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Chapter 5

B64 - Chapter 5

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Janelle Leboutillier

PSYB64 Chapter 5 Genetics and the Development of the Human Brain Genetics and Behavior  Genome: a set of instructions for constructing a human being  Genotype: personal set of genetic instructions  Phenotype: the genotype when it interacts with the environmental influence  Chromosomes: our genotype consists of 23 matched pairs of these o Each pair contains a chromosome from the mom and dad  DNA (Deoxyribonucleic acid): molecules that make up chromosomes  Genes are formed by smaller segments of DNA and each one contains instructions for making a particular type of protein  Gene expression: occurs when the genetic instructions are converted into a feature of a living cell  Most of the DNA is found in the chromosome, some are found in mitochondria o Mitochondrial DNA (mDNA) originates from the mother o Mutates are a regular rate  Alleles: different versions of a particular gene leading to different phenotypically traits o Max two versions of an allele but more than two can exist within a population (blood types)  Homozygous: if a person has two identical alleles, they are homozygous for that gene  Heterozygous: two different alleles for a gene  Recessive allele: will produce its phenotype only when it occurs as homozygous pair  Dominant: will produce its phenotype whenever it is present  Imprinted: only one allele is expressed o Identity depends on which parent supplied the allele o Have been implicated with genetic and behavioral disorders From Genetics to Proteins  Four nucleotides: adenine (A), cytosine (C), guanine (G) and thymine (T)  Sequences of bases of DNA are translated into proteins  Ribonucleic acid (RNA): a strand of DNA that make a copy of itself  Codons: bases along DNA and RNA that occur in groups of 3 o Each codon codes for an amino acid  Amino acids are joined together by ribosomes to form a chain  The chain is then folded into a shape based on its amino acid sequence and is officially a protein  Gene activity differs in different parts of the body and from different animals  Proteome: set of proteins encoded and expressed by the genome Sources of Genetic Diversity  Meiosis: process which forms egg and sperm cells o Parental chromosome are divided in half, leaving only one chromosome from each pair in an egg or sperm cell o Egg and sperm cells combine resulting in a zygote with 23 chromosome pairs  Linkage: when genes that are physically located close together on the same chromosome are often passed along to offspring as a group  Crossing over: chromosomes lining up prior to meiotic division physically cross one another and exchange equivalent sections of genetic material o Therefore, not all linked genes are inherited together Mutations PSYB64  Occur in the process of chromosome replication  Vast majority of them have little effect  If a segment of DNA that normally encodes a particular amino acid is somehow switched with another segment that produces the same amino acid there will be no effect  So this does not cause a mutation to occur  May occur in segment of DNA that do not appear to influence phenotypical traits  May result in a recessive allele  Inheriting a dominant mutant allele or two copies of the a recessive mutant allele will affect an organism’s phenotype  If the mutant allele is advantageous, it is likely to spread within species  Can be fatal and the mutation will disappear The Special Case of the Sex Chromosomes  Sex – linked characteristics: result from the genes on the X chromosomes that are not duplicated on the Y chromosome  Males are more likely to experience sex linked disorders than females o If there is a single recessive gene, it can influence the phenotype when there is no corresponding gene on the Y chromosome o Females can compensate the dominant gene on their second X chromosome  X – chromosome Inactivation: many gene on the X chromosome are not duplicated on the Y chromosome o Females can produce double the amounts of proteins than males o To compensate for imbalance, most of the genes on one of the X chromosomes are randomly silenced during development  X – inactivation in mothers has been linked to sexual orientations of sons Single Nucleotide Polymorphism (SNPs)  Single nucleotide polymorphism: occurs when a sequence of nucleotides making up one allele differs from the sequence of another at just one point The Roles of Heredity and Environment  Heredity always refers to a population, to individuals  Heredity also has to do with environment o If the environment is constant, the heritability of a trait is likely to be high  If the environment (everyone is treated the same way) is constant, the heritability of a trait is likely to be high  Studying monozygotic (MZ) and dizygotic (DZ) twins  MZ – shared the same genes  DZ – share the same number of genes  All twin shares similar environment unless they are separated o Regular siblings live in different environments at different times  Found that identical twins raised either apart or together were very similar, whether the correlation for a particular trait was high or low Development Growth and Differentiation of the Nervous System  Zygote: cell formed from egg and sperm  Embryo: Two to eight weeks after conception, it develops  Fetus: From eighth week until birth  Germ layers: a week after conception the human zygote has form three of these differentiated bands of cells  Ectoderm: outer layer which develops into the nervous system, skin and hair PSYB64  Mesoderm: the middle layer which forms into connective tissue, muscles, blood vessels, bone and the urogenital systems  Endoderm: develop many of the internal organs (stomach and intestines)  Neural plate: formed during the third week of conception, where cells in the ectoderm located along dorsal midline begin to differentiate into a new layer  Inducing factors: combination of genes or chemical signals that cause cells to differentiate  Neural tube: a structure formed by the developing embryonic neural plate that will eventually form the brain and spinal cord, with the interior of the tube forming the ventricle system of the adult brain  Six stages of development after neural tube is formed 1. Neurons and glia continue to form and grow 2. Migration of cells to their eventual locations in the nervous system 3. Differentiation of neurons into distinctive types 4. Formation of connections between neurons 5. Death of particular neurons 6. Rearrangement of neural connections The Formation of Neurons and Glia  Neurons and glia originate from cells located in the ventricular zone  Ventricular zone: a layer of cells lining the inner surface of the neural tube  Progenitor cells: a stem cell in the ventricular layer that gives rise to either additional progenitor cells or migrating neural cells o Divide by mitosis o One daughter cell remains and the other migrates out to form neuron/glial cell  If the progenitor cell produces an additional progenitor cell and a migrating cell, divide along a cleavage line parallel to ventricular zone o Parallel cleavage line means that daughter cell to the outside will not be attached to ventricular zone once division is complete o Cell is free to migrate Cell Migration  Radial glia: radiate from ventricular layer to the outer edge of the cerebral cortex, serving as a pathway for migrating neurons o Also retain ability to produce daughter cells  Some migrating cells wrap around radial glia and move along them o The rest do to follow radial glia and move in a horizontal direction  Migrating cells in cerebral cortex form in an inside – out fashion o Layers for the outside layers are first made inside and then migrate outside  Because of the regularity migration, researchers can determine timing of certain brain abnormalities Differentiation  Neural tube can go into two different process of further differentiation  First process differentiates dorsal and ventral halves of neural tube o Neurons in the ventral half develop into motor neurons  Sonic hedgehog proteins help keep the organization of the ventral neural tube into a motor system under control  Notochord: mesodermic structure that releases sonic hedgehogs  Eventually develops into vertebrae surrounding spinal cord o Neurons in the dorsal half develop into sensory neurons  Occur in response to BMP (inducing agent that was involved in differentiation of ectoderm) o Ventral sonic hedgehog and dorsal BMP organization goes up through hind- and midbrain PSYB64  Second differentiates the neural tube along its rostral – caudal axis o Division of nervous system into the spinal cord, hindbrain, midbrain and forebrain o Controlled by inducing proteins – Hox genes o Different Hox genes are expressed in different segment of hindbrain leading to development of specific cranial nerve nuclei  Mature organization of the cortex appears to be a combination of internal genetic factors and external inducing factors Growth of Axons and Dendrites  Growth cones: the swelling at the tip of a growing axon or dendrite that helps the branch reach its synaptic target o Have both sensory and motor abilities to help developing branch find pathway  Three basic structural parts 1. Main body containing mitochondria, microtubules and other organelles 2. Filopodia: long, fingerlike extensions f
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