Lecture 07 Flashcards
Sex-limited traits
(autosomal or x-linked) traits that affect a structure or function occurring only in one sex
ex: beard growth & milk production
sex-influenced traits
traits in which the phenotype expressed by a heterozygote is influenced by sex
allele is dominant in sone sex but recessive in the other
ex: heterozygous male is bald, but a heterozygous female is not
what the phenotype in sex-influenced traits expressed by?
A heterozygote
The inactivated x chromosome
barr body
X inactivation
females have two alleles but males have only one x chromosome gene
in mammals, x inactivation balances this inequality and one x chromosome is randomly inativated in each cell
When does x inactivation occur
early in prenatal development
What is x inactivation an example of?
epigenetic change - an inherited change that does not alter the DNA base sequence
the XIST gene encodes an RNA that binds to and inactivates the x chromosome
Manifesting heterozygote
female that expresses the phenotype corresponding to an X-linked gene
genomic imprinting
the phenotype of an individual differs depending on the gene’s parental origin
How are genes imprinted?
by an epigenetic event: DNA methylation
-methyl (CH3) groups bind to DNA and suppress gene expression in a pattern determined by the parent’s sex
Importance of genomic imprinting
- Function plays a role in development
- explains incomplete penetrance
- two opposite sex produces a healthy embryo (male genome controls placenta development and female genome controls embryo development)
When are (genomic) imprints erased and what happens next?
They are erased during meiosis and then reinstituted according to the sex of the individual
What are the two syndromes that result from a small deletion in chromosome 15?
1) Prader-Willi syndrome - deletion inherited from father
2) Angelman syndrome - deletion inherited from mother
the two syndromes may also result form uniparental disomy
How do nucleotide join
via a covalent bonddbetween the 5’ phosphate of one and the 3’ hydroxyl of another
-this creates a continuous phosphate backbone
What do two aligned polynucleotide chains form?
A double helix
-hydrogen bonds forming between complementary bases holding two strands together
What direction does the double-helix run?
One strand runs a 5’ to 3’ direction and the other strand runs in a 3’ to 5’ direction
Histones
proteins that is coiled with DNA, forming a bead-on-a-string like struction, called the nucleosome, which winds tigher to form chromatin
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When does DNA replication occur?
During the S phase of the cell cycle, prior to cell division
-replicates about 50 bases/sec
Replication fork
a site where DNA is locally opened
Enzymes in DNA replication
helicase binding proteins primase DNA polymerase ligase
How much protein-encoding genes does the human genome contain?
~20,325 protein-encoding genes (this only represents a small part of the genome)
What does much of the human genome control?
protein synthesis - including the time, speed, and location
Gene expression - difference between transcription and translation
transcription - production of mRNA
translation - production of protein using mRNA, tRNA, and rRNA
What type of enzyme does transcription require?
RNA polymerase
Transcription
RNA is synthesized from one strand, called the template strand
the complementary strand is called the coding strand of DNA
Two types of nucleic acids
RNA & DNA
Types of RNA
1) mRNA (500-4,500+ bases long) - encodes amino acid sequence
2) rRNA (100-3,000 bases long) - associates with proteins to form ribosomes, which structurally support and catalyze protein synthesis
3) tRNA (75-80) - transports specific amino acids to the ribosome for protein synthesis
