4.1 Genes, Chromosomes, Mutations
27/01/2013 § Leave a comment
Welcome to the seventh unit of Year One! This blog will be a bit of an introduction to the next three units, in my opinion, which includes Meiosis (Unit 7), Genetics (Unit 8), and Reproduction (Unit 9!!).
Meiosis is centered on genetics, which is the study of variation and inheritance, and the basic unit of inheritance is the gene, which is a heritable factor that controls a specific characteristic. The normal nucleus in any organism contains thousands of genes – the exact number is and has mostly always been unknown. The collective whole of the genetic information in an organism is the genome.
Chromosomes have the same genes as one another and are arranged in the same sequence but not entirely in terms of the alleles in the genes. Alleles are different forms of a gene, seeing as genes are made of DNA (remember the four – or five – bases that make up DNA and RNA molecules? Yep, those).
Now, during mitosis or meiosis, which are both processes of division, the DNA in nuclei are replicated – we know this. The two strands on the chromosome (called chromatids) are connected by a centromere, which can be found either in the middle of the chromosome or towards the end.
Alright, sorry about that “boring” stuff (that’s a joke, genetics and reproduction are my favorite part of biology) but the slightly more interesting part comes in now: genetic mutation. When genes are passed from parent to offspring, it’s better if they don’t change and stay the same, maintaining the parents’ qualities and passing them on to the offspring (Natural selection! Evolution! Daaaarrwiiiinnn!). A male parent’s gamete and a female parent’s gamete (gametes are haploid, which means the nucleus has only one set of chromosomes, therefore one chromosome of each type – think of it like half of a full chromosome) to form a zygote, which is diploid, wherein the nucleus has two sets f the chromosome, so there are two chromosomes of each type.
But moving on – when the base sequence of a gene is changed, that is a gene mutation. The smallest change is when only one base changes (called a base substitution), but that change can make an entire difference. My 8th grade science teacher told us that it’s the difference between typing D-U-C-K on an English report and accidentally making a typo, i.e. replacing “D” with another letter very close to it on the keyboard. There are thousands of genetic diseases caused by genetic mutations, that have been discovered in humans, and the most commonly used example is sickle cell anemia.
- Define the terms gene and allele and explain how they differ. 4 marks
- Describe the consequences of a base substitution mutation with regards to sickle cell anemia. 7 marks
- Outline the formation of chiasmata during crossing over. 5 marks
- Explain how an error in meiosis can lead to Down syndrome. 8 marks
- Karyotyping involves arranging the chromosomes of an individual into pairs. Describe one application of this process, including the way in which the chromosomes are obtained. 5 marks
- Compare the processes of mitosis and meiosis. 6 marks
- Outline one example of inheritance involving multiple alleles. 5 marks
- Describe the inheritance of ABO blood groups including an example of the possible outcomes of a homozygous blood group A mother having a child with a blood group O father. 5 marks
- Outline sex linkage. 5 marks
- Explain, using a named example, why many sex-linked diseases occur more frequently in men than women. 9 marks
DATA BASED QUESTIONS
Page 155, differences in chromosome number
1. There are many different chromosome numbers in the table, but some numbers are missing, for example, 5, 7, 11, 13. Explain why none of the species has 13 chromosomes.
There cannot be an odd number of chromosomes because each chromosome is a pair, wherein each chromatid is given by one parent (even the diploid zygote, which is half male gamete and half female gamete, which still makes it even), always doubling the number of chromosomes.
2. Discuss, using the data in the table, the hypothesis that the more complex an organism is, the more chromosomes it has.
We can look in terms of the number of chromosomes in a plant. A Michaelmas daisy is more extravagantly detailed than a field bean and the daisy has far more chromosomes than the bean (54 versus 12). With the animals, we see an even stronger relationship between the complexity of the organism and the number of chromosomes the organism has. A fruit fly, for example, has but 8 chromosomes, compared to an armadillo, which has 64 chromosomes. Fruit flies are quite obviously far smaller than armadillos and have less tasks to fulfill in its daily life. The armadillo would need more proteins and cells to accomplish its different functions, therefore it would require more chromosomes to code for those proteins and enzymes.
3. Explain why the size of the genome of the species cannot be deduced from the number of chromosomes.
We do not have the data of the sequences within each of the chromosomes. In each of a vampire bat’s 28 chromosomes, for example, there are different numbers of genes, and we don’t have those numbers.
4. Suggest, using the data in Table 3, a change in chromosome structure that may have occurred during human evolution.
The easiest change that can be pinpointed would be the close relationship between chimpanzees and humans. Both organisms have frequently been said to be distant ancestors of each other, which also makes sense because of not only the physical traits but because of the number of chromosomes (humans = 46, chimpanzees = 48). The small difference between the chromosomes (only one pair difference!) could suggest that humans evolved from chimpanzees because perhaps some of our chromosomes lost the need for certain proteins that became unnecessary for humans.