Chapter 13-15 Homework Review

Missense mutation: A point mutation where a single nucleotide change results in a codon that codes for a different amino acid. This can lead to a non-functional protein or a protein with altered function.
Nonsense mutation: A point mutation that changes an amino acid codon into a premature stop codon, leading to a truncated and often non-functional protein.
Silent mutation: A point mutation that changes a single nucleotide, but does not change the amino acid sequence of the protein. This occurs because of the redundancy of the genetic code, where multiple codons can code for the same amino acid.
Frameshift mutation: Caused by insertions or deletions of nucleotides in a DNA sequence that are not multiples of three. This shifts the reading frame during translation, leading to a completely different amino acid sequence downstream from the mutation and often resulting in a non-functional protein.
Mutations are broadly classified as point mutations (affecting a single nucleotide or a small number of nucleotides, like missense, nonsense, and silent mutations) or frameshift mutations (involving insertions or deletions that alter the reading frame).

Messenger RNA (mRNA) in eukaryotes undergoes several modifications after transcription, collectively known as post-transcriptional RNA processing.
- 5’ cap addition: A modified guanine nucleotide is added to the 5' end of the pre-mRNA. This cap is crucial for ribosome binding during translation, protects the mRNA from degradation, and aids in transport out of the nucleus.
- Poly-A tail addition: A sequence of approximately 50-250 adenine nucleotides is added to the 3' end of the mRNA. The poly-A tail protects the mRNA from enzymatic degradation, facilitates mRNA export from the nucleus, and helps in the initiation of translation.
Introns: These are non-coding sequences within a gene that are transcribed into pre-mRNA but are ultimately removed during RNA splicing. They are interspersed between coding regions.
Exons: These are the coding sequences that are retained in the mature mRNA and are eventually translated into protein.
Splicing: The process by which introns are removed from pre-mRNA and exons are ligated together. This precise removal is carried out by a complex molecular machinery called the spliceosome, composed of small nuclear ribonucleoproteins (snRNPs) and other proteins.

Ribosomes are complex molecular machines, composed of ribosomal RNA (rRNA) and proteins, that synthesize proteins by linking amino acids together in a specific order dictated by the mRNA sequence. They move along the mRNA molecule, reading the codons.
tRNA (transfer RNA): Small RNA molecules that act as adapters, carrying specific amino acids to the ribosome based on matching anticodons to the codons on the mRNA.
mRNA (messenger RNA): Carries the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm. It consists of a sequence of codons, each specifying a particular amino acid or a stop signal.
Amino acids: The fundamental building blocks of proteins, linked together by peptide bonds during translation.
Genetic code: The set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins by living cells. It is degenerate (redundant), unambiguous, non-overlapping, and nearly universal.
Start codon: AUG typically signals the beginning of protein synthesis and codes for the amino acid methionine (or N-formylmethionine in prokaryotes).
Stop codons: UAA, UAG, and UGA signal the termination of protein synthesis, and no tRNA carries amino acids for these codons.

Gene expression is a tightly controlled process that determines which genes are expressed and at what levels, allowing cells to adapt to their environment and perform specialized functions.
Primarily regulated by controlling transcription: This involves controlling the binding of RNA polymerase to the promoter region of a gene, often influenced by regulatory proteins called transcription factors (activators or repressors) that bind to specific DNA sequences (enhancers or silencers).
Alternative splicing: A post-transcriptional regulatory mechanism that allows one gene to produce multiple distinct protein variants (isoforms) by selectively including or excluding certain exons during the splicing process. This significantly increases the coding capacity of the genome.
Other levels of regulation include translational control, post-translational modifications, and mRNA stability.

Source of genetic variation: Mutations are the ultimate source of new alleles and genetic variation within a population. This variation is raw material for evolution.
Evolutionary adaptation: Adaptive mutations (beneficial mutations) provide an advantage to organisms in a particular environment, increasing their survival and reproductive chances. These mutations may then spread through populations over generations via natural selection, leading to evolutionary change.
Harmful mutations: Most mutations are neutral or harmful, typically decreasing an organism's survival or reproduction chances. Organisms with detrimental mutations may be selected against.
Disease: Many genetic diseases are caused by mutations in specific genes (e.g., cystic fibrosis, sickle cell anemia).

Nondisjunction: The failure of homologous chromosomes to separate properly during meiosis I, or the failure of sister chromatids to separate during meiosis II (or mitosis). This results in gametes with an abnormal number of chromosomes.
Leads to aneuploidy: The condition of having an abnormal number of chromosomes in a haploid set. Individuals with aneuploidy typically have either too many or too few chromosomes. Examples include:
- Trisomy: Presence of an extra chromosome (e.g., Trisomy 21, causing Down syndrome; having 3 copies of chromosome 21 instead of the normal 2).
- Monosomy: Absence of one chromosome (e.g., Monosomy X, causing Turner syndrome; having only one X chromosome in females).
Aneuploidy often results in developmental abnormalities, miscarriages, or genetic disorders.

Transcription: The first step of gene expression, where genetic information from a DNA template is copied into an RNA molecule by RNA polymerase. This occurs in the nucleus of eukaryotes and the cytoplasm of prokaryotes.
Translation: The process by which the genetic information carried by mRNA molecules is decoded to synthesize proteins. This occurs on ribosomes in the cytoplasm.
Splicing: The process in eukaryotic pre-mRNA processing where introns are removed, and exons are joined together to form a mature mRNA molecule.
Telomeres: Repetitive nucleotide sequences (e.g., five prime-TTAGGG-three prime in humans) that cap the ends of eukaryotic chromosomes, protecting the internal gene-carrying regions from degradation and preventing chromosomes from fusing with each other. They shorten with each cell division.
Telomerase: An enzyme that extends telomeres by adding more repetitive sequences to the ends of chromosomes. It is typically active in germ cells, embryonic stem cells, and cancer cells, helping to maintain chromosome length and cell proliferative capacity.