2 - Biology of Development Flashcards
Critical Periods, prenatal and natal
time periods (ages) in which specific experiences are necessary for typical development to occur, there are both biological and environmental experiences
Prenatal
- certain hormones required for proper sex organ development
Natal
- early exposure to visual input is requires for typical visual development
> generalisable
Factors of genes
- genes made of DNA
- 46 chromosomes (23 pairs)
- genes code for proteins which incur bodily functions
- structural DNA sequence (A-T, C-G) directs the assembly of proteins
- Regulatory DNA determines when and how much protein is produced
Meiosis
Cell division that produces 4 genetically distinct cells with half genetic information (23 chromosomes)
- produces germ cells (sperm and egg)
- one chromosome from each parent is duplicated
- there is a crossing over action (the order of genes stays the same, but which gene variant is inherited may vary)
Mitosis
General cell division producing 2 genetically identical cells via chromosome duplication
Cause of Genetic Variation
Due to Meiosis
- Segregation
> only 1 pair of chromosomes transmitted from each parent
- Independent assortment
> which 1 chromosome of that pair that is transmitted is random - Crossing-over
> between members of homologous pairs, causing variation in the alleles
Homozygous
A pair of the same alleles a chromosome, coding for a particular aspect
Heterozygous
A pair of different alleles within a chromosome, coding for a particular aspect
Recessive vs Dominant Alleles
Recessive Alleles will only induce their function if the chromosome is homozygous
Dominant alleles will induce their function in heterozygous chromosomes
Genotype and Phenotype
Genotype
- an individual’s unique combination of (all) alleles
- identical twins have the same genotype
Phenotype
- the observable trait produced by the genotype
> and environmental factors
Environmental effects on development (pre and post natal)
Prenatal
- hormones
- substances consumed by mother
- mother’s health
- sounds and light (in late gestation)
Postnatal
- alcohol/drugs
- environmental toxins
- infectious diseases
- parental love
- nutrition
Regulatory Cascades (in genes)
Due to environmental triggers and repressors
- an environmental trigger causes gene activation and influences level of expression
- causing a subsequent cascade of gene activation and expression
- environmental repressors stop this gene activity
Genetic Canalisation
- genetic or epigenetic restrictions to a small number of developmental outcomes
- increases cell specialisation through epigenetic regulation
- unidirectional
Epigenetic Regulation
- ‘on top of genetics’
- if DNA is hardware, the epigenome is software, regulating DNA utilisation in different cells at different times
- Epigenetic regulation often involved Methylation
- is often precisely timed, ensuring changes occur in the correct developmental window
(if this does not happen it can cause developmental errors)
Passive gene-environment Correlation
- genetic predisposition shapes environment
a genetically intellectual household may have many books around, stimulating intellectual development in the child
Evocative gene-environment correlation
- inherited tendencies evoke certain responses from others
a child with predisposed behavioural problems may evoke harsher parenting
Active gene-environemnt correlation
- genetic predisposition encourages ‘niche picking’
child with a predisposition for football talent actively chooses to join the football club
Describe the processes:
- Fertilisation
- Implantation
- Early Divisions
- Embryonic Period
- Foetal Period
Fertilisation
- 50-100 sperm reach the egg, only one normally penetrates
- chemical process prevents other sperm from penetrating
- this now forms a zygote
> monozygotic twins (identical)
> dizygotic twins (not)
Implantation
- about 6 days post fertilisation
- egg implants into the wall of the uterus
- by 2 weeks the egg is fully implanted
- now it is called an embryo
Early Divisions
- first division at 2 hours
- by day 4 there is a morula (16 cells)
- day 5 there is an outer and inner layer of cells
> blastocyst
+ outer cells will become the placenta
- day 16 is gastrulation
> blastocyst inner layer of cells differentiate into the ectoderm, mesoderm and endoderm
(ectoderm and mesoderm give rise to all other cells)
Embryonic Period
- weeks 3-8
- large changes in body structure
- large changes in size
- cells are differentiating
Foetal Period (prenatal)
- week 9 to birth
- heartbeat and facial features
- by 22 weeks some can survive with neonatal intensive care
- by 28 weeks, fully developed lungs
Homeobox genes
- responsible for the general structure and organisation of body parts in utero
> by encoding homeodomain proteins that regulate genetic activity - highly specific, resulting in species-specific characteristics
- there are also gene sequences that exist across many species
Teratogens
- harmful environmental factors that can disturb prenatal development
- Thalidomide
> was prescribed for morning sickness, ended up producing huge abnormalities in physical development
> this was dependent on when in the pregnancy the foetus was exposed to Thalidomide - current example is the Zika virus
- causing babies to be born with microcephaly (very small brain), associated with reduced mental ability
Foetal Alcohol Syndrome
- caused by excessive alcohol during pregnancy
- distinct facial features, small lips, microcephaly
- intellectual disability, emotional and behavioural problems
Brain Development timeline
Prenatal:
Week 3
- neural tube starts to form (brain development begins)
Week 4 - three distinct brain regions > forebrain > midbrain > hindbrain - neural tube is almost completely closed
Week 5
- cerebral vesicles are present
Week 10
- major CNS structures are visible
Week 20
- brain weighs 100g
- cortical surface appears smooth
Week 24
- apoptosis begins (programmed cell death)
Week 28
- cortical surface has clear folds
Week 38 - birth
- brain weighs 350-400 grams
Postnatal:
- in the first year the brain more than doubles in weight
- by age 5 it is about 1,250g
Brain development procedure and Pruning
Brain structures and systems that are life-essential develop first
- followed by structures supporting basic motion, sensing stimuli
- structures involved in complex reasoning and planning develop last
Brain structures that develop later help act by inhibiting earlier developed abilities (i.e. the ability to hold breath)
Brain structures get ‘pruned’ over development
- brain grows until age 5
- neural pathways are trimmed, only the essential ones are kept over development (ones that are used more heavily)
Brain Communication
Occurs via neurons, transmitting information in the brain
- communication between neurons occurs at the synapses
The Speed of synaptic communication increases over development
- between birth and adulthood, some increase 100x
Neuron Development
Prenatally:
Proliferation
- largely prenatally, lots of neurogenesis
Migration
- new nerve cells are made in the centra of the brain and move outwards
- mostly prenatally
Postnatally: Consolidation - synaptic pruning - sometimes via apoptosis - postnatal Myelination - enclosing the axon with a myelin sheath, increasing speed of transmission - some areas this occurs early, some areas continue to be myelinated into later life
Grey and White matter
Grey Matter
- around the edges
- contains neuronal cell bodies
White Matter
- centre of the cortex
- composed of axons surrounded by myelin (looks white)
Effects of early deprivation on brain development
- less development of grey matter volume, which remains that way
> low neural plasticity - less development of white matter, but can be caught up
> neural plasticity
Sheridan et al.
- study in 8-11 year old Romanian children
- 20 typically developing
- 29 exposed to institutional rearing (associated with deprivation)
- 25 previously exposed previously exposed to institutional rearing, subsequently raised in foster care
- all had their cortical volume measured with MRI scans
Results:
Grey Matter
- no difference in cortical volume matter in children exposed to institutional rearing
- significant increase in cortical volume matter in children never in institutional rearing
> suggesting a lasting effect on grey matter
White Matter - highest volume in typical children - less in those in foster care - even less in those in institution > suggesting neural plasticity
Limitations:
- prenatal and genetic risk factors
Neural Plasticity
The capacity of the brain to respond and adapt to inputs from the external environment
Resilience
- the ability of a child with abnormal birth conditions, to overcome these adverse effects and develop normally
- Poehlmann et al. found that resilient children are less likely to have experienced negative parenting earlier in childhood
Brain development in Puberty and risk taking
- overall total brain volume is larger in boys than girls
- Prefrontal cortex (PFC) matures latest
Changes in reward and control circuits:
- Striatum is involved in reward-processing
- PFC is involved in cognitive control
Children - both are immature > the striatum matures more quickly than the PFC Adolescents - Striatum is mature, PFC is not Adults - both are mature
These differences in maturation could explain why risk taking is most prevalent in adolescents
- since in adolescence, there is a developed system of reward seeking, but an immature system of cognitive control
Pleiotropy
One gene affects many different types of traits
Polygenic
One trait is affected by many genes
Genetic Heterogeneity and Autism
Different sets of genes produce the same trait in different people
Autism
- twin studies show it is highly heritable
- molecular genetic studies indicate there are different genes that cause autism
- it may be a combination of genes
