DNA and Genes Unit 4 Chapter 11

DNA structure DNA controls cellular activity because it regulates the production of proteins. DNA is the blueprint for proteins that are necessary for cellular metabolism.

Why are proteins so important? Some proteins become important structures, such as the filaments in muscle tissue. Other proteins, such as enzymes, control chemical reactions that perform key life functions—breaking down glucose molecules in cellular respiration, digesting food, or making spindle fibers during mitosis.

Who discovered that DNA is the blueprint for life? In 1952 Alfred Hershey and Martha Chase performed an experiment using radioactively labeled viruses that infect bacteria. Because viruses are protein and DNA only, they figured out that viral DNA (not viral protein) could force the bacteria to make new viruses. This was evidence that DNA can determine cell activity.

DNA is a polymer. Polymer: chemical structure made of repeating units DNA is made of repeating nucleotide units. DNA nucleotides always have a phosphate group, deoxyribose sugar, and a nitrogen base. DNA nucleotide

Four DNA nitrogenous bases A nitrogenous base is a carbon ring with nitrogen atoms and determines the name of the nucleotide. In DNA, there are four possible nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine (A) Guanine (G) Cytosine (C) Thymine (T)

Section 11.1 Summary – pages 281 - 287 The structure of nucleotides Nucleotides join together to form long chains, with the phosphate group of one nucleotide bonding to the deoxyribose sugar of an adjacent nucleotide. The phosphate groups and deoxyribose molecules form the backbone of the chain, and the nitrogenous bases stick out like the teeth of a zipper. Section 11.1 Summary – pages 281 - 287

DNA is a double helix and looks like a twisted ladder. The outer parts are the sugar-phosphate backbone. Two nitrogen bases of the nucleotides face inward and form the rungs of the helix ladder. Adenine always binds to thymine. Cytosine always binds to guanine..

Who discovered the double helix structure? In 1953, Watson and Crick proposed that DNA is made of two chains of nucleotides held together by nitrogenous bases and twisted together. They used Rosalind Franklin’s X-ray crystallography work to figure this out.

The importance of comparing DNA nucleotide sequences Each species has its own unique DNA sequence. The more closely related two individuals are, the more likely they will share the same DNA nucleotide sequence. Comparing DNA base pairs of two species will show their evolutionary history.

DNA replication – making copies of the DNA code Necessary before a cell undergoes mitosis, occurs in interphase Click on image to play video.

Copying DNA

How does DNA code for proteins? The sequence of nucleotides in each gene contains information for assembling the string of amino acids that make up a single protein. The ribosomes required to make proteins cannot read DNA. Therefore, for DNA to code for proteins, an RNA molecule must be made. Ribosomes can read RNA.

RNA is another nucleic acid, nucleotide polymer. RNA differs from DNA structure in three ways. Single stranded instead of double stranded Ribose sugar instead of deoxyribose Uracil instead of thymine nitrogen base Ribose sugar

Three types of RNA Messenger RNA: carries the DNA code (message) to the ribosomes Ribosomal RNA: makes up the ribosomes that reads the mRNA to build the correct amino acid sequence Transfer RNA: brings the amino acids to the ribosome

Click on the image to play the video. Transfer RNA Click on the image to play the video.

Click on image to play the video. Transcription The process of building an RNA strand from the DNA template In eukaryotes, this occurs inside the nucleus. In prokaryotes, this occurs in the cytoplasm. Click on image to play the video.

Transcription

mRNA processing in eukaryotes Since much of the DNA code is useless or codes for multiple proteins, the unnecessary portions of DNA that were coded into mRNA must be removed. The useless portions of RNA (introns) are removed. The coding portions (exons) are linked together to make the final mRNA.

mRNA codes for amino acids. Three mRNA nucleotides code for one amino acid, but more than one combination codes for the same amino acid.

Translation The process of ribosomes reading the mRNA code to properly make an amino acid chain that is folded into a usable protein

Translation The ribosome binds to AUG, the starting code (codon). The ribosome directs the methionine tRNA to bring transfer the methionine (met) amino acid.

Translation The ribosome read the next codon and directs the appropriate tRNA to transfer the amino acid.

Translation process The ribosome joins the amino acids together and continues this process until the codon indicates stop.

What happens if there is a mistake (mutation) in the DNA code? Possibly proteins won’t be made or are made improperly. If the mutations occur in the gametes, the offspring’s DNA will be affected positively, negatively, or neutrally. What can cause a mutation? Replication error Transcription error Cell division error Chemical agents (mutagens) Spontaneous changes

Point mutation A point mutation is a change in a single base pair in DNA. A change in a single nitrogenous base can change the entire structure of a protein because a change in a single amino acid can affect the shape of the protein.

Point mutations May change the amino acid code if the mutations occurs in the right place in the code. mRNA Normal Protein Stop Replace G with A Point mutation mRNA Protein Stop

Frameshift mutations Losing a single nucleotide base This mutation would cause nearly every amino acid in the protein after the deletion to be changed Deletion of U mRNA Protein

Changes to the chromosome When a part of a chromosome is left out, a deletion occurs When part of a chromatid breaks off and attaches to its sister chromatid, an insertion occurs. A B C D E F G H A B C E F G H Deletion A B C D E F G H A B C B C D E F G H Insertion

Changes to the chromosome When part of a chromosome breaks off and reattaches backwards, an inversion occurs. When part of one chromosome breaks off and is added to a different chromosome, a translocation occurs. A B C D E F G H A D C B E F G H Inversion A B C D E F G H W X A B C D E F G H W X Y Z Y Z Translocation

Repairing DNA Enzymes proofread the DNA and replace incorrect nucleotides with correct nucleotides. The greater the exposure to a mutagen such as UV light, the more likely is the chance that a mistake will not be corrected.