  Each polypeptide contains a combination of any or all of the 20 different amino acids.  The properties of proteins are determined by the order in which different amino acids are joined together to produce polypeptides.

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Codon – a group of three nucleotides on messenger RNA that specify a particular amino acid.

Since there are 4 different bases (A, U, C, and G) there are 64 possible three-base codons (4x4x4=64)

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Translation

Translation is the decoding of an mRNA message into a polypeptide chain.  Translation takes place on ribosomes.  During translation, the cell uses information from messenger RNA to produce proteins.

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The Roles of RNA and DNA

The cell uses the DNA “master plan” to prepare RNA “blueprints”.  The DNA molecule remains in the nucleus, while the RNA molecules go to the protein-building sites in the cytoplasm – the ribosomes

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13-2 MUTATIONS

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Mutations – changes in the DNA sequence that affect genetic information

Gene mutations result from changes in a single gene. Chromosomal mutations involve changes in whole chromosomes.

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Gene MutationsSome gene mutations involve several nucleotides, but the majority involve just one.

Point mutations

– mutations that affect one nucleotide

Point mutations occur at a single point in the DNA sequence

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When a point mutations involves the insertion or deletion of a nucleotide, much bigger changes happen. The genetic code is read in groups of three, called a codon. If a nucleotide is deleted, the base is still read in groups of three, but now the groupings are shifted for every codon that follows. Inserting an extra nucleotide has a similar affect. This is called

frameshift

mutations

because they shift the genetic message.

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Example of frameshift mutation:

THE FAT CAT ATE THE RAT

Delete the h

TEF ATC ATA TET HER AT

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Chromosomal MutationsA chromosomal mutation involves changes in the number or structure of chromosomes. Chromosomal mutations may change the locations of genes on chromosomes and even the number of copies of some genes.

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13-3 GENE REGULATION

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An expressed gene is a gene that is transcribed into RNA. Only a fraction of the genes in a cell are expressed at any given time.

Gene Regulation: An Example

Operon

– A group of genes that operate together

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Example: E. ColiThe protein-encoding genes in this bacterium include a cluster of three genes that are turned on or off together. Because these genes must be expressed in order for the bacterium to be able to use the sugar lactose as a food, they are called the lac operon.

The lac genes are turned off by repressors and turned on by the presence of lactose

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Eukaryotic Gene Regulation

Most eukaryotic genes are controlled individually and have regulatory sequences that are much complex than those of the lac operon.

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Development and Differentiation

Regulation of gene expression is especially important in shaping the way a complex organism develops. Each of the specialized cell types found in the adult develops from the same fertilized egg cell. So, cells don’t just grow and divide during embryonic development; they also undergo

differentiation

.

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Differentiation

– cells become specialized in structure and function

Hox

genes

– control the differentiation of cells and tissues in the embryo

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A mutation in a hox genes can completely change the organs that develop in specific parts of the body.

(Mutations affecting the

hox

genes in a fruit fly can replace the fly’s antennae with legs growing on its head)