You hear mutations - but we hear variation! Mutations have several causes and can be positive, neutral or negative to the phenotype.
You hear mutations - but we hear variation! Mutations have several causes and can be positive, neutral or negative to the phenotype. Grab a recap of the Central Dogma before diving in (1:40). Distinguish between point mutations and frameshift mutations (2:50). Mutations have a strong correlation to evolution(3:40). Prokaryotes have been around a long time, with many reproductive techniques (5:08). Genotypic changes are also the result of errors in cell division, as with non-disjunction (5:40).
The Question of the Day asks (6:25) “What metal atom is bound to a heme group in hemoglobin?”
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Hi and welcome to the APsolute Recap: Biology Edition.
Today’s episode will recap mutations
Lets Zoom out:
Unit 6 - Gene Expression and Regulation
Topic - 6.7
Big Idea: Information Storage and Transmission
Introduction:
Mutations get such a bad wrap - but mutations in and of themselve aren’t negative. Mutations are just genetic changes. There are many reasons for mutations - including errors in DNA replication, environmental or chemical interactions, or even at times random. These changes might be rough, even fatal - but they also might be neutral or provide some evolutionary advantage. Mutations are the primary source of genetic variation. Whether a mutation is beneficial, neutral, or harmful depends on environmental context. So let’s roll the genetic dice and look at a few examples.
Lets Zoom in:
Quick recap of the central dogma of biology. Genes are stored in DNA, transcribed into an mRNA transcript and translated into protein at a ribosome. This protein will be folded and processed into a specific shape for a specific function. One of our earliest episodes recaps biological molecules - Episode 5. The central Dogma is Episode 20. OK, back to mutations. The differences in the expression of genes account for some of the phenotypic differences among organisms. You are mostly made of proteins and so long as they are made properly, you should be in good shape to function normally. Disruptions in genes, and consequently gene products can cause new phenotypes. One example of the bad - mutations in CFTR gene disrupt the transport of ions, causing the lungs to clog with extra mucus, and results in cystic fibrosis.
As we said earlier, DNA mutations can be positive, negative or neutral. This is determined by the effect the mutation has on the nucleic acid sequence, resulting protein, and phenotype. Recall that the mRNA is translated by codons, three base pair sequences long. First - the mutation in question has to be within the coding region of a gene to have effect. There's alot of junk in DNA. Second, the location of the change within a codon matters. If the mutation changes the amino acid sequence of the polypeptide and alters the structure and function of the protein, then it affects the phenotype of the organism.
Point mutations substitute one nucleotide base pair for another. They can be silent, having no effect on the amino acid sequence, missense - causing an amino acid change, or nonsense - coding for a stop. There are three stop codons ( UAA, UAG, UGA). If the point mutation is in the third base of a codon, the chances of a silent mutation increase. For example, any codon that begins with CU will give you leucine. The third letter can be U,C,G or A and you still get leucine. This allows for mistakes in transcription and translation to occur, while still getting the intended protein product. A frameshift mutation is caused by the insertion or deletion of a base. This causes a shift in the ribosomes reading frame and every subsequent codon downstream of the mutation may be different. Frameshift mutations can also cause a stop of translation.
How about the good? Some genetic changes allow for antibiotic or pesticide resistance. Consider the process of evolution by natural selection - There needs to be an overproduction of offspring with genetic variation, some sort of struggle to survive and then successful reproduction. If the farmer had never sprayed pesticide on the crops, the gene that allowed for some insects to survive over others would never have had selective advantage. Another, seemingly less likely advantage is with the disorder sickle cell anemia. Again, we hear disorder and we think bad. Normally, individuals with sickle cell anemia struggle to carry sufficient oxygen in their misshapen red blood cells and frequent clotting. But in certain regions where malaria is prominent (fever, diarrhea, vomiting, often death if untreated) being heterozygous for sickle cell trait provides malaria resistance. And so, the allele for sickle cell anemia remains persistent in those populations.
Genetic changes that enhance survival and reproduction can be selected for by environmental conditions. Prokaryotes have been around longer - and have quite a few reproductive tricks up their sleeve. They have horizontal gene transformation where they take in naked DNA (like in the pGLO lab). They have transduction, a viral transmission of genetic information. They have conjugation, a form of cell to cell transfer of DNA through a temporary pilli connection. Bacteria can be very inappropriate. Lastly, transposition - where pieces of DNA can move within and between DNA molecules. All of these strategies increase variation. Viruses have been around a long time too (some theories suggest even longer than prokaryotes). They too can combine or recombine genetic information if they are closely related and infect the same host cell. It might be sneaky, and hard to combat (aka coronavirus pandemic), but it is successful.
You can also have an altered genotype, and subsequent phenotype, due to errors in cellular division. Nondisjunction during meiosis, when homologous chromosomes don’t separate can alter the chromosome number. Some changes in chromosome number cause human disorders that have developmental limitations, such as Trisomy 21, also known as Down syndrome, and Turner syndrome, where females are missing part of their sex chromosome and are often infertile.
To recap….
Mutations are the spice of life - or at least primary source of variation and subsequent evolution. Genetic mutations can be neutral, harmful or beneficial. Make sure to connect the genetic change with phenotypic effect.
Today’s Question of the day is about red blood cells.
Question “What metal atom is bound to a heme group in hemoglobin?”
Coming up next on the Apsolute RecAP Biology Edition: Biotechnology