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
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
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
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
DNA Codon Table:
Second Position of CodonT / C / A / G
F
i
r
s
t
P
o
s
i
t
i
o
n / T / TTT / Phe / [F]
TTC / Phe / [F]
TTA / Leu / [L]
TTG / Leu / [L]
/ TCT / Ser / [S]
TCC / Ser / [S]
TCA / Ser / [S]
TCG / Ser / [S]
/ TAT / Tyr / [Y]
TAC / Tyr / [Y]
TAA / Ter / [end]
TAG / Ter / [end]
/ TGT / Cys / [C]
TGC / Cys / [C]
TGA / Ter / [end]
TGG / Trp / [W]
/ T
C
A
G
/ T
h
i
r
d
P
o
s
i
t
i
o
n
C / CTT / Leu / [L]
CTC / Leu / [L]
CTA / Leu / [L]
CTG / Leu / [L]
/ CCT / Pro / [P]
CCC / Pro / [P]
CCA / Pro / [P]
CCG / Pro / [P]
/ CAT / His / [H]
CAC / His / [H]
CAA / Gln / [Q]
CAG / Gln / [Q]
/ CGT / Arg / [R]
CGC / Arg / [R]
CGA / Arg / [R]
CGG / Arg / [R]
/ T
C
A
G
A / ATT / Ile / [I]
ATC / Ile / [I]
ATA / Ile / [I]
ATG / Met / [M]
/ ACT / Thr / [T]
ACC / Thr / [T]
ACA / Thr / [T]
ACG / Thr / [T]
/ AAT / Asn / [N]
AAC / Asn / [N]
AAA / Lys / [K]
AAG / Lys / [K]
/ AGT / Ser / [S]
AGC / Ser / [S]
AGA / Arg / [R]
AGG / Arg / [R]
/ T
C
A
G
G / GTT / Val / [V]
GTC / Val / [V]
GTA / Val / [V]
GTG / Val / [V]
/ GCT / Ala / [A]
GCC / Ala / [A]
GCA / Ala / [A]
GCG / Ala / [A]
/ GAT / Asp / [D]
GAC / Asp / [D]
GAA / Glu / [E]
GAG / Glu / [E]
/ GGT / Gly / [G]
GGC / Gly / [G]
GGA / Gly / [G]
GGG / Gly / [G]
/ T
C
A
G
Mutations:
· a change in the DNA is called a mutation
· not all mutations are harmful
· there are two types of mutations: gene mutations and chromosomal mutations
· gene mutations:
o involve changes in one gene
o there are two types of gene mutations: point mutation and frameshift mutation
§ in a point mutation, one base is changed (a.k.a. base substitution mutation) so only one amino acid is changed; this type of mutation may result in no change in the protein or it can cause a disease; sickle-cell anemia and cystic fibrosis are two diseases caused by point mutations
§ in a frameshift mutation, one base is added (a.k.a. base insertion mutation) or one base is removed (a.k.a. base deletion mutation); this causes a shift in the reading frame of the codons so all the amino acids, starting at the site of the mutation, are changed; this results in an entirely different protein
· chromosomal mutations:
o involve changes in the entire chromosome
o there are 4 types of chromosomal mutations:
§ Deletion: a chromosome breaks and a piece of the chromosome is lost
§ Duplication: part of a chromosome breaks off and is incorporated into its homologous chromosome (recall that all chromosomes are paired since you receive one copy of the chromosome from your father and one copy from your mother; these paired chromosomes are called homologous chromosomes); the homologous chromosome now has an extra copy of one of its parts
§ Translocation: a part of a chromosome breaks off and attaches to a different, non-homologous chromosome
§ Inversion: a part of a chromosome breaks off, turns around, and reattaches in the reverse order
o these mutations are usually caused by nondisjunction, failure of the chromosomes to separate during cell division
§ if nondisjunction occurs during mitosis (the type of cell division that is used to make all cells of the body except the gametes; recall that in this type of division a cell starts with 46 chromosomes and the 2 new cells each have 46 chromosomes), the individual cell is affected, but the organism is not usually harmed
§ if nondisjunction occurs during meiosis (the type of cell division used to make gametes—sperm and eggs; recall that in this type of division a cell starts with 46 chromosomes and the 4 new cells each have 23 chromosomes), the entire organism is affected
§ nondisjunction results in:
· monosomy: the zygote (fertilized egg) has only one copy of a particular chromosome (has 45 chromosomes instead of 46)
· trisomy: the zygote has three copies of a particular chromosome (has 47 chromosomes instead of 46)
§ nondisjunction in all of the chromosomes is called polyploidy (has 69 chromosomes instead of 46)
Genetics:
· the study of heredity
· alleles are different forms of the same gene (ex: in plants the gene from height can have two alleles, one short and one tall)
· for all traits you have two alleles (one that you received from your father and one from your mother)
· if the two alleles are the same (both tall or both short), the organism is homozygous for that trait
· if the two alleles are different (one tall and one short), the organism is heterozygous for that trait
· in the heterozygous condition, the allele that is expressed or observed is said to be dominant; the allele that is not expressed is said to be recessive
· the phenotype is the outward appearance of the genes; it is what you see (ex: tall or short)
· the genotype is the gene combinations of an individual and can not be seen (ex: TT, Tt, or tt)
· Punnett squares are used to predict the outcome of a cross between two parents
Ex: a heterozygous tall parent and a homozygous dominant parent
T t
TT / TtTT / 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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