Mod 1- DNA & Gene Structure Flashcards
What is the composition of DNA?
DNA is a polymer of nucleotides.
Who discovered the double helix structure of DNA?
James Watson and Francis Crick in 1953.
What are the two roles of genes in the cell?
- Units of biological information
- Units of inheritance
What are the three parts of a nucleotide?
- Sugar = 2’-deoxyribose (2 prime is the position in the sugar)
- Phosphate groups
- Base = nitrogenous
What are the four bases found in DNA?
Attached to sugar with a β-N-glycosidic bond (base attached to sugar)
- Adenine (A)
- Guanine (G)
- Cytosine (C)
- Thymine (T)
A & G are purines
C & T are pyrimidines
What is Chargaff’s Rule?
The total number of purines (A & G) in a DNA molecule is equal to the total number of pyrimidines (C & T)
What is the directionality of DNA strands?
DNA has a direction, e.g., 5’ to 3’.
What type of structure does DNA have in living cells?
DNA is a double helix made up of two polynucleotide strands.
What is the significance of the DNA double helix being antiparallel?
The two strands run in opposite directions.
What bonds hold the two strands of DNA together?
Hydrogen bonds.
What is complementary base-pairing in DNA?
A pairs with T and G pairs with C.
This stabilises the DNA but allows for separation
What is GC content?
The amount of G + C nucleotides in an organism’s DNA.
What is the GC content of human DNA?
40.3%.
This is never 50/50 with A-T content
What are the three types of DNA double helix?
B-DNA = normal/ most common form of DNA = right-handed
A-DNA = most rare DNA (‘messy’) = right-handed
Z-DNA = found in our DNA in small amounts = left-handed
What is the most common form of DNA?
B-DNA.
Under what conditions does A-DNA form?
When there is not enough water around – dehydrating conditions.
What is known about the biological activity of Z-DNA?
Found naturally with B-DNA only in certain regions but its function is still not clear.
Associated with some diseases
Fill in the blank: DNA is vital for all _______.
[living things]
True or False: The double helix structure of DNA was solely based on Watson and Crick’s work.
False.
What are the names of the four nucleotides in DNA?
- 2’-deoxyadenosine 5’-triphosphate (dATP)
- 2’-deoxyguanosine 5’-triphosphate (dGTP)
- 2’-deoxycytidine 5’-triphosphate (dCTP)
- 2’-deoxythymidine 5’-triphosphate (dTTP)
What is a gene?
A segment of a DNA molecule containing biological information
(A very small portion of the genome)
Genes are the functional units of the genome
How is the length of a DNA molecule measured?
In base pairs (bp)
1000 bp = 1 kilobase (kb),
1000 kb = 1 megabase (Mb)
What is the length range of genes?
Shortest genes: about 100 bp (eg tRNAs)
Longest genes: about 2,400,000 bp (eg.human muscle protein dystrophin)
Do the longest genes contain 24,000 times more biological information than the shortest genes?
No
This is a misconception
What are the two main components of a gene?
Genes are discontinuous so are split into:
Exons and introns
Exons contain information to make a protein; introns contain information not needed for protein synthesis
What is the average number of introns and exons in a human gene?
8 introns and 9 exons
Mean intron length is 3365 bp, mean exon length is 145 bp
In many genes, the introns can make up almost 90% of the total length
What is gene expression?
The process by which the information in a gene is read
This leads to the production of proteins and RNAs
What are the functional end products of gene expression?
Proteins
Proteins play many roles in the cell; noncoding RNAs also play important roles
What distinguishes unique genes from gene families?
Unique genes have one copy
other genes occur in families = related or same gene
Gene families can be simple or complex
What are simple multi-gene families?
Families where all genes are the same
They are identical and produce the same product (amplify)
Example: ribosomal RNA genes
What are complex multigene families?
- Families where genes are not identical but have similar DNA sequences
- Code for similar but non-identical proteins
- Provides proteins with slightly different functions which allows increased organismal complexity
Example: human globin genes
How do multigene families arise?
Through gene duplication (during meiosis)
After duplication, sequences gradually change so the members of the gene family have slightly different sequences
What is the molecular clock?
A measure of the rate at which the sequence of a gene changes
It helps determine when a pair of genes were formed by duplication
If more similar = shorter amount of time
What are pseudogenes?
Genes that have lost their function and no longer make sense
There are four pseudogenes in the alpha-globin family and one in the beta-globin family
Fill in the blank: The human globin genes are located on chromosome ___ and chromosome ___.
16- Alpha- globin genes
11- beta-globin genes
Why is DNA important?
- it is vital for all living things
- it holds instructions for an organisms development, survival and reproduction due to its structure
- long term storage of information = DNA is very stable
Where does a phosphodiester bond form?
Bonds to oxygen
What is the typical process of gene expression?
Gene —> RNA —> Protein
Gene to RNA = via transcription by polymerase
RNA is the intermediate = 96% is non-coding RNA, 4% is coding RNA that turns into protein
RNA to Protein = via translation by ribosome
This is the classic pathway BUT sometimes stays as RNA which have different independent functions
What are some examples of non-coding RNA?
tRNA- adds amino acids to chain
rRNA- makes up part of ribosome
snRNA- involved in splicing
snoRNA- causes brain disorders when mutated
