D2.2 Gene Expression Flashcards
[D2.2.1] Outline gene expression
Gene expression is the process of turning the genotype into the phenotype. This happens in a series of stages:
1. Transcription: a selected genotype is produced into a mRNA, which is transported to the ribosomes in the cytoplasm.
2. Translation: ribosomes translate the mRNA to form a specific polypeptide chain, where non-polypeptide components are also sometimes added.
3. Protein function: proteins can change appearance of organism, the phenotype, or can have other functions inside the organism like catalysing reactions by becoming an enzyme.
[D2.2.2] Explain how the rate of transcription is regulated in a cell
For the transcription of a gene, transcription factors binds to a promoter sequence, upstream of the target gene sequence. A common feature of the promoter is the TATA box, including a short sequence of thymine and adenine bases as a part of the promoter. These transcription factors indicate where RNA polymerase should start transcription.
Transcription factors are DNA-binding proteins that regulate transcription. Their binding sites are upstream of the promote and the target gene. The transcription factors that act as activators or repressors to affect the rate of transcription:
- Enhancers: activators that increase the rate of transcription
- Silencers: repressors that decrease the rate of transcription
[D2.2.3] Explain the importance of the poly A tail in regulating translation
The degradation of the mRNA by nuclease enzyme is regulated by the long poly A tail. The length of the poly A tail can be shortened, which decreases the chance of the mRNA being translated. This may occur if the protein is only needed for a short period of time. The rate at which the poly A tail degrade varies from 30 nucleotides per minute to 1 nucleotide per hour.
[D2.2.4] Outline epigenesis, epigenetic tags and its impact on the geneotype and phenotype of an organism
Epigenesis is the concept of multicellular organisms like plants and animals developing from undifferentiated cells. Differentiation of cells are achieved by the activation and silencing of certain genes.
Epigenetic tags are chemical modification to the DNA, and proteins associated with the DNA determines which gene is activated or deactivated. Epigenetic modification does not influence the genotype of an organism because DNA sequences are not altered. Only the phenotype is affected during epigenesis.
[D2.2.5] Distinguish between genome, transcriptome and proteome
Genome: entire genetic information of a cell that includes both coding and non-coding sequences.
Transcriptome: entire set of mRNA that can be transcribed in a cell. However, no cells express all of the genes
Proteome: entire set of proteins that can be produced by a cell.
[D2.2.6] Explain the role of methylation and acetylation by describing the structure of nucleosomes. Provide example of methylation of a specific base sequence
DNA is coiled using nucleosomes, containing 8 histone proteins. These histones have histone tails, which is made out of amino acids. The amino acids can be methylated making the DNA more tightly coiled to decrease the rate of transcription since transcription factors cannot access the DNA. Methyl group is an epigenetic tag, and methylation patterns does not alter the genome since complementary base pairing is not affected, but patterns change during individual’s lifetime due to environmental factors. Methylation of cytosine in the promoter region tends to repress transcription.
Acetyl groups are also epigenetic tags that can make DNA loosely coiled. This increases the rate of transcription since transcription factors can easily access the DNA.
[D2.2.7/ D2.2.8] Describe the inheritance of epigenetic tags. Provide a definition of epigenome.
The sum of all epigenetic tags in a cell is the epigenome. Each type of cell in a multicellular organism have distinctive patterns of epigenetic tags to produce specific proteins. When haploid gametes are formed by meiosis from diploid cells, the epigenetic patterns are conserved and therefore is passed on to the daughter cells. The epigenome can be affected by environmental factors such as air pollution, which decreases DNA methylation across the genome. This changed epigenome is also passed on to daughter cells, increasing the chance of the offspring having heart disease or asthma.
[D2.2.9] Outline the consequences of genomic imprinting, using an example.
During the production of sperms and eggs in humans, approximately 99% of the epigenetic tags are removed, however, some tags are still inherited to the offspring, impacting the phenotype of the offspring. This is genetic imprinting, where only 1 of the 2 genes for a trait is expressed since 1 copy is silenced by epigenetic tags during egg and sperm formation.
Ex. Ligers and tigons:
Male lions have epigenetic tags that allow its offspring to be large as possible. Female lions have epigenetic tags that allow big litters. Thus, when a male lion and a female tiger forms a liger, their offspring are very large. On the other hand, when a female lion and a male tiger forms a tigon, their offspring are smaller.
[D2.2.10] Describe how dizygotic and monozygotic twins are formed
Twin can arise when 2 eggs are released during ovulation and both are fertilised. Such twins are dizygotic and have about 50% of their genetic information common. More rarely, there are monozygotic twins, where the cells of very early embryo become separated and each develops into an individual. There has been studies with monozygotic twins to investigate the impact of the environment on their gene and phenotypes.
[D2.2.11] Describe how variants in environmental factors can impact gene expression by using 2 examples
Variations in environmental factors that impacts gene expression is seen in:
1. Lactose metabolism
When lactose is present, a repressor protein is deactivated, allowing genes in E.coli to be expressed and metabolise lactose. On the other hand, if lactose is absent, the repressor protein is activated again, preventing gene expression. In this case, the breakdown of lactose regulate gene expression.
2. Oestrogen and progesterone hormones When oestrogen passes through the phospholipid bilayer of cells in the uterus, it binds to an oestrogen receptor within the nucleus, This allows a target gene to be expressed. This target gene includes the production of progesterone receptors.
