3.1 Genes Flashcards
Why are living organisms what they are?
Because of the expression of genetic information in their cells.
Where is genetic information stored?
In molecules of DNA that can be copied and passed to offspring.
What is an organism’s genome?
What is an organism’s genome?
The whole of the genetic information of an organism.
Explain how DNA codes for proteins
-DNA stores its information in the sequence of its four nitrogenous bases, abbreviated as A, T, C, and G.
-The bases form a code that is read in groups of three (triplets) that specify which amino acid to use in building proteins.
-These proteins then regulate cellular reactions and much more.
What is a gene?
-A heritable factor that consists of a length of DNA and influences a specific characteristic.
-It is a short stretch of DNA that influences a specific characteristic.
How do organisms store their genes?
On long stretches of DNA called chromosomes.
How many genes may a single chromosome have?
Hundreds or thousands of genes, each in a specific location or locus (plural: loci).
Diagram of human chromosome 11, showing locations (loci) of two genes, HBB and TYR.
It has a length of about 135 million DNA base pairs, about 4% of the total human genome.
Chromosome 11 has around 1 300 protein-coding genes. Each gene contains information that our cells can use.
Explain what alleles are (using the example of chromosome 11)
-Each of the 1,300 genes on chromosome 11 contains information, but individuals may have different versions of that information.
-In a hypothetical example, at the locus for petal color, a rose might have instructions that produce red petals, or a different version that produces yellow petals.
-The different, specific forms of a gene are called alleles, and they often vary from each other by only one or a few bases.
Explain how albanism can occur
-For the TYR gene shown in Figure 1, one allele codes for an enzyme that converts the amino acid tyrosine into melanin, a pigment in human skin.
-Another allele with a slightly different DNA sequence codes for an altered enzyme that cannot convert tyrosine into melanin.
-The result of the second allele is albinism, or the absence of melanin.
What are alleles?
The various, specific forms of a gene that usually vary from each other by one or a few bases.
What is a gene locus?
This refers to the specific position of a gene on a chromosome.
What is the difference between the diploid state and haploid state?
Having a pair of each kind of chromosome is called the diploid state; having one copy of each chromosome is the haploid state.
Are humans diploid or haploid?
-Like most eukaryotes, humans are diploid and inherit one copy of each chromosome maternally (from the mother) and the other paternally (from the father).
-As a result, humans have two copies of every gene. (There is an exception related to the sex chromosomes that will be covered in section 3.4.4).
-You may inherit the same allele from both parents, or you may inherit two different alleles.
What does it mean to be homozygous and heterozygous for a gene?
You are homozygous for a gene if you have two copies of the same allele; you are heterozygous if you have two different alleles.
What is your genotype?
The alleles you have for a gene.
What is your phenotype?
The trait you have as a result of your alleles.
Give an example of an organism’s genotype and phenotype
For example, if a plant inherits a red petal colour allele from both parents, its genotype would be homozygous for ‘red’ at the petal colour locus, and its phenotype would be flowers with red petals.
What is the heirarchy of genetic information? (biggest to smallest)
-Genome – consists of the entire genetic material of an organism.
-Chromosomes – long DNA molecules, which contain many genes.
-Genes – short stretches of DNA, which usually code for one characteristic.
-Allele – a specific form of a gene.
What is always different between the alleles of a gene?
Their base sequence.
A particular gene has three alleles across the human population.
At maximum, how many different alleles can you, an individual human, have for that gene?
2
This question is really asking how many of each type of chromosome do humans have. Humans have two of each chromosome, meaning two copies of each gene. This means that the copies you have can either be the same allele, or two different alleles. This makes two the maximum number of alleles you can have for a particular gene.
For the hypothetical ‘hair length’ gene, a dog has one copy of a ‘long hair’ allele and one copy of a ‘short hair’ allele.
How would you describe the dog’s genotype?
Heterozygous
What are mutations?
-When DNA is replicated, errors are sometimes made.
-The daughter cells then contain DNA that differs from the DNA in the parental cell.
-These changes in DNA sequence are called mutations.
-All new alleles are created by mutation.
-However, once a new allele exists it is copied and passed on in the same manner as the original sequence.
What are the causes of mutations?
-We know that the DNA replication mechanism can make mistakes; it is a very fast enzymatic process, where several hundred new bases per second are incorporated in a new strand.
-Even with ‘proofreading’ enzymes, occasionally a mistake will be made.
-Other possible causes of mutations include exposure to radiation and some chemicals, known as mutagens.
-If the mutagens cause tumorigenesis, then they are also known as carcinogens.
What can mutations cause to happen in the DNA?
-Mutations can cause bases to be deleted from or added to the DNA sequence.
-Mutations can also cause a different nitrogenous base to be used in a particular place, for example guanine could be used where adenine should have been
What is a base-substitution mutation?
A mutation that changes one nitrogenous base in a sequence.
Diagram of a base-substitution mutation event during DNA replication
What factors does the result of a base-substitution event depend on?
-Some mutations occur in a stretch of DNA that does not code for a polypeptide.
-Some mutations in protein-coding genes cause no change because the genetic code includes multiple codons (triplets) for the same amino acid.
-If an amino acid is changed, the polypeptide formed might work differently or not at all.
Diagram showing the effect of a hypothetical base-substitution mutation
Explain how new alleles formed by mutation can be harmful or advantageous
-The new alleles formed by mutation are usually harmful or neutral to an organism’s ability to survive and reproduce.
-Occasionally, an allele is formed that has some advantage.
-Evidence suggests that all life on Earth shares a common ancestor.
-From that original species, every difference between each and every living thing on Earth – from palm trees to E. coli to humans – has arisen by mutation.
Table showing a list of some organisms whose genomes have been sequenced
Diagram of a comparison of DNA, mRNA and amino acid sequence in Hb A (normal) and Hb S (sickle cell)
Diagram explaining the relationship between the mutation in sickle cell anemia and the structure of hemoglobin
Map showing the frequency of sickle cell allele and malaria
Picture of BLAST tools allowing comparison between nucleotide and amino acid sequences
Picture of a step-by-step method for comparing cytochrome C sequences
Genbank® was used to compare the cytochrome C gene of humans and Old World monkeys. The following table was generated as output, showing the levels of similarity with Homo sapiens.
State the percentage of nucleotides that are an exact match between the cytochrome C gene of Macaca cyclopis and that of Homo sapiens.
93%
You should find the species by reading the description. In this case, the 7th listed is correct. You should read the percentage from the ‘Ident’ column – second from the right – which gives the percentage of identical nucleotides.
Genbank® was used to compare the beta hemoglobin gene of humans and gorillas. The following table was generated as output.
State the percentage similarity between the beta-globin gene of Gorilla gorilla and that of Homo sapiens.
99%
Find the correct gene by reading the description. In this case, the first one is correct. Find the percentage similarity by reading the ‘Ident’ column – second from the right.
The BLAST below shows a portion of the cytochrome C sequence for humans compared to chimpanzees (Pan troglodytes).
How many differences are there between the subject (Humans) and chimps for nucleotides 1562–1503?
1
The selected range is the second row of data. There is one nucleotide difference, seen where the vertical connecting line is absent. In this case, humans have a ‘C’ where chimps have a ‘T’.
