17.4 Molecular Evolution

What are researchers comparing when they use a molecular clock?

• versions of genes

• alleles on chromosomes

• stretches of DNA

• sister chromatids

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What do scientists use a molecular clock for?

• They use DNA sequences to estimate how long a species has been on earth.

• They use mutation rates in DNA to estimate the time that two

  species have been evolving independently.

• They use mutations in the alleles to figure out how different chromosomes have become over time.

• They use amino acid chains to determine RNA sequences in the original species ancestor.

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Old-fashioned pendulum clock mark time with a swinging pendulum. A molecular clock also relies on a repeating process to mark time it is called a ____________

• gene sequence

• allele

• chromosome pair

• mutation

Some mutations have a major positive or negative effect on an organism’s phenotype. Many mutations, however, have no effect on phenotype. What are these mutations called?

• Neutral mutations

• Point mutations

• Inversion mutations

• Reverse mutations

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The more differences there are between the DNA sequences of the two species, the more time has elapsed since the two species shared a common ancestor.

• True

• False

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The use of molecular clocks is not simple, because ...

• there are too many genes to work with.

• genes are hard to read because they change quick.

• there are many reasons that mutations happen in DNA.

• there is not just one molecular clock in a genome.

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There are many different clocks, each of which “ticks” at a different rate.

• This is because some chromosomes are different sizes.

• This is because some genes accumulate mutations faster than others.

• This is because some cells do not mutate at the same rate.

• This is because some cells use clocks that work on a different scale.

Researchers check the accuracy of molecular clocks by trying to estimate how often mutations occur. In other words, they estimate how often the clock they have chosen “ticks.”

• To do this, they compare the number of mutations in a particular gene in species whose age has been determined by other methods.

• To do this, they compare the number of cell changes in a particular gene in species whose age has been determined by fossils.

Where did the roughly 25,000 working genes in the human genome come from?

• Modern genes probably came from other species that existed before and were copied.

• Modern genes probably started with the mutations that happened around the turn of the millennium.

• Modern genes probably descended from a much smaller

  number of genes in the earliest life forms.

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What’s so important about gene duplication?

• A gene needs to be able to segregate during the process of duplication.

• A gene can mutate while the original gene remains unchanged.

• A gene needs to have multiple copies for it to function properly.

• Genes need to be duplicated in order to make new cells.

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