DNA Review - Socorro Independent School District



DNA Review

Deoxyribonucleic Acid (DNA):

• nucleic acid that carries genetic information

• DNA remains in the nucleus of cells

• in the nucleus, DNA is in the form of chromosomes

• the chromosomes can be divided into smaller pieces called genes

• genes code (carry the information) for traits (ex: hair color, eye color, etc.)

• the information in DNA is used to make proteins

DNA Structure:

• made up of subunits called nucleotides

• nucleotides are made up of a phosphate group, a sugar (deoxyribose), and a base

• the bases in DNA are adenine, thymine, guanine, and cytosine and are abbreviated A, T, G, and C

• the shape of DNA is described as a double helix

• a double helix resembles a “twisted ladder”

• the “backbone” of the ladder is made up of alternating sugar and phosphate groups

• the “rungs” of the ladder are made up of the bases joined by weak chemical bonds; the bases joined together are called a base pair; the only base pairs that occur in DNA are A-T and G-C

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1. deoxyribose

2. base

3. nucleotide

4. base pair

5. hydrogen bonds

6. phosphate group

DNA Replication:

• when a cell divides, the DNA must make an exact copy of itself so that each cell will have one complete copy of the DNA

• the process of making a copy of DNA is called replication

• the steps in replication are:

o the DNA “unzips” as the weak chemical bonds holding the bases together are broken

o the parent strand (the original DNA) acts as a template (something from which a copy can be made)

o the enzyme DNA polymerase attaches free-floating nucleotides in the nucleus to the parent strand to create the daughter strand

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Transcription:

• recall that DNA is found in the nucleus and cannot exit

• because proteins are made in the cytoplasm of a cell, another nucleic acid, which can leave the nucleus is needed; this nucleic acid is RNA or ribonucleic acid

• RNA is similar to DNA with only 3 exceptions:

o RNA has only one strand not two

o RNA contains the sugar ribose instead of the sugar deoxyribose

o RNA contains the bases A, C, and G like DNA but it does not contain T; instead it has the base uracil (U) which base pairs with A

• transcription is the process by which the genetic information in DNA is copied to RNA

• the steps in transcription are:

o a protein known as RNA polymerase binds to a strand of DNA and “unzips” it

o RNA polymerase brings in the complementary nitrogen bases to match up with the template strand of DNA

o the sugar-phosphate backbone of the mRNA is formed

o the mRNA is released and the DNA “re-zips”

o the mRNA leaves the nucleus and travels to the ribosomes in the cytoplasm

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Translation:

• the process by which the genetic information in mRNA is used to make a protein

• THE GENETIC INFORMATION IS BASED ON THE SEQUENCE OF THE BASES IN THE mRNA

• proteins are made up of subunits called amino acids

• there are 20 different amino acids

• a set of 3 bases in the mRNA is called a codon

• each codon codes for an amino acid

• with the 4 bases in RNA (A,C,G U), there are a total of 64 possible codons; this means that the genetic code is redundant, meaning that some amino acids have more than one codon that code for it (ex: CUU, CUC, CUA, and CUG all code for the amino acid leucine)

• The genetic code is universal, meaning that it is the same for all organisms (ex: UGU will always code for the amino acid cysteine and GUC will always code for valine)

• only 61 of the 64 codons code for an amino acid; the other 3 codons are stop codons and signal the end of the protein

• steps in translation:

o the large and small subunits of a ribosome attach to a strand of mRNA

o the first codon in a mRNA strand is AUG; this is known as a start codon

o the first tRNA with the anti-codon matching the AUG start codon brings in the first amino acid--methionine

o another tRNA with a matching anti-codon brings in another amino acid

o a peptide bond is formed between the two amino acids

o the ribosome moves down the mRNA chain one codon (3 bases)

o a new tRNA can now bring in the next amino acid

o when the ribosome encounters a stop codon, a release factor is brought in instead of a tRNA

o the release factor causes the large and small subunits to release the mRNA strand

o the newly formed amino acid chain or protein is released into the cytoplasm

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DNA Codon Table:

| | |Second Position of Codon | | |

| | |

|TT |Tt |

T phenotypes: all are tall*

T genotypes: TT, Tt in a ratio of 2:2

*notice that all the plants are tall because one of the parents is homozygous dominant; when a parent is homozygous dominant for a trait all the offspring will be dominant for the trait

• the above square is a case of simple inheritance; complex inheritance involves alleles that are not simply dominant and recessive; complex inheritance explains why traits such as eye color and height don’t have only 2 choices (brown or blue or tall or short); eye color is inherited through polygenic inheritance, meaning that more than one gene is controlling this trait

In this example of incomplete dominance, the heterozygous condition (Bb) results in gray plumage.

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